form 4 – igcse physics - thermometers
DESCRIPTION
Notes about Physics lessons. Topic: Thermometers. IGCSE Physics Syllabus: 2.2 (b).TRANSCRIPT
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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
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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
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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
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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.
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Thermometers
• Bulb thermometers
• Bi-metal thermometers
• Thermocouple
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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
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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
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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.
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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
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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
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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
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Demonstration: Thermocouple
• Observe your teacher’s demonstration1. Draw a diagram of the demonstration in
your notebook
2. Record the values
3. Write your conclusions
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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.