Form 4 – IGCSE Physics - Thermometers

Form 4 – IGCSE Physics Thermometers (5.02, 5.03 - Temperature (1 & 2)) Cambridge IGCSE Physics Syllabus 625: 2.2 (b)

St. Paul’s Form 4 IGCSE Physics Mr. Andre Borges 2010 / 2011

Learning objectives At the end of this lesson, you should be able to…

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… understand how a physical property that varies with temperature may be used for measurement of temperatures, name examples of such properties. Also demonstrate understanding of sensitivity, range and linearity. … recognise thermometers as tools based on such properties.

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… recognise the need and identify fixed points.

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… describe the structure and action of liquid-in-glass thermometers

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… describe the structure of a thermocouple and understand how to use it

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Physical properties may vary with temperature • Some materials have properties that changes with temperature:

 Liquids can expand if the temperature increases, or contract if it decreases;  Metals and alloys also expand and contract if the temperature changes;  Some substances change their colours when the temperature increases  Some substances change their electrical conductivity when its temperature rises

Thermometers o Devices that measure temperature or temperature gradient (variation) using substances’ physical properties that vary with temperature

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o A glass thermometer (clinical thermometer) contains a liquid (either mercury or coloured alcohol) whose volume increases when the temperature increases.

Thermometers • Bulb thermometers

• Bi-metal thermometers

• Thermocouple

Fixed points • Before it can be used, a thermometer must be calibrated (ºC marks need to be put on it). • To do that, we need to mark the fixed points.  Lower fixed point or “Ice point”: temperature of pure melting ice  Upper fixed point or “Steam point”: temperature of steam above pure boiling water at standard pressure (Remember!! 100 kPa)

• Then, we calculate the scale: Y=24cm X=6cm

Properties of Thermometers • Range : How many °C the thermometer can measure from the biggest to the smallest. – oil thermometer -10ºC – clinical thermometer 35°C

400°C large range 42°C small range

• Sensitivity: The change in length per change in To nearest temperature. – Liquid in glass thermometer measures to ± 0.5°C More sensitive – Clinical thermometer measures ± 0.1°C  It has a thinner tube (bore)

Properties of Thermometers • Linearity: The change in temperature produces the same change in length. y

Length (mm) c

x Temperature (°C)

e.g. In a thermometer, a 2°C rise in temperature, causes a 5 mm increase in length.

Kelvin Scale • When a material is cooled, its particles lose kinetic energy and move more slowly. • Until a point is reached where it has no more energy left to loose. At this point it is not possible to lower the temperature any further. This point is -273°C and is called Absolute Zero. • In scientific work this temperature is used as the basis for a temperature scale called Kelvin Scale. • In the Kelvin Scale, the fixed points are 273 K (melting ice) and 373 K (boiling water).

• To convert from Celsius (ºC) scale to Kelvin scale: Kelvin temperature = Celsius temperature + 273°C

or T(K) = T(ºC) + 273°C

Thermocouple • A thermocouple consists of a mechanical junction of two dissimilar metals. This junction generates a small electrical potential (voltage), the value of which depends upon the temperature of the junction. Thus with calibration, and an appropriate choice of metals, one can obtain a thermometer for the desired temperature range. Iron wire

Copper wire

Hot junction

Copper wire

Cold junction

Thermocouple • The bigger the temperature difference between the two junctions, the greater the electric current (the thermocouple is not linear). • A thermopile is lots of thercouples joined together. • Advantages of thermocouple: 1. Measure rapidly changinge temperatures 2. Measures higher temperatures 3. Can be reader and logged (recorded) in a computer 4. Sensitive

Demonstration: Thermocouple • Observe your teacher’s demonstration 1. Draw a diagram of the demonstration in your notebook 2. Record the values 3. Write your conclusions

Homework Due date: Wed Aug 25 Mr. Borges’ tray • Read pages 100 to 103. Make notes in your notebook. • Answer questions 1, 2, 3 (page 101) and 1, 2 (page 103). Show all the work, when appropriate.