Physics Practical Notes 1. Duri Durin ng the the tes test, read ead the the ques uestio tion and and all all info nformat rmatiion give given n caref areful ully ly.. Make Make sure that you understand the experiment given. Certain part arts of the question require you to record the readings from the experiment in a table. You should draw draw the the tabl table es befo efore carry arryiing out out the the exp experi eriment ment so that that you you can rec record ord your your readings in the table straight away during the experiment. Then, carr arry out the experiment by following the steps given in the question exactly. You need to apply your Physics practical skills when carrying out the experiment. 2. Do not panic if the context of the practical experiment appears unfamiliar. Wher Where e appr approp opri riat ate, e, the the ques questi tion on pape paperr will will tell tell you you exac exactl tlyy what what to do and and how how to do it. 3. If you you find ind yours oursel elff in real real diff ifficul iculty ty setti etting ng up you your prac practi tica call equi equipm pmen entt you you may may as askk your our supe upervis rvisor or for help help,, alt althou hough you you may lose lose one or more ore mark markss for for this. 4. Whe When reco record rdiing read readiings ngs fro from an meas measur urin ing g ins nstr trum ume ent (exc (excep eptt metre etre rule, ule, vern verniier cali alipers pers and and mic microm rometre etre scre screw w gaug gauge) e),, the the nu num mber ber of dec decimal mal plac laces used us ed sh shou oulld be equal qual to half alf of the the smal malles lest divi divission ion of scal scale e of the the ins nstr trum ume ent (Fo (For example, if the instrument's smallest division of scale is 0.1, you should record the reading to the nearest 0.05, which is 2 decimal places.) For digital instruments (except digital stopwatc atch), the number of decimal places used
2
should be the same as that shown on the display. The reading should be recorded to the nearest 0.1cm for metre rule, 0.01cm for vernier calipers, 0.01 0.01mm mm for for micr microm omet eter er scre screw w gaug gauge e and and 0.1s 0.1s for for stop stopwa watc tch h (bot (both h anal analog ogue ue and and digital). In all cases, do not give more or less number of decimal places. You must also write the correct units. 5. To save ave time time,, draw draw a sing singlle tab table for for your your res results ults bef before ore takin aking g any any read readiings ngs and enter your readi adings in the table as you take them (so that you do not waste time having to copy them later). 6. In most most case cases, s, you you sh shou ould ld take take each each read readin ing g twic twice, e, then then calc calcul ulat ate e and and reco record rd the mean of the 2 readings. Some questions are worth 2 marks, which means you get one mark for repeating. Ensure that you show in your answer both read readin ings gs and and the the calc alculat ulatio ion n of thei theirr mean. ean. Howev wever, er, for for the the part part in Qu Ques esti tion on 1 whic which h requ requir ires es you you to reco record rd read readin ings gs in a tabl table, e, ques questi tion onss that that carr carryy only only 1 mark mark and and ques questi tion onss whic which h stat state e that that repe repeat ated ed read readin ings gs are are not not requ requir ired ed,, you you only only need need to take each reading once and record it straight away. 7. All the raw readings of a particular quantity should be recorded to the same numb nu mber er of deci decima mall plac places es whic which h shou should ld in turn turn be cons consis iste tent nt with with the the un unce cert rtai aint ntyy in the the read readin ings gs.. Each Each colu column mn head headin ing g in your your tabl table e must must cont contai ain n both both a quan quanti tity ty and and its its un unit it.. For For inst instan ance ce,, if you you have have meas measur ured ed time time “t” “t” in seco second nds, s, your your colu column mn heading would bewritten as “t/s”. 5. When When plot plotti ting ng a grap graph, h, draw draw both both the the hori horizo zont ntal al and and vert vertic ical al axis axis on the the grap graph h paper correctly. Label both axis correctly and state the unit (if any). Use a suit su itab able le scal scale e for for both both axis axis and and do not not us use e any any odd odd scal scales es su such ch as 3:10 3:10.. Both Both the the x-ax x-axiis and and y-axi -axiss need need not not star startt fro from 0. The scal scales es sh shou ould ld be chos chosen en su suc ch that that the the poi points nts plott lotted ed on grap graph h cove coverr at least east hal half of the the graph raph paper aper.. Th The e marki arkin ngs on the scales should not be more than 3 large squares apart. Plot all points on the the grap graph h accu accura rattely. ely. Th The e poin points ts shou hould be accu accura rate te to half alf a small mall squa square re.. For For all all the the poin points ts,, thei theirr diam diamet eter er sh shou ould ld not not be larg larger er than than half half a smal smalll squa square re.. Th Then en,, draw draw the the corr correc ectt stra straig ight ht line line or curv curve. e. When When draw drawin ing g the the stra straig ight ht line line or curv curve, e, it shou sh ould ld pass pass thro throug ugh h all all poin points ts on the the grap graph h if poss possib ible le.. If this this is not not poss possib ible le,, the the line line or curv curve e shou should ld pass pass thro throug ugh h as many many poin points ts on the the grap graph h as poss possib ible le,, all all the the
3
poin points ts sh shou ould ld be clos close e to it and and the the nu numb mber er of poin points ts abov above e and and belo below w the the line line or curve should be almost equal. 6. When When dete determ rmin inin ing g the the grad gradie ient nt of the the line line of grap graph, h, choo choose se 2 poin points ts on the the line line and and draw draw a tria triang ngle le.. Th The e dist distan ance ce betw betwee een n the the 2 poin points ts chos chosen en shou should ld be at leas leastt half half the the leng length th of the the line line.. When When dete determ rmin inin ing g the the y-in y-inte terc rcep eptt of the the line line of grap graph, h, if the the x-ax x-axis is star starts ts fro from 0, you you can can read ead it off dire direc ctly tly from rom the the y-ax y-axiis of grap graph h, or if the the x-ax x-axis is does does not not star startt from from 0, you you sh shou ould ld choo choose se a poin pointt on the the line line,, pref prefer erab ably ly one one of the the poin points ts that that you you us used ed to calc calcul ulat ate e its its grad gradie ient nt,, and and su subs bsti titu tute te its its x and and y valu values es as well well as the the grad gradie ient nt into into the the equa equati tion on y=mx y=mx+c +c to dete determ rmin ine e the the valu value e of c which is the y-intercept. 7. For For any any ques uestio tions invo nvolvin lving g calc alculat ulatio ion, n, the the nu num mber ber of signi igniffican icantt figur igures es of your answer should be equal to or one more than the number of significant figures of the raw value used in the calculation with the least number of sign signif ific ican antt figu figure res. s. You You sh shou ould ld sh show ow all all work workin ings gs and and do not not skip skip any any impo import rtan antt steps. You must also write the correct unit for the final answer if it is not provided. You are not allowed to write extra solutions or answers. If you do so and any of the answers or solutions is wrong, marks will be deducted. For a cal calcula culati tion on quest uestiion which hich requ equires ires you you to us use e you your ans answer wer fro from the the prev previious ous ques questi tion on,, even even if your your ans answer wer for for the the previ revio ous ques questi tion on is wron rong and and you you us use e it for for this this ques questi tion on caus causin ing g your your answ answer er for for this this ques questi tion on to be wron wrong, g, us usua uall llyy you you will still get full marks for this question as long as your calculation for this question is correct. This is known as 'error carried forward'. 8. For the part of Question 2 which asks you to estimate the percentage unce un cert rtai aint ntyy in a part partic icul ular ar valu value, e, in most most case casess the the abso absolu lute te un unce cert rtai aint ntyy us used ed to calculate the percentage uncertainty should be equal to twice the smallest divi divisi sion on of scal scale e of the the inst instru rume ment nt us used ed to meas measur ure e the the valu value e (For (For both both anal analog ogue ue and digital stopwatch, the absolute uncertain ainty used should be 0.2s). This is because the measurement for this part is often difficult to be done accurately. 9. For the part of Question 2 which asks you whether your results support the sugges suggested ted relati relations onship hip betwee between n 2 variab variables les,, you have have to calcul calculate ate the percen percentag tage e dif differe ferenc nce e betw betwee een n 2 value aluess of a cons nsta tant nt which hich is obtai btaine ned d in pre previo vious part art of
4
the question. The suggested relationship is supported if the percentage dif differe ferenc nce e is 5% or less less and and not su sup pport ported ed if the the perc percen enta tag ge dif differe feren nce is more ore than 5%. The percentage difference is equivalent to the difference of the two values divided by the largest value.
10. 10. For For all all ques questi tion ons, s, you you shou should ld us use e the the corr correc ectt expe experi rime ment ntal al and and Phys Physic icss term termss in your your answ answer er.. Do not not repl replac ace e them them with with othe otherr term termss that that are are inap inappr prop opri riat ate, e, even even if their meaning are are the same. You should spell all experimental and Physics term termss corr correc ectl tly. y. If you you can' can'tt do so, so, try try to spel spelll it in su such ch a way way wher where e it soun sounds ds the the same as the actual term when read out. Marks are usually not deducted for spe spellin lling g erro errors rs in exp experim erimen enta tall and and Phys Physic icss term termss as long ong as it sti still sound oundss the the same sa me and and that that it is not not easi easily ly conf confus used ed with with othe otherr term terms. s. If you you spel spelll othe otherr term termss wrongly or if you make grammatical errors in your answer, marks will not be dedu deduc cted ted for for as long ong as the the exami xamine nerr can un unde ders rsta tand nd what hat you you are are wri writing ting.. You are allowed to use suitable short forms in your answer, especially for representing physical quantities or their units. Limitations and improvements: 1. Two/three readings are not enough to make a valid conclusion - Take more readings and plot a graph 2. The value of (a physical quantity) is small so the percentage uncertainty of (the quantity) is large - (The way to make the value of the quantity larger) 3. The movement/oscillation of (something) is affected by wind movement - Use a wind shield when carrying out the experiment 4. Difficult to determine the start and end of oscillation/movement of (something) because it moves too fast - Use a video camera with slow motion feature and timer to record the experiment with scale, then view the video playback frame by frame. 5. Difficult to release (something) without applying a force - Use a mechanical hand to release the (thing)
5
6. Difficult to shape the plasticine into the shape of (something) - Use a mould to shape the plasticine 7. Heat loss through the sides and bottom of beaker/container - Use polystyrene container or insulate the beaker/container using a lid or lagging. 8. The (measuring instrument) is not precise enough - Use Use anot anothe herr (ins (instr trum umen ent) t) with with grea greate terr sens sensit itiv ivit ityy and and prec precis isio ion. n. You You must must stat state e the precision of the suggested instrument. 9. The length/diameter/thickness of (something) is not uniform - Measure the length around/along the (thing) and calculate the mean 10. Difficult to measure (something) due to (specific reason based on experiment) - (Suggest a better way to measure it) 11. Parallax error when measuring (something) - (Suggest a better way to measure it, such as use mirror scale) 12. (Something) moves - (Way to keep it in the original position) 13. 13. Dif Difficu ficult lt to main mainta tain in (so (somethi ething ng)) at (a part partiicula cularr posi positi tion on)) / mai maintai ntain n rul ruler vertical - Use a clamp 14. Friction at pulley - Apply oil to lubricate the pulley 15. 15. Diff Diffic icul ultt to dete determ rmin ine e when when (som (somet ethi hing ng)) reac reach h the the maxi maximu mum m heig height ht beca becaus use e it remains there for too short a time Diff Diffic icul ultt to take take the the read readin ing g of newt newton on--mete meterr immed mmediiatel atelyy when when (so (somethi ething ng))
6
starts to move because it moves suddenly Diff Diffic icul ultt to start tart or stop top the the sto stopwat pwatc ch imme immed diate iatelly when hen (so (someth methin ing g) pas assses through (somewhere) - Use a video camera with slow motion feature and (the measuring device) to video the experiment with scale, then view the video playback frame by frame. General Notes: Oscillations:
* Pe Perf rfor orm m 3 se sets ts of me meas asur urem emen ents ts fo forr ea eac ch di diff ffer eren entt osc scil illa lati tion on,, (y (yo ou wi will ll get mor ore e marks if you do 2 sets of repeats). * Measure at least 30 periods in total. * Preferably make each measurement 20 periods long. * Precaution: minimize oscillations in any other plane other than the one being observed * Always write times to two d.p., never to the nearest second. * Precaution: do small amplitude oscillations * Precaution: say that you did several periods at once to minimize reaction time errors. Moments Precaution:
* balance the ruler being used first. * Use distances from the pivot of greater than 25 cm (250 mm). * Weigh the unknown and known masses in your hands before using the ruler, and place the lighter mass as far from the pivot as possible. * Measure distances from the pivot to the centre of mass of the object. * All measurements should be to 1 mm accuracy. * To make sure the rule is balanced, pull each end down. If it comes up again then the rule must have equal moments acting on it on either end.
Density Measurements
* When using a micrometer, one full rotation of the barrel is 0.5 mm (50 on the barrel scale).
7 * The micrometer reads to 0.01 mm accuracy. * Vernier calipers can read to 0.05 mm accuracy. However, it is advisable to quote the figure to 0.1 mm accuracy, as this will give a larger uncertainty, which will make comparisons in later parts of the question easier. * If measuring a very small dimension, measure several "thicknesses" of it. Remember to divide by the number of thicknesses after the measurement! * Precaution: check the zero error on the micrometer and/or Vernier calipers. Wipe the jaws of the micrometer to remove grease. * Give answers to 2 or 3 s.f. : it is meaningless to write, for example, "density is 7785.654 kg m-3"! * Always take at least 3 readings for each measurement, and take an average. * When measuring string diameters or foil thicknesses t hicknesses (or similar), use a minimum of 10 thicknesses.
Uncertainties:
* % Uncertainty = * When multiplying or dividing quantities, add their % uncertainties together. * When adding or subtracting quantities, add their absolute errors together, then divide by the result of the addition/subtraction of the measurement, e.g. for , where , and , the absolute errors added = 0.102 mm. Therefore the % uncertainty is: * If you have to calculate the error in, e.g. , the absolute error in d must be multiplied by pi and then added to the absolute error in x. The percentage uncertainty is this total error divided by the calculated and the result multiplied by 100. * If a measurement is to be raised to a power, then multiply the % uncertainty in the t he measurement by the power to get the % uncertainty in the overall term. * If two values, for say, a density are available, calculate the % difference between them. If a value is given by the examiner, then use this as the "correct" value, and calculate the % difference the following way: where x is your measured value, and c is the examiner's value. If you have obtained two values, then the expression changes: where x1 and x2 are your measured values, and is the median of the two, (not necessarily the mean!). * Compare the % difference with your % uncertainty. Any relationship suggested, such as that the two densities should be equal, can be considered correct if your % difference is less than your % uncertainty. This will gain you marks!
8 * If your % uncertainties look small, check that you have multiplied by 100! Electrical Experiments:
* With capacitor discharges, either take readings every 5 seconds for the first part of the discharge, or I think that every 10 seconds is sufficient. * If a range is specified over which you should take measurements, do not exceed it: you will be penalised. * With an analogue ammeter, use the top scale. This reads (generally), from 20, to 0, to 10. These are in fact divisions of 10 m A, and the meter actually reads from -20 to 100 m A. If you are out by a factor of 10, (e.g. ( e.g. you get a calculated cell voltage of 0.15 V), check that you have read the meter correctly. Always remember that the polarity on the meter must be correct. * Any small discrepancy in your results can be explained by "electrical resistance at the contacts in the circuit". * With most electrical experiment where a curve will be obtained (e.g. the V/I characteristic of a diode), 9 points on a smooth curve are sufficient. Graphs:
* With any graph, a minimum of 6 to 8 points are needed, and you must have at least 4 points on a curve. * When measuring the gradient of a graph, carry your tangent on to the sides of the graph paper, however big your graph. The triangle you use should be greater than 10 cm in length and height, although in some mark schemes 100 cm2 is fine. * Your graph does not have to go through the origin. If the data does not indicate that it does so, do not force it to. Comment on the fact that there must have been a systematic error. * When choosing values to read of a graph, it is better to take them from the middle part of the curve, as this is where you will have more points per change in y co-ordinate. * When describing your "plan" in Experiment C, state that the graph you plot will be a straight line through the origin (if this t his is the case!), of gradient = to an expression which will help you confirm the relationship suggested by the examiner. * Turning points on graphs require at least 4 points. * If points near the origin deviate substantially from your line of best fit, point out that
9 for small measurements there is a greater uncertainty. Experiments Involving Temperature:
* Readings should be accurate to fractions of a degree. * Stir any liquid being heated. * Insulate the apparatus if possible. * The thermometer should not be touching the sides of the container it is in, and should be in the middle of the liquid you are measuring the temperature of. * Precaution: read the thermometer at eye level to avoid parallax error. * The bulb of the thermometer should be completely submerged. * Comment on the result obtained being the right order of magnitude. * The uncertainty on a vernier caliper is either 0.1 mm or 0.01 cm. * The smallest value on a newton meter is usually 0.2 N so its uncertainty will be 0.1 N * One of the age old precaution and improvement in every paper is that 2 values or reading are not sufficient for a conclusion. Its improvement will be take a set of readings and plot a graph. * Whenever you're given two values of k and asked if the relation is proportional, if the difference is within 10% then it is proportional.
Hope that helps. :)
-Huzaifah Punjani
