Guidance for the A2 Practical Assessment

Tutor support materials

GCE

Edexcel GCE in Physics Guidance for the A2 practical assessment February 2009

Edexcel, a Pearson company, is the UK’s largest awarding body, offering academic and vocational qualifications and testing to more than 25,000 schools, colleges, employers and other places of learning in the UK and in over 100 countries worldwide. Qualifications include GCSE, AS and A Level, NVQ and our BTEC suite of vocational qualifications from entry level to BTEC Higher National Diplomas, recognised by employers and higher education institutions worldwide. We deliver 9.4 million exam scripts each year, with more than 90% of exam papers marked onscreen annually. As part of Pearson, P earson, Edexcel continues to invest in cutting-edge technology that has revolutionised the examinations and assessment system. This includes the ability to provide detailed performance data to teachers and students which helps to raise attainment.

Acknowledgements This guide has been produced by Edexcel on the basis of consultation with teachers, examiners, consultants and other interested parties. Edexcel would like to thank all those who contributed their time and expertise to its development.

References to third-party material made in this specification are made in good faith. Edexcel does not endorse, approve or accept responsibility for the content of materials, which may be subject to change, or any opinions expressed therein. (Material may include textbooks, journals, magazines and other publications and websites.)

Authorised by Roger Beard Prepared by John Crew

All the material in this publication is copyright © Edexcel Limited 2009

This document should be read in conjunction with the GCE Physics specification — Issue 3 (publications code UA018902)

Contents

Introduction

1

How science works

1

General considerations

1

Preparing students for the practical assessment

3

Introduction

3

Safety

3

Planning: General

3

Planning: Identifying equipment

3

Planning: Identifying techniques to use

4

Implementation: Measurements

6

Accuracy and precision

6

Implementation: Recording results in tables

7

Analysing: Graphs

7

Analysing: Limitation of results

8

Evaluating

9

Advice for students

10

Plan

10

Implementation and measurements

11

Analysis

11

Conclusion

12

Uncertainties in measurements

13

What are uncertainties? Why are they important?

13

Calculating uncertainties

13

Calculating percentage uncertainties

14

Compounding errors

14

Using error bars to estimate experimental uncertainties

16

Carrying out the practical work

16

Providing guidance to students during the practical pr actical session

17

Carrying out the analysis

17

Returning work

18

Exemplar of assessed work: Interacting magnetic fields

19

Briefing

19

Exemplar for an able student using own plan for interacting magnetic fields

20

A2 Marking grid for interacting magnetic fields — able student

24

Exemplar for a less able student using own plan for interacting magnetic fields

27

A2 Marking grid for interacting magnetic fields — less able student

29

Examiner’s comments for interacting magnetic fields

32

Exemplar of assessed work: Guitar strings

33

Briefing

33

Exemplar for an able student using own plan for guitar strings

34

A2 Marking grid for guitar strings — able student

39

Exemplar for a less able student using own plan for guitar strings

42

A2 Marking grid for guitar strings — less able student

45

Examiner’s comments for guitar strings

47

Exemplar of assessed work: Linked oscillators

48

Briefing

48

Student exemplar using own plan

49

A2 Marking grid for linked oscillators

53

Examiner’s comments for linked oscillators

55

Exemplar of assessed work: Temperature control

56

Briefing

56

Student exemplar using own plan

57

A2 Marking grid for temperature control

60

Examiner’s comments for temperature control

63

Training Exercise: Safety in hospital

64

Introduction

64

Safety in hospital

64

Frequently asked questions

65

Questions relating to written work

65

Questions relating to the practical session

65

Questions relating to marking work

66

Other questions

67

Further advice

68

Plagiarism and collusion

68

Annotation of student work

69

Glossary

70

Appendix 1: Exemplar centre devised plans for candidates

71

Plan for experiment for interacting magnetic fields

71

Plan for experiment for guitar strings

72

Plan for experiment for linked oscillator

73

Plan for experiment for temperature control

74

Plan for experiment for safety in hospital

75

Appendix 2: Precision, accuracy and sensitivity

77

Introduction

All A2 students are required to carry out one piece of assessed practical work that is based on an application of physics. This book provides guidance and e xamples for the practical work. It includes a section that discusses how students should be prepared for this assessment, advice for students and some notes on uncertainties that may be issued to students, and suggestions and exemplars of practical assessments.

How science works The practical assessment gives students the opportunity to address some of the ‘How science works’ themes. These themes are about how scientists go about investigating the world about us. It has nothing to do with content and is a development that builds on and extends the science skills from Key Stages 3 and 4 through to AS. Students can use this opportunity to demonstrate: •

•

•

•

•

•

their knowledge and understanding to pose scientific questions, define scientific problems, and to present scientific arguments and ideas their ability to use appropriate methodology to answer scientific questions and solve scientific problems their ability to carry out experimental and investigative activities, including appropriate risk assessment their ability to analyse and interpret data to provide evidence, recognising correlations and casual relationships their ability to evaluate methodology, evidence and data their ability to communicate information and ideas in appropriate ways using appropriate technology

•

a consideration of ethical issues

•

an appreciation of the ways in which society uses science to inform decision-making

•

a consideration of applications and implications of science.

General considerations It is important to ensure that all students have the opportunity to gain marks for all the assessment criteria for Unit 6 when selecting the practical work. It would be beneficial to the students to be given a practical on a topic within the A2 or AS course but this is not a requirement of the assessment criteria (however it is expected that this work will show progression from AS). The practical work can be completed at any time during the A2 course but it would be more appropriate to administer the assessment near the end of the course. The practical work should take no more than two hours to complete. The practical work needs to involve the variation of two interdependent quantities that can be measured. Students need to be able to produce a graph that will usually be a straight line and derive the relationship between the two variables or derive a constant. For example this might involve one variable plotted against the square root of the other. It is a development from AS that students will often plot log graphs.

Tutor support materials – Edexcel GCE in Physics – Guidance for the A2 practical assessment – Issue 1 – February 2009 2009 © Edexcel Limited Limited 2009

1

Edexcel does not specify a list of equipment that should be made available to students and therefore the practical assessment may be achieved by using basic laboratory apparatus; this does not preclude students from using more complex equipment and centres are encouraged to use equipment such as signal generators, oscilloscopes and data logging devices, where these are available, to reflect the improving skills of the candidates. The practical work has been designed to be flexible so that centres may use their existing resources. If many students in large centres require the use of expensive equipment then different groups of students may have to do the practical assessment at different times of the year. If a staggered approach is taken then different groups of students should do different experiments to avoid collaboration.

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Tutor support materials – Edexcel GCE in Physics – Guidance for the A2 practical assessment – Issue 1 – February 2009 2009 © Edexcel Limited 2009

Preparing students for the practical assessment

Introduction The practical work will assess each student’s ability to: •

plan

•

implement

•

analyse and

•

evaluate.

Centres should devise and implement a suitable programme of practical work throughout the A2 course to ensure that students acquire the skills and experience that will be needed for them to succeed in each of t hese aspects of the practical assessment. The specification suggests experiments that students could carry out to enable them to experience a wide range of practical skills. The suggestions are not exhaustive and centres could use different experiments to those suggested to reflect the equipment that they have available. Students should be encouraged to calculate percentage uncertainties (discussed in another section) whenever possible in experiments that t hey do throughout the course. They should be given the opportunity to combine uncertainties and carry out more complex error analysis than at AS.

Safety Teachers should emphasise the importance of safety in all practical work throughout the course as a matter of good practice.

Planning: General The plan should include all aspects of the practical from selection of the apparatus through methods employed to how the data will be used and it should include some indication of how the aim, stated in the briefing, will be achieved. The intentions should be clear with few grammatical or spelling errors and will benefit from subheadings that divide the text into clear sections.

Planning: Identifying equipment Students should be able to identify apparatus and materials that are needed to achieve a particular aim. This includes the identification of the most appropriate measuring instruments for a particular task. For example, if a student needs to measure the width and thickness of a rule then they would be expected to select vernier callipers for the width and a micrometer for the thickness (or a suitable digital device for both).


3

Students should be aware of the precision of instruments, in general: mm scale

0.50 mm

vernier

0.10 mm

micrometer

0.01 mm

If measuring a mass such as the mass of a coin students should identify an appropriate instrument to use. Different digital top pan balances have different ranges and different precisions. Students should select the most appropriate top pan balance to use. Where appropriate, students should calculate/estimate the values of equipment needed, eg, resistors and their power rating in electrical circuits or suggest a range of values, eg weights, that will be needed for their experiment.

Planning: Identifying techniques to use Students should develop their knowledge and understanding of a variety of techniques in order to produce results which are as accurate and reliable as is reasonably possible. Experience shows that students who do this are more likely to gain higher marks for the better results that this achieves. The following list (which is by no means exhaustive) contains some common techniques that should be experienced several times during normal practical work: •

zero error checks

•

repeat measurements (at different places if appropriate)

•

difference methods (eg for extension of a spring)

•

eye level to avoid parallax error

•

use of marker at centre of oscillations to aid timing

•

use of set square for checking vertical or horizontal arrangements

•

interpolation of analogue scales

•

trigonometric methods for measuring angles.

4


D Technique for measuring the diameter of a cylinder that is several cm across

Tan θ = y/x y

θ

x

Using a trigonometric method for measuring angles

Pin Cork

Using a marker at the centre of an oscillation to aid timing


5

Implementation: Measurements During the course, students should develop their skills for making valid, reliable measurements using appropriate techniques. Students should provide written evidence when writing up their assessed practical work to show the techniques that they have used to ensure that they get the appropriate credit; it is recommended that students be encouraged to do this with the normal practical work that they do throughout the course so that it becomes a habit. Students should realise that a liquid must be stirred before using a thermometer to record its temperature and this should be mentioned in the notes that the students produce. Before taking measurements, students should check instruments for zero error and record that this was done. If measuring a fixed quantity, eg diameter of a rod, then students should take repeat measurements in at least three different places at different orientations (recording all these measurements to provide evidence they have done this). Students should make and record sufficient relevant observations over a suitable range of values with appropriate precision. What is a ‘sufficient’ number of observations cannot always be defined - it depends on the nature and context of the experiment and is in itself a ‘skill’ which is acquired through experience. For example, for a mass oscillating on a spring with a period of about 1s it might be appropriate to time, say, 20 oscillations and then repeat this measurement. However, with a heavily damped motion it might not be possible to count more than a few oscillations, in which case it might be necessary to repeat 5 oscillations at least 4 times. Students should be prepared to modify their planned procedures in response to their experimental observations. Students should realise that in some experiments (eg, plotting a cooling curve) it is not possible to take extra measurements after obtaining a set of readings and therefore they should plan to take as many readings as possible (eg by taking readings every 30 s rather than every minute). It may actually be counter productive to take repeat readings in some cases, for example in an electrical experiment a component may heat up and so a repeat set of readings would be completely different from the first set of readings. Where it is difficult to make a precise measurement, eg timing a ball rolling down a slope (which is likely to be in the order of two seconds and subject to considerable subjective error) then several readings should be taken and averaged.

Accuracy and precision Students should be aware of the difference between the accuracy and precision of measurements, for example although a stopwatch can read to high precision (0.01 s) timings will be subject to error because of the reaction time in starting and stopping the stopwatch. This will give rise to random errors, which can be reduced by taking several readings. When measuring the resistance of a length of wire the contact resistance can lead to a systematic error. Repeat readings might not do anything about this but plotting a graph of resistance against length of wire should reveal a value of the contact resistance when length is zero. Thermometers are notoriously inaccurate: although 0 – 100 ºC thermometers can be read (by interpolation) to a precision of 0.5 ºC or better they are unlikely to be accurate (due to their manufacture) to within 1 ºC, or even worse. This has more effect when measuring a temperature difference (eg determining the rise in temperature when a beaker of water is heated) and so students should still be trained to attempt readings to 0.5 ºC or better.

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Students should recognise that even though an instrument is capable of high precision (eg digital meter, electronic balance, digital stopwatch), its accuracy may well be in doubt (particularly if the student hasn’t checked for any zero error) or there may be a further uncertainty due to human error.

Implementation: Recording results in tables Students should present work appropriately in writ ten, graphical or other forms. In particular, results should be tabulated with data columns headed by the corresponding units with the data expressed to the appropriate precision, eg:

h1 /

mm

h2

/mm

/mm x /mm

20T / s

20T / s

T ²

/ s²

327.5

321.0

6.5

19.52

19.64

0.96

327.5

314.5

13.0

27.64

27.50

1.90

All readings should be shown and recorded to the precision of the instrument. It is not essential to record ‘intermediate’ calculations (of, for example, the mean value of 20 T and T ), ), but the 2 required quantity, T , should be expressed to a suitable number of significant figures. The number of significant figures is deemed to represent the precision of the value, eg 0.96 s 2 indicates a value of 0.96 + 0.005 s 2. The correct heading for a column of figures that is the logarithm of another column is, for example, log ( x/mm).

Analysing: Graphs Plotting graphs is an important part of practical work that students must be familiar with before they make routine use of software. It is also going to be difficult to ensure that candidates use any computers solely for graph plotting when doing assessed practical work. All assessed graphs should therefore be plotted by hand and they should be drawn using a large scale, but avoiding ‘awkward’ scales, particularly scales of three. A rule-of-thumb definition of ‘large’ is that the points should occupy at least half the grid in both the x and y directions (or else the scale could be doubled!); this may include the origin if appropriate. The axes should be labelled with the quantity being plotted (or its symbol) and its units (if applicable), eg T 2 / s 2, ln (V / cm 3) , l / D 2 / m-2. Points should be plotted with precision (interpolating between grid lines) and denoted by a dot with a small circle round it or a small cross. Error bars are not expected, although students could be made aware of them since they can be useful in determining uncertainties. Students should be taught to draw the line of best fit, whether it be a straight line or a smooth curve, preferably with a sharp pencil. If a straight line graph is anticipated, it is appropriate initially to take six or more measurements over as wide a range of values as possible. Having plotted the graph it might be necessary to take extra measurements, perhaps in a region where there is some doubt as to the nature of the line. This is particularly so in the case of a curve where more points are generally required, especially in the region of a maximum or minimum. It is therefore a good idea to plot the graph before dismantling the apparatus and considering how the graph compares with the theory and prediction.


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X X

X

X

(i)

X

(ii)

Does graph (i) curve to the origin, or continue as a straight line and give an intercept? More readings would be needed (if possible) to decide. In graph (ii) extra readings in the region of the maximum would help to define its shape more precisely. At A2 students are expected to relate linear graphs to y = mx + c and to understand that a straight line graph must pass through the origin to confirm a directly proportional relationship. They should, however, bear in mind that not all relationships in physics are linear! It is a development from AS that A2 students are expected to be able to plot logarithmic graphs in order to test for exponential relationships or power laws. Students should be able to interpret information from a graph, allocating units where appropriate to the gradient, intercept and area under the curve where these represent physical quantities. When a gradient is being determined, whether from a straight line or by drawing a tangent at the appropriate point on a curve, as large a triangle as possible should be used and its co-ordinates should be recorded in the calculation of its value. The student’s graph may not pass through the origin, from which they might infer that there could be a systematic error, eg there may be an additional constant term in the expression that they are using.

Analysing: Limitation of results In analysing their observations, students should be aware of the limitations of t heir experimental measurements. They should understand that certain types of measurement are more reliable than others. For example, finding the period of a mass oscillating on a spring from 20 oscillations (say 20 s) should be a reliable, reproducible measurement, whereas the time for a ball to roll down a slope is likely to be fairly unreliable for a number of reasons: human error in measuring a time of about 2 s, the ball may not roll in a straight line and the ball might skid. Simple electrical measurements using digital meters should be reliable, whilst thermal experiments may be less so due to thermal energy losses and inaccurate and insensitive thermometers. They should understand how repeat measurements and graphical methods can reduce random and systematic errors and how such techniques can invariably improve the reliability of their data. Students should be aware of the precision of instruments as discussed previously. They should recognise that if a measurement is the result of the difference of two readings (eg the depression of a cantilever as measured by a metre rule), it would be unreasonable to quote an uncertainty of better than 1 mm (ie 0.5 mm for each reading).

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Evaluating In drawing their conclusions, students should be aware that as well as possible instrument errors (even with high precision devices such as digital meters and electronic balances), values stated on components (eg masses, resistors and especially capacitors) are only ‘nominal’ values, subject to manufacturers’ tolerances. For example, electrolytic capacitors may have a tolerance of 10% or more. They should also be aware of factors inherent within their apparatus or experimental arrangements which limit the reliability of their measurements, eg friction, air resistance, contact resistance, fluctuating power supplies and change of temperature during the experiment. Students should assess the reliability of their data by considering the uncertainty of their measurements. In general terms this should be taken to be half the range of their measurements if several readings are taken or else the precision to which the instrument can be read if only a single reading is taken. However, if human error is likely to exceed this (eg reaction time starting and stopping a stopwatch) then this should be taken into consideration (eg although a stopwatch can read to a precision of 0.01 s, a more realistic uncertainty when using it to time oscillations might be 0.1 s to reflect reaction time). Uncertainties are usually of little value unless expressed as a percentage, eg a 0.1 s uncertainty in timing 20 oscillations (say 20 s) would give rise to a percentage uncertainty of only 0.5%, whereas a realistic uncertainly of 0.2 s in timing a ball rolling down a slope (say 2 s) would result in a 10% uncertainty. If the gradient of the graph is not to be used to calculate a further numerical value such as the Young Modulus then the mark for compounding errors is not readily accessible. In this case error bars can be drawn and best fit and worst fit lines can be drawn. These can then be used to determine an uncertainty in the gradient. Thus if the power relationship between two variables was sought the uncertainty in the numerical value could be derived from the difference between these two gradients. Conclusions, wherever possible, should be based on quantitative evidence. For example, in an experiment to determine acceleration of free fall, the student might get a value for g of 10.4 ms-2. A valid conclusion would be that the experiment confirms the relationship within experimental error because the value of g g obtained is within about 4% of the accepted value and the experimental uncertainty is 10% from just the timing. Comments such as ‘close to the right value’ get no credit! Finally, students need to apply their knowledge and understanding of physics, together with common sense. For example if in an experiment to determine a value for the density of a golf ball it was found it to be 140 kg m -3 they should stop and think ‘but doesn’t a golf ball sink in water?’ A check of their calculations might enable them to discover, perhaps, that they had used the diameter of the ball instead of its radius and hence found a volume that was eight times too large (‘is the volume really 320 cm3?’). If a careful check does not reveal such an error, then a suitable comment should be made to indicate that the student is somewhat surprised by the result.


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Advice for students

Plan List all the materials that you require for your experiment. State how you will measure two different types of quantities using the most appropriate instrument. For example, you could write: •

I will use a top pan balance to measure the mass.

•

I will use a micrometer screw gauge to measure the diameter of the wire.

Explain why you have chosen two of the measuring instruments that you have listed. For example, you could write: •

•

I will use vernier callipers to measure the internal diameter of the test tube as no other instrument has this facility. I will use a multimeter to measure the resistance of the thermistor since it has a variety of ranges so I will be able to select the one that gives me the best precision.

Describe at least two measuring techniques that you have used to make your measurements reliable. For example, you could write: •

•

I will look horizontally across the wire with the metre rule behind in order to measure the position of the node. I will remove the Bunsen to slow the rate of heating as I measure the temperature of the thermistor. This will allow it to come to thermal equilibrium.

You need to identify other variables that could affect your results and state how these were controlled to ensure that you carried out a fair test. For example, you could write: •

I increased the pressure of the gas slowly so that the temperature stayed the same.

If you will not be taking repeat readings you should explain why. For example, you could write: •

I will be recording the temperature of the liquid as it warms up the thermistor so it will not be able to repeat my readings. I will check each reading carefully before replacing the bunsen.

Identify any safety hazards in your experiment and any precautions you may take. For example, you could write: •

I will use a stand to make sure the beaker of boiling water is kept securely on the tripod and gauze.

Indicate how you intend to use the data that you collected. For example, in an experiment to find out how the period, T , of a pendulum varies with its length, l , you could write:

I will plot the log of the time against the log of the length and find the gradient to give me the value of n in the equation T = kl n. Include a diagram showing the arrangement of the apparatus that you will use. Mark important distances on this diagram and, in particular, mark any distances that you will measure.

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The sources of uncertainty and error should be commented on. For example you could write: •

•

The uncertainty in my measurement of the period comes from the range of my repeated readings. It is caused by my judgement of when the pendulum actually stops. The thermometer might introduce a systematic error since I am unable to check whether it reads 00C in melting ice. I will get an indication when the water boils and I can see if it reads 100 0C even though the water is not pure.

Finally, remember that your plan should show logical thought by describing what you intend to do in sequence. The plan should be written in the future tense but this is not essential.

Implementation and measurements Record all your results in an appropriate table. If you take the average of, say three readings, then you should ensure that you write down each individual reading, not just the average value to show the examiner that you have taken an appropriate number of measurements. If you are plotting a graph then you should aim to take at least six readings and repeat these if necessary. It is a good idea to draw a rough graph as you are taking the measurements so that you can investigate anomalous readings or to take extra readings near any turning points in any curves that you obtain. Make sure that you take measurements over as wide a range as possible. For example, if you are determining the distance between two nodes that are separated by a few centimetres then you should not measure the distance between two nodes only. Instead, measure the distance occupied by several nodes and then calculate the average distance between two of these nodes. Think critically about your plan as you carry it out. Record any changes that you make to the plan with a reason. Record any techniques that you use but might not have written in your plan.

Analysis When you draw your graph, you should use more than half the graph paper in both the x and y directions. The graph need not necessarily include the origin; this depends on the measurements that you are carrying out. Use a sensible scale; for example avoid the use of a scale that goes up in steps of three as this will make it difficult for you to process any readings that you take from your graph. Make sure that you label each axis with the quantity being plotted (or its symbol) and its units if it has any, eg log (T/s). Plot points accurately, using either a dot surrounded by a small circle or a small cross. Make a brief comment on the trend shown by your graph, eg as temperature increases, resistance increases linearly. Remember that a straight line graph must pass through the origin to confirm a directly proportional relationship. If you need to obtain the gradient of your graph you should draw as large a triangle as possible on your graph paper to show how you worked out a value for the gradient. If the gradient is to be used to calculate a value for a physical quantity then you must read the units carefully from the axes.


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You will need to discuss the sources of error and calculate the uncertainties that these contribute to the result(s) of your experiment. At A2 you will need to compound your errors to estimate their combined effect on the final result. You might use error bars on your graph to do this. You should comment on the precision of your measurements and how these contributed to the precision of your result. It might be that some of your readings were more precise than others in which case the least precise determines the result. The likely accuracy of your result might be commented on by reference to the uncertainties or by numerical comparison with the accepted value of a quantity such as the acceleration due to gravity. Suggest at least one realistic non-trivial modification that you could make to reduce the errors in your experiment or to improve your experiment. Trivial suggestions such as if I had more time I would have taken more readings will not score this mark. Vague suggestions such as I would use a digital meter are only of use if they go on to describe how they improve the experiment. Considering the precision of your readings is an appropriate way to do that. Similarly you might consider using a more sensitive device. Certainly the accuracy of your result merits comment. You should suggest further work that will develop the investigation that this work started, often it will involve changing different variables with the same apparatus. You should explain how this work will add to your understanding of the investigation and what you might expect to find.

Conclusion It is important to make a clear concise statement of your final conclusion. Make sure it is easy to find the conclusion in your report. For example, draw a box round it, give it a prominent heading, or underline it in a bright colour. The conclusion should relate your results to the original aim of the experiment and should include your final numerical result with its uncertainty. For example you could write: From my measurements I found a value of 6.2 +/- 0.5 x 10 -34 J s for the Planck constant. or The results from these experiments indicate that there is a power-law relationship between wave speed v and tension T : v = kT a where a = 0.48+/- 0.03. Theoretical analysis suggests that a = 0.5 (ie v = k √T ), ), which is consistent with the data. Briefly mention any physics principles that y ou use in your calculations and/or conclusion. This might involve algebraic manipulation of equations or a discussion of the phenomenon you have been investigating. For example why the wire was resonating at all in an experiment to measure resonant lengths.

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Uncertainties in measurements measurements

What are uncertainties? Why are they important? When you repeat a measurement you often get different results. There is an uncertainty in the measurement that you have taken. It is important to be able to determine the uncertainty in measurements so that their effect can be taken into consideration when drawing conclusions about experimental results.

Calculating uncertainties Example: A student measures the diameter of a metal canister using a ruler graduated in mm and records three results:

66 mm, 65 mm and 61 mm.

The average diameter is (66 + 65 + 61) / 3 = 64 mm.

The uncertainty in the diameter is the difference between the average reading and the biggest or smallest value obtained, whichever is the greater. In this case, the measurement of 61 mm is further from the average value than 66 mm, so the uncertainty in the measurement is: 64 – 61 = 3 mm.

Therefore the diameter of the metal canister is 64 + - 3 mm. Even in situations where the same reading is obtained each time there is still an uncertainty in the measurement because the instrument used to take the measurement has its own limitations. If the three readings obtained above were all 64 mm then the value of the diameter being measured is somewhere between the range of values 63.5 mm and 64.5 mm.

In this case, the uncertainty in the diameter diameter is + -0.5 mm. Therefore the diameter of the metal canister is 64.0 + -0.5 mm.


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Calculating percentage uncertainties The percentage uncertainty in a measurement can be calculated using: Uncertainty of measurement x 100% Measurement taken

The percentage uncertainty in the measurement of the diameter of the metal canister is: Uncertainty of measurement x 100% = 0.5 Measurement taken 64

x 100% = 1 %

The radius of the canister = diameter/2 = 32 mm.

The percentage uncertainty for the radius of the canister is the same as its diameter ie 1%.

Compounding errors Calculations often use more than one measurement. Each measurement will have its own uncertainty, so it is necessary to combine the uncertainties for each measurement together to calculate the overall uncertainty in the result of the calculation.

The total percentage uncertainty is calculated by adding together the percentage uncertainties for each measurement if (1) all the measured quantities are independent of one another AND (2) they are multiplied together.

Example 1: Calculating the percentage uncertainty for the area of a square tile.

A student using a rule to measure the two adjacent sides of a square tile obtains the following results: Length of one side = 84 + - 0.5mm Length of second side = 84 + - 0.5mm Show that the percentage uncertainty in the length of each side of this square tile is about 1%. Calculate the area of the square. (The above two calculations are left as an exercise for the student.) [Area of square A = 84 x 84 = 7100 mm] The percentage uncertainty in the area of the square tile is calculated by adding together the percentage uncertainties for its two sides.

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Percentage uncertainty in the area of the square tile is:

Δ A/A = 1% + 1% = 2% Example 2: A metallurgist is determining the purity of an alloy that is in the shape of a cube by measuring the density of the material. The following readings are taken:

Length of each side of the cube l = 24.0 + - 0.5mm Mass of cube m = 48.230+ - 0.005g Calculate (i) the density of the material (ii) the percentage uncertainty in the density of the material. Solution 2:

(i) Density of alloy = mass/volume = 48.230 x 10

-3

kg/ (24.0 x 10 -3)3 = 3500 kg m-3.

(ii) Percentage uncertainty in the length of each side of the cube

Δl/l

= 0.5 24

x 100% = 2 %

Percentage uncertainty in mass of cube

Δm/m = 0.005 x 100% = 0.1 % 48.2

Therefore total percentage uncertainty = 2% + 2% + 2% +0.1% = 6.1% We normally ignore decimal places in calculating uncertainties so the percentage uncertainty in the density of the material is 6%.

Example 3: Calculating the percentage uncertainty for the cross sectional area of a canister.

If the student determines that the radius of the metal canister is 36 mm with an uncertainty of 1% then the cross sectional area A of the canister is:

A = π r 2 A = π (36) 2

A = 4.1 x 10 3 mm2.

Notice that the result has been expressed using scientific notation so that we can write down just two significant figures. The calculator answer (4071.5...) gives the impression of far greater precision that is justified when the radius is only known to the nearest mm.


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The cross sectional area was calculated by squaring the radius (ie multiplying the radius by the radius). Since two quantities have been multiplied together, the percentage uncertainty in the value of the cross sectional area is found by adding the percentage uncertainty of the radius to the percentage uncertainty of the radius:

Percentage uncertainty in cross sectional area

Δ A/A = 1% + 1% = 2%

Using error bars to estimate experimental uncertainties The equation v = kT a relates the speed of a wave, v in a string to its tension, T . In an experiment to verify this relationship, a graph of ln ( v/ms-1) against ln ( T /N) /N) is plotted and the gradient of the straight line is the constant a. To determine the uncertainty in constant a, the uncertainties in v and T can be compounded by considering the difference between the best fit and worst fit lines that can be plotted through the data using error bars. To produce error bars in ln( T /N) /N) you need the uncertainty in T . You then calculate the logarithm of your data point with the uncertainty applied and draw the error bar to this value. Suppose you measure T as T = 3.4N +/- 0.2N. Then the length of the error bar is [ln(3.6N)-ln(3.2N)]. This need only be calculated for one data point and the same size error bar used for each value of T . The uncertainty in ln ( v/ms-1) can be calculated in the same way and error bars drawn in that direction to give, in effect, an error box around each plot. The best fit line is the line that passes closest to all the plots. The worst fit line just passes through all the error boxes. It is not intended that this should be a particularly lengthy procedure but it is one way of finding an estimate of the uncertainty in an experiment.

Carrying out the practical work Students must carry out the practical work individually under supervised conditions. It is advisable to have spare parts available, particularly for vulnerable components. It should be possible for students to set up their equipment and record all necessary measurements in one normal practical session. If it is not possible to complete the practical in one session then the teacher may decide to use the following session to complete the practical. The unmarked plan should be returned to students at the beginning of the lesson. Teachers may give students a copy of the assessment criteria (marking grids) from the specification and briefing documents at the start of the session; students must not bring their own copies of any documents to the session to prevent them from accessing annotated versions that they may produce. Teachers may provide students with any formula that are needed during the session without penalty.

16


Teachers should remind students of health and safety issues before they begin the practical work and advise students to have, for example, electrical circuits checked before the power is switched on. Relevant warnings should be given, eg warning students that a component may get very hot during the course of the experiment. Students must work individually. Teachers must collect in all the work that the student has produced at the end of the lesson.

Providing guidance to students during the practical session The specification states that ‘Teachers may provide guidance to students without penalty. Guidance is feedback that a teacher might reasonably be expected to give to a student who asks questions about the work that they are carrying out. In effect, the teacher is being used as a resource.’ For example, the student may ask the teacher to check whether apparatus has been set up correctly if the apparatus does not appear to be working correctly. For example, a student carrying out an experiment using an electrical circuit might sensibly ask the teacher whether the circuit is correct before switching on the power supply. The teacher should check the circuit and tell the student if it is incorrect. The error still needs to be identified and corrected by the student and this advice would carry no penalty. If however after several attempts the teacher feels the error needs to be explained and corrected then this should be noted clearly on the Candidate Record Sheet. The specification continues: ‘Students may require assistance whereby the teacher needs to tell the student what they have to do. Assistance in this respect carries a penalty. The teacher should record details of any assistance provided on the Candidate Record Sheet.’ It may be necessary to tell a student how to connect up a circuit so that they can carry out the experiment and record some measurements. In this situation, students will be penalised. If the teacher has to explain how to use an instrument, eg micrometer, then the help given should be recorded and the student should lose the mark for P6: States how to measure a second quantity using the most appropriate instrument . However, if the student provides a satisfactory reason for the choice of this measuring instrument they will not lose the mark for P7: Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken.

Carrying out the analysis The analysis may be carried out in a separate lesson under supervision. At the beginning of the lesson, teachers should return the work that students produced for the experiment. Teachers may also give students a copy of the assessment criteria, briefing documents and formula that may be needed. Working individually under supervised conditions, students should analyse their results and write up their conclusions. Teachers must not assist students with the analysis or presentation of their results, or provide any hints about possible conclusions. At the end of the session the teacher should collect in all the documents that students have in their possession.


17

Returning work Teachers must not return work to t o students to improve. However, students may do more than practical assessment and some training exercises are highly recommended. Their best piece of work should be submitted to Edexcel for assessment purposes.

18


Exemplar of assessed work: Interacting magnetic fields

Briefing Magnetic resonance imaging (MRI) is a medical tool that uses a magnetic field and the natural resonance of nuclei in the body to obtain images of human tissues. The patient is placed in a constant magnetic field and radiofrequency radiation is then applied to the system which causes certain nuclei within the patient to resonate. This can be modelled by using a small bar magnet as the nucleus and freely suspending it in an external magnetic field. At first it lines up with the external field as a compass needle does. When the magnet is rotated about its centre and released it experiences equal and opposite forces at each end that rotate it back into line. The strength of these forces depends on the strength of the external magnetic field. When released the magnet then oscillates about the centre with a resonant frequency that depends on the strength of the magnetic field. You can produce a magnetic field by passing electric current through a flat coil. This can be made from a length of wire wrapped around a 250 ml beaker to form a flat coil with 10 turns and held together with sticky tape. Your teacher will give you such an arrangement. Make sure there is no current flowing in the flat coil and suspend the magnet so that it is at the centre of the flat coil. Rotate the magnet about its centre and release it and you will observe oscillations with a definite period T . This period T is affected by the strength of the total external field in which it is placed ie the strength of the Earth’s magnetic field and the strength of the magnetic field in the coil depends on the current I flowing in it. Plan an experiment to determine how the period T varies with current I in the coil. It is suggested that they are related by 1 /T 2 = k I I where k is a constant. You should plan to find out how well your data follows the suggestion.


19

Exemplar for an able student using own plan for interacting magnetic fields X

Apparatus Coil of wire – as advised by teacher Small bar magnet Thread Stopclock Ammeter Power supply unit and leads Retort stands

n A

N

s

Method 1

Make the coil and hold it with a retort stand. Attach the power supply supply unit and ammeter using crocodile clips. Use thread to suspend the magnet in the centre of the coil. See diagram. I will rotate the coil so that it lies East – West, this means the magnet will line up with the Earth’s field.

2

Place a marker at the equilibrium equilibrium position and rotate the magnet magnet about about 200 and release it. Use the stopclock to time 10 oscillations to reduce the uncertainty in T. Record the time in the table below; take repeat readings and find an average.

3

Turn on the power supply unit and adjust adjust the current to read 0.5 A on the ammeter.

4

Rotate the magnet about 200 and release it. Time 10 oscillations and record the current and time in a table like this.

I / A

10 T / s

10 T / s

10 T / s

Mean T /s

1/ (T 2) / s-2

0

5

Increase the current in steps of 0.5 A and repeat 4 for each each current.

6

Plot a graph of 1/ T 2 against I . The plots should be on a straight line and the intercept should be very small.

20


Carrying out I let the magnet swing freely so that it pointed North and South, I then placed the coil around it using a second retort stand. I used wooden retort stands so that there was no influence on the magnets swinging. When I turned the current on I made sure the magnet did not turn round, I was then sure the coil field reinforced the Earth’s field. I recorded the following readings

I /

A

2

-2

10 T / s

10 T / s

10 T / s

Mean T /s

1/ (T ) / s

0

12.32

12.22

12.28

1.23

0.661

0.5

9.68

9.68

9.72

0.969

1.065

1.0

8.28

8.32

8.22

0.827

1.462

1.5

7.38

7.37

7.34

0.737

1.841

2.0

6.72

6.75

6.72

0.673

2.208

2.5

6.18

6.24

6.32

0.625

2.56

3.0

5.82

5.78

5.82

0.581

2.962

3.5

5.46

5.50

5.46

0.547

3.342

4.0

5.22

5.15

5.22

0.520

3.698

4.5

4.96

4.97

4.91

0.495

4.081

5.0

4.72

4.75

4.78

0.475

4.432


21

Analysis I plotted a graph of 1/T 2 against I as shown. The current readings are to a precision of 0.01 A and the timings all agree very well suggesting the uncertainty in I is a maximum of 2% - 0.01 in 0.5. Uncertainty in T – taking the 4.5 A readings – 0.03/4.95 so 0.6% ( using half the range). So the uncertainty in 1/T 2 is 1.2%. These will be too small to plot as error bars on the graph.

22


Gradient

(4.45 – 0.70) / 5.00 = 3.75/5.00 = 0.75

So the value for k is 0.75 A -1 s-2 with an uncertainty of about 3%. The plots all lie very close to the Best Fit Line suggesting that this value for k is reliable and that the measurements are accurate. It certainly supports strongly the equation describing the motion especially since the uncertainty in the readings is so small. When the coil current increases there is a stronger field and this will have a greater force on the poles of the oscillating magnet. There is a greater restoring force on the rotating magnet and so the period of oscillation is smaller. The actual value for k depends on the shape of the coil and the number of turns, amongst other things. This could be developed by using coils of different shapes and radii. The field could also be varied by moving the magnet out along the axis of the coil.


23

A2 Marking grid for interacting magnetic fields — able student A: Planning Ref

Criterion

Mark

Marking notes

P1

Identifies the most appropriate apparatus required for the practical in advance.

1

List shown, assume that the power supply is variable and so a series adjustable resistor is not necessary.

P2

Provides clear details of apparatus required including approximate dimensions and/or component values (for example, dimensions of items such as card or string, value of resistor).

0

Does not specify, eg, likely ammeter range or typical power supply value.

P3

Draws an appropriately labelled diagram of the apparatus to be used.

1

P4

States how to measure one quantity using the most appropriate instrument.

1

Ammeter is appropriate

P5

Explains the choice of the measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken.

0

With no reason

P6

States how to measure a second quantity using the most appropriate instrument.

1

Stopclock is fine

P7

Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken.

0

Precision not mentioned

P8

Demonstrates knowledge of correct measuring techniques.

1

Describes use of fiducial mark.

P9

Identifies and states how to control all other relevant quantities to make it a fair test.

1

Takes account of the effect of Earth’s magnetic field and orientation of coil

P10

Comments on whether repeat readings are appropriate for this experiment.

1

Plans to take repeats

P11

Comments on all relevant safety aspects of the experiment.

0

None mentioned

P12

Discusses how the data collected will be used.

1

Mentions how variables will be used to plot graph.

P13

Identifies the main sources of uncertainty and/or systematic error.

1

Explains use of repeats to reduce uncertainty

P14

Plan contains few grammatical or spelling errors.

1

24


Ref

Criterion

Mark

Marking notes

P15

Plan is structured using appropriate subheadings.

1

Structured plan

P16

Plan is clear on first reading.

1

Logical progression through method

Mark for this section.

12/16

B: Implementation and measurements Ref

Criterion

Mark Marking notes

M1

Records all measurements with appropriate precision, using a table where appropriate

1

All readings to 0.01 s although final column is optimistic

M2

Readings show appreciation of uncertainty

1

Repeats taken, with small variation, for mean

M3

Uses correct units throughout

1

Units good

M4

Refers to initial plan while working and modifies if appropriate

1

Coil orientation described

M5

Obtains an appropriate number of measurements

1

Plenty of readings, almost too many, students need to be careful that they don’t run out of time

M6

Obtains measurements over an appropriate range

1

Good range of current although differences in 10T are quite small at the end

Maximum marks for this section

6/6

C: Analysis Ref

Criterion

Mark

A1

Produces a graph with appropriate axes (including units)

1

A2

Produces a graph using appropriate scales

1

Plots fill page and are spread widely

A3

Plots points accurately

1

Six plots checked

A4

Draws line of best fit (either a straight line or a smooth curve)

1

Easily drawn

A5

Derives relation between two variables or determines constant

1

k determined


25

Ref

Criterion

Mark

A6

Processes and displays data appropriately to obtain a straight line where possible, for example, using a log/log graph

1

Inverse square used

A7

Determines gradient using large triangle

1

Triangle stretches across page

A8

Uses gradient with correct units

1

Unusual units

A9

Uses appropriate number of significant figures throughout

0

Mostly fine not so for 1/T 2

A10

Uses relevant physics principles correctly

1

Motion discussed

A11

Uses the terms precision and either accuracy or sensitivity appropriately

1

Precision and accuracy mentioned

A12

Discusses more than one source of error qualitatively

0

Mentions wooden retort stands without saying why, might have discussed the position of the magnet.

A13

Calculates errors quantitatively

1

Considers uncertainty in readings and combines these to get an uncertainty for k.

A14

Compounds errors correctly

1

Doubles the uncertainty in T for 1/T2

A15

Discusses realistic modifications to reduce error/improve experiment

0

Nothing suggested

A16

States a valid conclusion clearly

1

Conclusion valid and…

A17

Discusses final conclusion in relation to original aim of experiment

1

..related to value for k

A18

Suggests Suggests relevant further work

1

Develops variables


Total marks for this unit

26

15/18

33/40


Exemplar for a less able student using own plan for interacting magnetic fields I will wrap the wire around the beaker and make a flat coil of wire. Which I will connect to the labpack and ammeter to read the current in the wire. The labpack is safe for me to use in the lab. I will tie the magnet with thread to the top part of the coil so that it hangs in the middle. I will then twist the magnet and time 10 swings and record them in a table. I will turn on the labpack and measure the current. I will twist the magnet again and record 10 swings each time. I will plot a graph of T against I and see if it is a straight line. Coil of wire Small bar magnet Stopclock Ammeter PSU and leads

I / A

10 T / s

10 T / s

Mean T /s

0

12.32

12.22

1.23

1.0

8.28

8.32

0.830

2.0

6.72

6.75

0.673

3.0

5.82

5.78

0.580

4.0

5.22

5.15

0.519

5.0

4.72

4.75

0.474


27

Analysis The readings for time are pretty close to each other so I think these are accurate. The ammeter was accurate. I plotted the graph as shown. The plots all lie on a nice smooth curve which suggests that the theory is correct and that the readings are accurate and with a small uncertainty. The graph shows that as the current increases the period gets less. It was quite difficult to ensure the magnet was in the centre of the coil and this might have affected the readings. To improve the experiment I would take more readings and change the way I hung the magnet.

28


A2 Marking grid for interacting magnetic fields — less able student A: Planning Ref

Criterion

Mark

Marking notes

P1

Identifies the most appropriate apparatus required for the practical in advance

1

List shown, assume that the power supply unit is variable and so a series adjustable resistor is not necessary, ignore missing apparatus that is standard and not specific to this practical.

P2

Provides clear details of apparatus required including approximate dimensions and/or component values (for example, dimensions of items such as card or string, value of resistor)

0

Does not specify ammeter range or stopclock precision

P3

Draws an appropriately labelled diagram of the apparatus to be used

0

No diagram

P4

States how to measure one quantity using the most appropriate instrument

1

Ammeter is appropriate

P5

Explains the choice of the measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken

0

With no reason

P6

States how to measure a second quantity using the most appropriate instrument

1

Stopclock is fine

P7

Explains the choice of the second measuring instrument with reference to the scale of the instrument as appropriate and/or the number of measurements to be taken

0

No details of stopclock

P8

Demonstrates knowledge of correct measuring techniques

0

No mention of fiducial mark or size of swing

P9

Identifies and states how to control all other relevant quantities to make it a fair test

0

No mention of the effect of Earth’s magnetic field and orientation of coil

P10

Comments on whether repeat readings are appropriate for this experiment

1


P11

Comments on all relevant safety aspects of the experiment

1

Shows awareness of labpack as a potential hazard


29

Ref

Criterion

Mark

P12

Discusses how the data collected will be used

0

Table produced with columns but no awareness of need to process data and so inappropriate graph planned

P13

Identifies the main sources of uncertainty and/or systematic error

0

Nothing mentioned

P14

Plan contains few grammatical or spelling errors

1

P15

Plan is structured using appropriate subheadings

0

Any structure is not clear or helpful

P16

Plan is clear on first reading

0

Plan shows path through practical but ignores too much detail to be clear

Mark for this section

Marking notes

6/16


Criterion

Mark

M1


1

Ammeter readings to 0.1 A in the table. To match the time measurement it would be appropriate to have three significant figures if they were available. Since candidate does not specify give benefit of the doubt.

M2


0

No mention of uncertainty, possible confusion with accuracy.

M3


1

Units ok

M4


0

No mention of plan

M5


1

Readings are sound in number (just)….

M6


1

…and in range


30

4/6


C: Analysis Ref

Criterion

Mark

A1


1

Graph displays data adequately and has units

A2


1

Scales allow data to cover half the page – just

A3


1

Six plots checked

A4


1

Smooth curve drawn although line is thick

A5


0

Misses this aspect of the experiment

A6


0

A7


0

A8


0

A9


1

SF ok

A10


0

No Physics mentioned

A11


0

Accuracy often used – not always correctly – but no mention of precision

A12


0

No mention of errors

A13


0

A14


0

A15


0

More readings by itself does not merit the mark. Reference is made to magnet position but without any physics of the reason for the change.

A16


1

Clear conclusion…

A17


0

..but not based on proper analysis

A18

Suggests relevant further work

0

Nothing suggested


Cannot draw gradient

6/18



16/40

31

Examiner’s comments for interacting magnetic fields Help from the teacher is perfectly permissible where this concerns the setting up of complicated apparatus that is unfamiliar to the candidate. In this case the teacher may provide help on the manufacture of the coil and the way it is supported. However, the teacher should not help the candidate to use the apparatus. In this case the teacher should tell candidates about the effect of the Earth’s magnetic field and how the plane of the coil should be oriented. The briefing should not tell the candidate about other apparatus as this will preclude the award of P4, States how to measure one quantity using the most appropriate instrument . Time is one of the resources the candidate should control and so the number of readings taken should be considered carefully; teachers should not allow candidates an open ended time scale. The final part of the briefing is open ended allowing the candidate freedom to test the data as they see fit. From the assessment criteria they will be able to decide how to do that appropriately and score marks but it is unlikely that the candidate will be able to compound errors. However, error bars could be drawn on the graph to show the uncertainty in the gradient.

32


Exemplar of assessed work: Guitar strings

Briefing When designing a guitar it is important that the structure is strong enough to hold the strings tight without collapsing. To do this you must find out how much tension is required in a string to make it resonate with a particular frequency at the length it will be on the guitar. We can model this by using a metal wire as the string and placi ng it in a magnetic field. It is made to vibrate by passing an alternating electric current though it, it then vibrates at the frequency of the ac. In our model we shall use mains electricity from a lab power supply unit (psu) so the frequency will be fixed at 50 Hz – it is possible to vary this if you have access to a signal generator. You should stretch the wire horizontally over two supports and by hanging masses on the wire you can vary the tension T . By varying the tension the speed c of the waves on the string varies μ)1/2 c = (T / μ) where μ is a constant. Since c = f x λ variations in T cause the wavelength λ to change. The first resonance (when there is one antinode) occurs when l = λ/2 λ/2 where l is the length between supports when there is one antinode. Plan an experiment to find how the first resonant length l varies with T . You should use 0.27 mm diameter (32 swg) constantan wire to give a wavelength of about a metre when you hang 100 g on the end.


33

Exemplar for an able student using own plan for guitar strings Apparatus Bench mounted pulley Bridge support for the wire 2 blocks of soft wood G clamp 1.5 m length of 0.27 mm diameter (32 swg) constantan wire Low voltage ac psu 2 magnadur magnets and yoke – to produce magnetic field Slotted masses and hanger- 100 g to 500 g in 50 g increments Metre rule Crocodile clips and connecting leads

Method Safety - The low voltage supply will give no safety problems since it is too low to give a shock. The hanging masses need to be secure on the wire but should not present a hazard. Use of a wire under tension requires me to use safety goggles. I will use a metre rule to measure the resonant length as it i s large but under a metre. I will connect up the apparatus as shown below and put the yoke with the magnets near the middle for best effect. I will make sure the magnets have opposite poles facing.

wire clip X

bridge

l magnets on yoke

G

2 wood blocks clip X bench

G-clamp m power supply unit

I will start with 100 g on the end of the wire and connect up the electric circuit so that current flows safely through the wire, the frequency cannot change since it is the mains. I turn on the current and move the bridge back and forwards to find the position of maximum vibration. I will check for this by placing my eye alongside the wire and looking horizontally. When the vibrations are biggest I will measure the distance between nodes using the metre rule. I will then remove the bridge and repeat my reading. I will increase the mass hanging on the wire and take repeated readings and record my results in the table.

34


Mass/g

l1/cm

l2/cm

l3/cm

Mean l/cm

100 150 200 250 300 etc

Analysis From the briefing sheet

c2 = f 2 x λ 2 = T/μ

At the first resonance position So

f 2 x 4 l2 = T/μ

λ = 2l

so l2 = T/ 4 μ f 2

cf y = m x + c

I will plot a graph of l 2 against T which should give a straight line through the origin with a gradient of 1/(4 μ f 2). From this I will be able to read off the tension for the length of my guitar.

Carrying out I found a piece of dark card behind the wire helped me to see the vibrations. I looked vertically down on to the ruler when measuring the length to avoid parallax. I did not go over 500 g because the length was over a metre and difficult to measure with a metre rule – I had enough readings for my graph anyway. Mass/g

l1/cm

l2/cm

l3/cm

Mean l/cm

100

43.0

43.9

43.9

43.6

150

54.2

53.5

54.0

53.9

200

61.7

61.3

62.7

61.9

250

68.4

68.9

69.1

68.8

300

75.9

75.7

75.4

75.7

350

82.0

81.2

81.3

81.5

400

88.0

87.4

88.3

87.9

450

92.0

92.8

93.4

92.7

500

97.6

97.2

98.3

97.7

I decided that I would plot the length in centimetres squared against the mass in grammes, since I was not concerned about the value of the gradient. Since the tension is just mass x g, the line on the graph should still be straight.


35

I drew up the following table

36

m/g

l2/cm2

100

0.191

150

0.291

200

0.383

250

0.473

300

0.573

350

0.664

400

0.773

450

0.859

500

0.955



37

The gradient is 0.955 / 500 = 1.91 x 10 -3 cm2 g-1 As the current alternates it produces an alternating force since the current is flowing through a magnetic field. The alternating force is perpendicular to both field and current and so is up and down. When the frequency of this force matches that of the natural frequency of the wire a large oscillation is observed. By changing the tension the natural frequency changes and this needs the length to change for resonance. We have found that as the tension increases so does the length squared in proportion because the wavelength changes. There is a large uncertainty in the length since it is possible to get the wire to resonate over quite a range of distances. It is also difficult to measure the length once resonance is determined since the rule is not along the wire. The uncertainty in l is at least 1 cm so the uncertainty in l will be 2% and in l 2 will be 4%. The uncertainty in the mass is about 2% my teacher says. To improve my experiment I would reduce the interval in the mass readings so that the graph had more points. To develop the experiment I could use a signal generator to vary the frequency and find how the resonant length varies with frequency or keep the length fixed and vary the tension and frequency. This would tell me more about the way the guitar works.

38


A2 Marking grid for guitar strings — able student A: Planning Ref

Criterion

Mark

Marking notes

P1


1

List shown

P2


1

Although the safe value for the voltage or current is not specified the range of masses and length of wire is.

P3


1

P4


1

Metre rule is appropriate

P5


1

With reason

P6


0

Does not mention second measurement

P7


0

Might have checked calibration of masses or measured the frequency of the mains using an oscilloscope

P8


1

Eye level and card background

P9


1

Mentions frequency is mains

P10


1

P11


0

Shows awareness of safety aspects but fails to recognise need for ammeter to monitor current at safe levels

P12


1

Plans to plot l2 against T

P13


0

Although they plan to repeat they do not mention the large uncertainty in establishing the resonant length


39

Ref

Criterion

Mark

P14


1

P15


1

P16



Marking notes

1

12/16


Criterion

Mark

M1


1

Precision ok

M2


1

Repeats show variation

M3


1

Units ok

M4


1

Changes plan in limiting mass to 500 g to keep length under 1 m

M5


1

Almost too many

M6


1

Good range with reason


6/6

C: Analysis Ref

Criterion

Mark

A1


1

Axes fine

A2


1

Scales have sensible numbers and plots fill half the page in both directions

A3


0

250 g plot is wrong

A4


1

40


Ref

Criterion

Mark

A5


1

Derives equation

A6


1

Plots graph of length squared

A7


1

Whole page

A8


1

A9


1

3 SF ok throughout

A10


1

Uses wave equation and explains phenomenon later

A11


0

Doesn’t mention precision at all

A12


1

Considers percentage uncertainty in both variables

A13


1

In l squared

A14


0

Doesn’t compound errors

A15


0

More readings on graph is unlikely to help the conclusion

A16


1

Conclusion stated ‘as the tension increases so does the length squared in proportion because the wavelength changes’

A17


0

States proportionality but does not relate findings back to the aim of the experiment – the tension in the guitar string.

A18


1

Discusses detail of the experiment.


13/18



31/40

41

Exemplar for a less able student using own plan for guitar strings Apparatus Bench mounted pulley Bridge support for the wire 2 blocks of soft wood G clamp 32 swg constantan wire Low voltage psu 2 magnets and holder Metre rule

Method I will connect up the apparatus as shown in the diagram and make sure that everything is safe. I will hang 100 g on the end and I will connect up the power supply. When everything is ready I will turn on the psu and move the bridge support until I can see resonance. I will look carefully to see when the wire is vibrating at its maximum and then measure the length of the wire. I will increase the mass hanging on the end and repeat my readings in the table.

Mass/g

l1/cm

l2/cm

Mean l/cm

100 200 300 400 500

Analysis From the briefing sheet

c2 = f 2 x λ 2 = T/μ

At the first resonance position So

f 2 x 4 l2 = T/μ

λ = 2l

so l2 = T/ 4 μ f 2

l2 is proportional to T and I will plot a graph of l 2 against T.

42


Carrying out I carried out the experiment as I said, it was difficult to find the resonance position. I got the following readings

Mass/g

l1/cm

l2/cm

Mean l/cm

l2

100

43.3

43.9

43.6

1901

200

61.7

61.3

61.5

3782

300

75.9

75.7

75.8

5746

400

88.0

87.4

87.7

7691

500

97.6

97.2

97.4

9487


43

I plotted the graph and the gradient was 9.55/500 = .0191. The straight line shows shows that there is 2 a strong correlation between l and mass and that as the mass is increased the resonant length gets longer. The plots are all close to the line of best fit which means the experiment was a success.

44


A2 Marking grid for guitar strings — less able student A: Planning Ref

Criterion

Mark

Marking notes

P1


1

List shown is adequate

P2


0

Wire length not specified

P3


0

No diagram

P4


1

Metre rule is appropriate…

P5


0

… but not explained

P6


0

Does not mention second measurement

P7


0

Might have checked calibration of masses or measured the frequency of the mains using an oscilloscope

P8


1

‘Looking carefully’ is not enough, the actual method must be specified

P9


0

Doesn’t realise that frequency might be a variable

P10


1

Text is unclear about what will be repeated but table is clear

P11


0

Very vague about safety – specific precautions are needed.

P12


1

Correct graph plotted

P13


0

Although they plan to repeat they do not mention the large uncertainty in establishing the resonant length


45

Ref

Criterion

Mark

P14


1

P15


1

P16


1


Marking notes

8/16

B Implementation and measurements Ref

Criterion

Mark

M1


1

Table with units and precision appropriate

M2


0

Only one repeat is not really enough in this experiment

M3


0

The units in the last column have been omitted

M4


0

Mentions plan but with no detail

M5


0

Five readings is just not the minimum number for a graph – a minimum of six readings is expected

M6


1

Range is appropriate but without justification.


2/6

C: Analysis Ref

Criterion

A1


0

No unit and power of ten for l2 confused

A2


1

Scales ok – plots over half of page in both directions and sensible numbers

A3


1

Plots checked

A4


1

Line of best fit is good

A5


0

Analysis shows relationship derived in terms of T but plots m without showing link

46

Mark


Ref

Criterion

A6


1

Plots l Squared vs T for straight line

A7


1

Triangle crosses page

A8


0

No units for gradient

A9


0

Four significant figures are too 2 many for l for l

A10


1

Manipulation of equation in planning section

A11


0

No mention of either term

A12


0

…or errors

A13


0

A14


0

A15


0

A16


0

Simple conclusion based on graph but type of correlation not described ie proportionality should be mentioned

A17


0

Needs to develop simple T with m conclusion and relate T with

A18


0

Nothing mentioned


Mark

6/18


16/40

Examiner’s comments for guitar strings On some occasions it is possible to award a mark for something a candidate has done but not said explicitly. Here repeats were not stated as planned but since they are seen in the table in the plan P10 Comments on whether repeat readings are appropriate for this experiment can be awarded. Similarly A10 Uses relevant physics principles correctly can be awarded because the candidate has successfully manipulated the equations from the briefing sheet. The candidate will not be able to score A14 Compounds errors correctly with the briefing in its present form. If the briefing had asked candidates to find the density of the wire, requiring candidates to come up with a value whose uncertainty could be investigated, then the candidate has a chance. Briefings must be carefully drawn up to allow candidates to access all the marking points.


47

Exemplar of assessed work: Linked oscillators

Briefing In ski resorts the chair lifts often have notices telling skiers not to swing the chairs as this is dangerous. This is because when one chair swings it passes its energy on to the next chair and soon one of the chairs might be swinging enough to collide with a support. The reason the energy passes from one chair to the next is because they have the same resonant frequency. How much energy passes from one chair to the next depends on how well they are joined together, or coupled. We can model this behaviour by using two simple pendulums to represent two oscillating ski chairs and coupling them with a rubber band. The pendulums are made by hanging a small mass by a piece of string from a horizontal bar, such as metre rule. The rubber band is then placed around both strings – the initial distance between the strings should be about 1.5 times the unstretched length of the band, so that the pendulums are pulled together when at rest. When one pendulum is set in motion its energy is passed to the other and then back in a cyclic manner and each pendulum is seen to stop briefly. The period of the cycle is the time between one pendulum stopping and the next time the same pendulum stops. This model will be better if the pendulums have the same resonant frequency. The strength of the coupling depends on the vertical distance x from the metre rule to the rubber band and this affects the periodic time T of the cycle according to the formula.

T = k xn where k and n are constants Plan an experiment to investigate how T varies with x and determine a value for n.

48


Student exemplar using own plan Apparatus 2 Retort stands, bosses and clamps 2 Metre rules 2 x 100 g masses Thread and scissors Pin in cork Stopclock – reading to 0.01 s Elastic band about 8 cm in length, unstretched

Method 1.5l

x

1

Support a metre rule using the retort stands. Check that it is horizontal by measuring the distance above the bench at each end. Ensure that this arrangement is unlikely to topple – a heavy weight can be placed on the base of the stand if necessary to keep it safe.

2

Tie one of the masses on to the metre rule so so that it is about 80 80 cm below the rule. Measure the time for 20 complete oscillations and calculate the period.

3

Measure the length, l, of the unstretched elastic band.

4

Tie the other mass on to the rule so that it is 1.5 x l away from the first. Adjust the length so that the period is the same as that measured in 2.

5

Set the band band around both pendulums pendulums so that it is horizontal– as shown in the diagram – use the second metre rule to check this. Set the distance x to 2 cm.

6

Place the pin underneath underneath one of the pendulums to act as a fiducial mark. mark.

7

Hold one pendulum stationary stationary and set the other in motion, then release the first. When the pendulum stops over the fiducial mark start the stopclock. When this same pendulum next stops over the mark, stop the clock. Record the time T in the table below. Repeat this for a mean value of T .

8

Vary x in 5 cm steps up to 57 cm and repeat 7 for each value.

9

Plot a graph of ln T against ln x. The readings should be on a straight line and the gradient will be equal to n.


49

Carrying out I set up the horizontal metre rule. The rubber band had an unstretched length of 9 cm. I tied one of the 100 g masses on to the rule so that it was about 80 cm long. I measured 10 swings and got got 10T/s = 17.91 , 18.02. I tied the other pendulum loosely and secured it with sticky tape so that I could easily easily change the length. I recorded 10T/s = 17.93, 18.01. These give means of 1.796 s and 1.797 s so they are identical to within 0.1%.

x / cm

T / s

T / s

10

89.2

20

T / s

Mean T / s

ln(T / s)

ln( x / cm)

88.0

88.6

4.484

2.303

37.7

38.0

37.9

3.635

2.996

30

22.8

22.3

22.6

3.118

3.401

40

13.3

13.6

13.5

2.603

3.689

50

10.5

10.5

10.5

2.351

3.912

I recorded the data shown. I tried 5 cm but it took too long and over 50 cm the oscillations were too quick. The readings were very close so I took only one repeat. I recorded x to the nearest cm since it was impossible to measure any more precisely due to the thickness of the band itself. The uncertainty in the timings is small - < 1% - it was difficult to tell exactly when the oscillation stopped so this is quite surprising.

50



51

Analysis The graph is a little disappointing in that the points are quite widely spread around the Best Fit Line. The timings were close together but the plots off the line suggests that there was something else going on. Gradient

(2.21 - 4.69) / (4.00 – 2.20) = -2.48/1.80 = -1.38

So n = -1.38 The value of n is negative which suggests that as the band is moved down it is nearer the swinging masses it has more effect on coupling them so that the periodic time reduces. Although there is a clear trend on the graph I do not think my value for n is reliable but it is certainly negative. I should have taken a second repeat reading and taken more values as the graph is rather sparse and this contributes to the unreliability of any value for n. To improve I would like to try different amounts of stretch to the band as the tightness might have an effect.

52


A2 Marking grid for linked oscillators A: Planning Ref

Criterion

Mark

Marking notes

P1


1

List of main items shown, although some items mentioned in the text, eg weights to be placed on the base of the clamp stands, are not included

P2


1

Includes length of band and size of masses

P3


1

Diagram shows key distances

P4


1

Metre rule is appropriate

P5


1

mentions thickness of band later.

P6


1

Stopclock is fine

P7


1

Precision of clock detailed

P8


1

Describes use of fiducial mark.

P9


1

Ensures the period of each pendulum is the same for both

P10


1


P11


1

Discusses how to keep apparatus from toppling

P12


1

Plans to plot ln T vs ln x

P13


1

Appreciates need for special measures to ensure band horizontal


53

Ref

Criterion

Mark

P14


1

P15


1

Structured plan

P16


1

Logical progression through method


Marking notes

16/16

B Implementation and measurements Ref

Criterion

Mark

M1


1

Mentions precision of length x

M2


1

Uncertainties discussed

M3


1

Units good, including the logarithm

M4


1

Plan modified with reasons

M5


0

Only 5 readings so too few readings for a convincing graph, especially given the space between the first two

M6


1

Good range


5/6

C: Analysis Ref

Criterion

Mark

A1


1

Axes fine

A2


1

Scales good, plots fill the paper and scales are sensible

A3


1

Plots accurate

A4


1

Best fit line a good choice

A5


1

Finds value for n

A6


1

Straight line produced using log version of equation

54


Ref

Criterion

Mark

A7


1

Best fit line fills page

A8


1

No units for gradient of log graph

A9


1

SF are just ok as fourth SF is only just usable on the graph

A10


1

Discusses significance of negative value for n

A11


0

Precision of x x discussed in terms of the thickness of the band but neither accuracy nor sensitivity mentioned

A12


1

See above and difficulty in determining stopping point

A13


1

Uncertainty in times calculated

A14


0

Could have drawn error bars

A15


1

Just about achieves this mark since extra readings have a reason

A16


1

Conclusion clear

A17


1

Conclusion expressed in terms of the value for n

A18


1

Tightness of band


16/18


37/40

Examiner’s comments for linked oscillators For P9 P9 Identifies and states how to control all other relevant quantities to make it a fair test the candidate might have mentioned keeping the mass the same but this does not affect the period and so keeping T constant is more important – there is a very strong hint in the briefing.

M4 Refers to initial plan while working and modifies if appropriate is justified by the mention of the difficulties in timing if the length x gets too large or too small. This is another briefing that makes it difficult for the candidate to be awarded A14 Compounds errors correctly. Error bars should be drawn and the worst fit line drawn for obtaining a percentage uncertainty in the value for n.


55

Exemplar of assessed work: Temperature control

Briefing The electrical pump that pushes the hot water through the boiler and the pipes is the heart of a house’s central heating system. The pump needs to be switched on when the house is cool and off again when it is warm. To do this designers need to find a means of producing an electrical signal that varies with temperature and can be adjusted externally. The device they use is called a thermostat.

Thermistors can be used to produce a voltage that varies with temperature and when set up as one leg of a potential divider the voltage V in the middle varies as the resistance of the thermistor varies. This might be used in a thermostat to control the pump in a domestic central heating system. E is the supply voltage, which is fixed, and V is the output voltage that varies with temperature and it is V that is used to control the pump. In order to set up a thermostat it is important for the designer to know how a thermistor behaves as the temperature varies.

E

V

The resistance of a thermistor is given by bT R = R0 exp – bT

where T is the temperature in Kelvin and b and R0 are constants. Plan an experiment to show that the resistance of a thermistor behaves as shown above in the temperature range 273 – 373 K and find a value for b.

56


Student exemplar using own plan Apparatus Thermistor Ohmmeter Beaker Thermometer Ice Beaker

Method I will use an ohmmeter to measure the resistance of the thermistor and connect them using leads. I will put the thermistor in a beaker with some ice and measure the resistance. I will then heat the beaker to melt the ice and as the water warms up I will record temperature and resistance in the table below. I will make sure my eye is level with the thermometer to make the reading as accurate as possible and I will be careful when the beaker gets hot. I will plot a graph of resistance against temperature. I will do this because the independent variable should go on the x-axis.

Carrying out I took readings as said above. I stirred the water and made sure I didn’t touch the hot beaker. I got these readings. Temperature / 0C

Resistance / k Ω

21

5.92

22

5.53

34

3.29

44

2.19

55

1.48

61.5

1.089

76

0.698

86

0.556

96

0.428

100

0.308


57

58


Analysis I found that the resistance dropped a lot at first and continued dropping as the temperature rose but less and less. As you can see on the graph and this is what the equation on the briefing sheet says should happen. I think my readings are accurate because they all fit on a curved line. The thermometer was accurate because it measured boiling water as 100 0C and it was precise to better than 1 0C. I made sure that the thermometer was close to the thermistor when I took a reading. To improve what I did I would heat it slowly to make sure it was all at the same temperature and I would use a computer to take my readings.


59

A2 Marking grid for temperature control A: Planning Ref

Criterion

Mark

Marking notes

P1


1

List shown,

P2


0

No indication of size of beaker and no mention of heating apparatus.

P3


0

No diagram

P4


1

Thermometer is appropriate

P5


0

No explanation of choice and doesn’t mention range

P6


1

Ohmmeter is fine

P7


0

The scale would probably need to be determined by a trial before starting. Student should comment on the need to check that the scale covered at least the initial value and that this value would be close to the maximum.

P8


1

Appreciates need to avoid parallax on thermometer and ensures that thermometer is close to thermistor.

P9


0

No reference to need for gentle heating or temperature equilibrium when taking reading

P10


0

No such comment, in fact this experiment can be repeated but only with the same actual readings with great difficulty. Student should have explained why repeat readings were not planned.

60


Ref

Criterion

P11


0

A beaker containing two items in hot water on a tripod requires more detailed consideration of safety than is given here. If an electrical heater was used then this will get hot in operation and should have been commented on

P12


1

Plans to plot R vs T which is not the best idea but is awarded mark here for making a plausible suggestion, will lose marks later for this choice, eg student will not be able to determine a value for b

P13


0

No mention of uncertainty or error

P14


1

The little that is written has few errors

P15


0

No real structure to plan, just a list of activities

P16


1

These activities are clear


Mark

Marking notes

7/16


61


Criterion

Mark

M1


1

One reading shown to 0.5 0C and resistance shown to precision of meter – assumed range changed at 60 0C

M2


0

Resistance readings might well drift but nothing mentioned and no repeats possible. Temperature not mentioned either

M3


1

Units ok

M4


0

Mentions stirring but without reason and does not state that the liquid should be stirred just before taking a reading. Time lag for heat to penetrate case and safety aspects would merit the mark

M5


1

Plenty of readings…..

M6


1

….over a good range


4/6

C: Analysis Ref

Criterion

Mark

A1


0

No units on the axes

A2


1

Although the proportions are odd the graph displays the data in a way that can be read

A3


1

Six plots checked

A4


0

No line drawn

A5


0

No relationship suggested

A6


0

Data not processed

A7


0

No straight line and so no gradient ……

A8


0

…..and no units

62


Ref

Criterion

Mark

A9


1

SF mostly fine but three decimal places are not usable on this graph scale.

A10


0

Not much Physics in the analysis

A11


1

Precision and accuracy of the thermometer discussed

A12


0

Thermometer close to thermistor and water is stirred but without explaining why this is done.

A13


0

No error analysis is really possible here

A14


0

A15


1

Improves his experiment by suggesting heating slowly. Ignore computer reference as it is not suggested why this will help.

A16


1

Interprets graph correctly…

A17


0

…but fails to say why this is what the equation suggests. Doesn’t find a value for b.

A18


0

Nothing suggested


6/18


17/40

Examiner’s comments for temperature control It is difficult for the students to access some of the marks here, although if the teacher suggested a value for b in the briefing this would enable the student to find a percentage difference to use in commenting on accuracy. Any student might still score heavily if they appreciate the need for data processing in drawing valid conclusions. When referring to a multimeter the terms scale and range become indistinguishable. In this case the candidate cannot be expected to guess the resistance of a device they have not seen before – thermistors at room temperature can range from a few Ohms to a few kilohms. So they will need to investigate the appropriate range (or scale) before starting. This activity will be awarded P7.

M4 Refers to initial plan while working and modifies if appropriate. Very often candidates will modify their plan without recording the fact. When the plan is as thin as this one then it is quite likely that the candidate will do some good physics that is unrewarded by being unrecorded. Candidates must be in the habit of writing down their thoughts.


63

Training Exercise: Safety in hospital

Introduction It is likely that students will need to develop their planning skills. To develop planning skills beyond AS some training exercises are a good idea. This exemplar is based on an experiment that is probably best done as a teacher demonstration in which case students do the Planning and Analysis sections and the teacher does the implementation and measurements. This is also a good exercise to emphasise the Health and Safety aspects of planning and the part played by Risk Assessment. Teachers can of course use any exercise for training t raining and different ones can focus on different aspects of the Unit 6 assessment.

Safety in hospital Single Photon Emission Computed Tomography ( SPECT) is a nuclear medicine procedure in which a gamma camera rotates around the patient and takes pictures from many angles, which a computer then uses to form a tomographic (cross-sectional) image. Gamma rays have many other uses in archaeology and industry but the ability of gamma rays to affect individual cells makes them a serious hazard to those who use them regularly. In order to keep the users safe it is important that they should receive as little radiation as possible. The best way of doing this is to place a screen between the source and the user that absorbs as much of the radiation as

The screen can take the form of protective clothing or a physical wall and these will have a lead lining, since lead is very good at absorbing radiation because it is a dense material. It is important to know how much radiation is absorbed by how much lead. The key factor is the thickness of the lead between source and detector. You are to plan an experiment to determine the thickness of lead that reduces the count rate to half its initial value. When a detector is placed in front of a source the count rate, C , will decrease when a thickness, x, of lead is placed between them according to the equation

C = C 0 exp- x α

Where α is a constant and C 0 is the count rate when there is no lead between source and detector. You should plan to measure how the count rate varies with the thickness of the lead to determine a value for α . Using your value for a find the thickness of lead that reduces the count rate to half its initial value, this is called the half thickness. Your laboratory report should include a description of the steps you took to make your experiment as safe as possible.

64


Frequently asked questions

Questions relating to written work Can students submit draft work for checking?

No. The assessment has been designed to enable students to show that they have appropriate skills, knowledge and understanding for this level of study. If, for example, a student does not use the appropriate number of significant figures and this is pointed out to them, then the assessment will not be a realistic measurement of the student’s own knowledge. Consequently, students must not submit draft work for checking. Should students show all their workings?

Yes. If students enter numbers into a calculator and write down a result without showing their calculations then if the result is wrong it will not be possible to allow for the accidental pressing of the wrong button(s) on their calculator. For this reason, it is recommended that students show their workings in full. Should the plan be written in the future tense?

Yes, but a student should not be penalised for using a different tense. Do error bars have to be used on graphs?

Use of error bars could be encouraged where the variables plotted are simple since they can give an estimate of the uncertainty in a result. However, their use is not specifically required by the assessment criteria and therefore students will not be penalised if they are not used.

Questions relating to the practical session Can apparatus be set up for students?

No. Teachers will need to sign a form to verify that students have been able to handle equipment themselves. Consequently apparatus cannot be set up for students. However, teachers may check that the apparatus has been set up correctly before students use the equipment to take measurements. This will give teachers the opportunity to check, for example, that electrical circuits have been wired up correctly and to warn students of any health and safety risks, for example components that may become hot. If a student experiences difficulties with this aspect of the work then it should be noted on the Candidate Record Sheet. Can students use a physics simulator?

No. Teachers will need to sign a form to verify that students have been able to handle equipment themselves. If students use a software package to simulate an experiment, then they will not handle any laboratory equipment. Consequently the teacher will not be able to verify that students have been able to handle equipment and therefore the student will not pass the assessment for unit 6.


65

Can students have more time than is available in one lesson to complete the practical work?

Yes but in general this should not be necessary. Edexcel does not prescribe the amount of time that the practical work should take. It should be possible for the practical aspect of the unit 6 assignment to be completed within one lesson; however, teachers may allow students to complete the practical in the following session. Can work be done in pairs?

No, all aspects of the work that is produced for the practical assessment must be done individually. Can I give the student any help?

If the student is doing something dangerous the teacher must intervene. If a student requests a formula then this may be given without penalty. If apparatus is being used incorrectly and the student is unlikely to obtain any measurements, help may be given in order to ensure that the student will have some data to process. Any help given of this nature must be noted on the Candidate Record Sheet.

Questions relating to marking work Will I receive class sets of the marking sheets?

No, the templates provided by Edexcel should be copied for each student. Will the marking grids be returned?

No. Please note that since they are removed during the moderation process it is essential that the actual work of the student be clearly marked with the centre and student details. How much annotation is needed?

Brief annotation only. It is highly recommended that you make use of the codes given on the Edexcel marking grids. Do I need to use the Edexcel marking grids?

Yes, this has been seen to lead to more accurate marking. Can work for one skill be credited in another?

Yes, for sections B (Implementation and measurements) and C (Analysis). Can I award a half mark if a criterion has not been fully met?

No. If the criterion has not been fully met then no mark should be awarded.

66


Other questions How do I know if an experiment is A2 standard rather than AS or GCSE?

As a guide, consider the answer to the following (or similar) questions: Does the experiment use A2 physics theory? Does the experiment use ideas that are post AS? Does it lend itself to some mathematical analysis of errors and particularly combination of errors? Can students use ICT?

The report of the practical work must be hand-written and graphs must be hand-drawn. Computers may have materials on them that is of use to candidates other than for word processing or drawing graphs. ICT may be used for collecting data, eg the use of data loggers is permitted. ICT must not be used for processing results. If a student uses a spreadsheet package to produce a graph then it will be assumed that the student has used its facilities for automatically selecting an appropriate scale, drawing the best line through the points, etc, and hence the student will lose the relevant marks.


67

Further advice

Plagiarism and collusion Teachers must be able to declare that the work submitted by the student is solely the work of that student. Any work submitted which does not have a record sheet signed to that effect will be returned for such authentication. Edexcel is likely to penalise any student that deliberately copies information and attempts to pass it off as original work of their own. Since 2006, Edexcel has been using new software to identify any potential cases of plagiarism. Plagiarism is defined by the Joint Council for Qualifications as ‘The failure to acknowledge sources properly and/or the submission of another person’s work as i f it were the student’s own.’ For example, this would apply if the student has included an extract copied from an internet site without suitable identification of the material and acknowledgment of its source. The Joint Council publish very useful leaflets for teachers and for students, which are available on the JCQ website www.jcq.org.uk . This includes advice on how to detect plagiarism: Keeping watch on content •

•

•

•

Varying quality of content is one of the most obvious pointers. Well-written passages containing detailed analyses of relevant facts alternating with poorly constructed and irrelevant linking passages ought to give rise to suspicion. Another practice is for candidates to write the introduction and conclusion to an assignment to make if fit the question, and then fill in the middle with work which has been lifted from elsewhere. If the work is not focused on the topic, but presents a well-argued account of a related matter, this could be a sign that it has been used elsewhere. The same applies if parts of the work do not fit well together in developing the response to the assignment. Dated expressions, and references to past events as being current can also be indications of work which has been copied from out-of-date sources.

Keeping watch on vocabulary, spelling and punctuation •

•

•

The use of a mixture of English and American vocabulary or spellings can be a sign that the work is not original. If the piece contains c ontains specialised terminology, jargon, obscure or advance words, the internal assessors should ask if this is typical of this level of candidate and reasonable, or if it is because the candidate did not write the passage. Is the style of punctuation regular and consistent?

Keeping watch on style and tone •

•

68

Look for differences in the style or tone of writing. If a candidate uses material from textbooks alongside items from popular magazines the change of tone between the two should be marked. Look at level of sophistication of the sentence structure. Is this the sort of language that can be expected from a typical student? Is the use of language consistent, or does it vary? Does a change in style reflect a change in authorship at these points?


Keeping watch on presentation •

•

•

Look at the presentation of the piece. If it is typed, are the size and style of font uniform? What about use of headers and sub-headers? Are the margins consistent throughout? Does the text employ references and if so is the style of referencing consistent? Are there any references, for example, to figures, tables or footnotes, which don’t make sense (because they have not been copied)? Lack of references in a long, well-written section could indicate that it had been copied from an encyclopaedia or similar general knowledge source. Look out for quotations that run on beyond the part which has been acknowledged.

Other techniques •

•

•

Type in phrases or paragraphs into ‘Google’ (use the ‘advanced search’ option) and see if this comes up with a website that matches closely, if not entirely. Search parts of the bibliography for suspicious websites that are too closely matched to the title. Use free software as described on www.plagiarismdetect.com, www.turnitin.com, www.plagiarism.com, www.wordchecksystems.com or www.canexus.com/eve/index.shtml.

Remember that the centre, as well as the student, is liable for any plagiarism because the teacher will have signed a declaration stating that the student’s work is their own. Collusion

Collusion includes excessive help from teachers or parents or collaboration with other students. A student must not work with another student to carry out an assessed experiment.

Annotation of student work The QCA Code of Practice requires that internal assessors show clearly how credit has been assigned in relation to the criteria defined in the specification. The codes in the marking grids provided by Edexcel are designed to facilitate this. Some teachers also send commentaries, which are useful but are not required. The annotation codes should be placed in the margin of the work at the point where it was decided to award that criteria.


69

Glossary

Accuracy

The degree to which a measurement matches the true value of the quantity that is being measured. This is a qualitative term only.

Dependent variable

A variable physical quantity, the values of which are not chosen by the person doing the experiment, but change with another variable ie the independent variable.

Error

An offset or deviation (either positive or negative) from the true value.

Independent variable

A variable physical quantity, the values of which are chosen by the person doing the experiment.

Percentage uncertainty Percentage uncertainty =

Uncertainty of measurement x 100% Measurement taken

Precision of an instrument

This is a term meaning 'fineness of discrimination'. In practice, it is the smallest scale division on an instrument that can be read.

Random error

An unpredictable error that has no pattern or bias. To reduce the effects of random errors when measuring a quantity it is necessary to take the mean of several values.

Range

The difference between the smallest value and the largest value of a set of readings.

Reliability

The extent to which a reading or measurement gives the same value when a quantity is measured several times under the same conditions.

Sensitivity

The change in response of an instrument divided by the corresponding change in stimulus. For example, the sensitivity of a thermometer is expressed in mm/oC

Systematic error

An error that has a pattern or bias, for example, errors caused by background lighting. This type of error adds or subtracts the same value to each measurement that is taken.

True value

The value that would be obtained if there were no errors in the measurement of that value.

Uncertainty

A range of values which are likely to contain the true value.

Validity

The level of confidence that is associated with a measurement or conclusion.

Zero error

An error that is caused when an instrument does not read true zero, eg a spring balance may not read zero when there is nothing hanging from it. This type of error is a form of systematic error.

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Appendix 1: Exemplar centre devised plans for candidates

It should be remembered that the plan must not give anything away for later sections and should allow access to all the marks. They should allow candidates to improve upon them fairly easily.

Plan for experiment for interacting magnetic fields Apparatus Flat coil of wire Small bar magnet Ammeter Dc psu Thread Stopclock Two retort stands

Method 1

Suspend the magnet using thread so that it lies in the centre of the coil. Rotate the magnet, release it and take measurements to find the period T of the resulting oscillation of the magnet about its centre. It will oscillate due to the Earth’s magnetic field.

2

Turn on the power supply unit and increase the current to 0.50 A and repeat 1 above.

3

Repeat 2 up to 5.0 A to enable you to plot a meaningful graph. graph.


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Plan for experiment for guitar strings Apparatus Bench mounted pulley Moveable bridge - support for the wire 1.2 m length of 32swg constantan wire Low voltage ac power supply unit 2 blocks of soft wood G clamp 2 magnadur magnets and yoke – to produce magnetic field Slotted masses and hanger Metre rule Crocodile clips and connecting leads Ammeter.

Method 1

The wire is to be stretched across the bench so that it hangs over the bench mounted pulley. The other end is held between two blocks of wood by a G clamp. Hang 100 g on the end of the wire to tension it.

2

Place the moveable bridge bridge under the wire near the blocks of wood. The The distance l between the bridge and pulley should be about 1 m.

3

Use crocodile clips at each end of the wire to connect the power supply unit to the wire and pass an alternating current of less than 1 A through it.

4

Place the yoke and magnets around around the wire so that the wire passes through the magnetic field.

5

Turn on on the power supply supply unit unit and increase the voltage until until the wire clearly vibrates.

6

Adjust the position of the moveable bridge until until resonance is found. found. Measure Measure the length l .

7 Vary T by increasing the hanging mass and measure the new resonant length. Do this until you have enough data for a graph and record your data in a table.

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Plan for experiment for linked oscillator Apparatus Retort stands, bosses and clamps String Scissors Two identical masses Rubber band 2 Metre rules Stopclock

Method 1

Use the retort stands stands to support support one of the metre metre rules horizontally.

2

Measure the unstretched length of the rubber band.

3

Use the string and masses to make two pendulums that hang apart a distance about 1.5 times the unstretched length of the rubber band. Both pendulums should be about 80 cm long. Use the stopclock to help ensure that the pendulums have the same period.

4

Place the rubber band horizontally around both strings and measure the distance x of the band below the support bar.

5

Hold one pendulum vertical, displace the other about 10 cm and release it. Record the time T between one pendulum stopping and stopping again.

6

Vary x and measure T until you have enough data to plot a graph.


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Plan for experiment for temperature control Apparatus Thermistor Ohmmeter 100 ml Beaker Tripod, gauze and heatproof mat Bunsen burner Ice Retort stand Thermometer

Method 1

Use the retort stand to hold the thermistor in the beaker. Place the thermometer close to the thermistor and pack ice around them.

2

Connect the thermistor to the ohmmeter and read the resistance when the thermometer reads 273 K.

3

Replace most most of the ice with water and record record the temperature and and resistance. resistance.

4

Heat the water and record the temperature and resistance resistance as suitable intervals intervals as the water heats up to boiling point. Record your results in a table.

5

Process your results to plot an appropriate straight straight line graph and use this graph to find a value for b.

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Plan for experiment for safety in hospital Apparatus Sealed source of Gamma radiation GM tube and counter Pieces of lead of varying thickness from 5 mm to 5 cm Metre rule Means of safely supporting the above so that they are in line Stopclock Micrometer screw gauge

Method 1

Support the detector at a suitable distance from the source and use the stopclock and counter to determine a value for the count rate, C .

2

Use the screw gauge to measure measure the thickness of of the thinnest piece of lead.

3

Using a suitable handling tool place this piece of lead between the source and detector and take measurements to determine a new value for C .

4

Increase the number of pieces of lead until you have enough data to plot a graph – record your results in a suitable table.

The plan should not include tables as this gives too much away on sections B and C.


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Appendix 2: Precision, accuracy and sensitivity

Precision is a term meaning 'fineness of discrimination' but is often used erroneously to mean 'accuracy' or 'uncertainty'. It relates to the smallest division that can be read from an instrument. A thermometer that is marked in 1 oC steps is less precise than one that is marked in 0.1 oC steps because the latter has a more finely divided scale. Accuracy relates to the difference between the measured value of a quantity and its ‘true’ value. Suppose that the temperature of a boiling liquid is actually 60 oC and it is measured with two mercury-in-glass thermometers, one of which reads 59 oC and the other reads 57 oC; the first thermometer is the most accurate of the two because its reading is the closest to the actual value of the boiling liquid. Accuracy is a qualitative term only.

Accuracy can be improved by removing or compensating for the cause of a systematic error eg checking an instrument for a zero reading error and either adjusting the instrument to eliminate the error or noting the error and deducting its value from readings. Sensitivity is defined as the change in response of an instrument divided by the corresponding change in stimulus. So for example, the sensitivity of a thermometer is expressed in mm/ oC.


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A note on precision and accuracy

Precision and accuracy are often confused with each other. One instrument may be more precise than another, but it may not be as accurate. The diagrams show two thermometers that are being used to measure room temperature. The first thermometer is marked in 1 oC steps and reads 22 oC. The second thermometer is marked in 0.1oC steps and reads 20.2 oC. If the room temperature is actually 23 oC then the first thermometer gives the more accurate reading because it is closest to the true temperature. The second thermometer is more precise because t he scale has finer divisions.

25

20

22°C

More accurate

21

20.2°C 20

More precise

2054ma020309S:\LT\PD\Support\GCE Physics gdc for the A2 pract assesm TSM.doc.1-87/3

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Guidance for the A2 Practical Assessment

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