Form 4 – IGCSE PhysicsThermometers

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

St. Paul’s Form 4 IGCSE Physics

Mr. Andre Borges2010 / 2011

Learning objectives

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

• … 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.

• … recognise the need and identify fixed points.

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

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

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

Thermometerso Devices that measure temperature or temperature gradient (variation)

using substances’ physical properties that vary with temperature.

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:

X=6cm

Y=24cm

Properties of Thermometers

• Range : How many °C the thermometer can measure from the biggest to the smallest.– oil thermometer -10ºC 400°C large range

– clinical thermometer 35°C 42°C small range

• Sensitivity: The change in length per change in temperature.– Liquid in glass thermometer measures to ± 0.5°C

– Clinical thermometer measures ± 0.1°C It has a thinner tube (bore)

To nearest

More sensitive

Properties of Thermometers

• Linearity: The change in temperature produces the same change in length.

y

x

c

Length(mm)

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 toloose. At this point it is not possible to lower the temperatureany further. This point is -273°C and is called Absolute Zero.

• In scientific work this temperature is used as the basis for atemperature scale called Kelvin Scale.

• In the Kelvin Scale, the fixed points are 273 K (melting ice) and373 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 wireCopper wire

Hot junction 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 temperatures2. Measures higher temperatures3. Can be reader and logged (recorded) in a

computer4. Sensitive

Demonstration: Thermocouple

• Observe your teacher’s demonstration1. Draw a diagram of the demonstration in

your notebook

2. Record the values

3. Write your conclusions

HomeworkDue 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.