Remind yourself…
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What are fixed points?
Describe the thermal expansion of a solid.
Describe the thermal expansion of a liquid.
Describe the thermal expansion of a gas.
State some everyday applications and consequences
of thermal expansion.
Answers at the end of the slides
Remind yourself…
• What are fixed points?
• Known temperatures which can be used in calibration.
We use the freezing and boiling point of water.
• Describe the thermal expansion of a solid. Describe the
thermal expansion of a liquid. Describe the thermal
expansion of a gas.
• All three expand when heated. The heat energy loosens
the bonds between particles, but since solids are more
strongly bonded to begin with, they expand the least,
while gases expand the most.
• State some everyday applications and consequences of
thermal expansion.
• Use to make thermometers. Railway lines and bridges
have expansion joints to allow for expansion.
• Power cables are slack, to allow for contraction when is
gets cold.
You need to know….
• Know the meaning of:
– Sensitivity;
– the smallest amount of change a thermometer can
measure;
– Range;
– the highest and lowest temperatures a thermometer can
measure;
– Linearity;
– how for each degree of temperature the measurement
(length of liquid or number of volts) will change by the
same amount.
• Also….
You need to know the following about thermometers
1: Liquid thermometers are linear.
Each degree of temperature raises the liquid by the same distance
up the tube.
2: What its range is
This is the maximum and minimum temperatures that the
thermometer can measure. This is limited by…..
The freezing point and boiling point of the liquid in the tube. (just
above freezing to just below boiling.
3: How a liquid-in-glass thermometer can be made more sensitive
• A thinner bore (narrower tube) means that the liquid travels a
greater distance up the tube for each degree of temperature.
4: Thermometers can be made using liquids such as mercury or
water, or using gases, or even different metals to create a voltage.
Types of thermometers
Any physical property that varies with temperature can be
used to measure temperature.
There are different kinds of thermometer, each type being
best suited to certain jobs.
• A liquid-in-glass thermometer uses thermal expansion of
a liquid.
• A thermistor uses the resistance of a semiconductor.
• A thermocouple uses a voltage between two wires with a
hot and cold junction.
Thermometers
Video explaining how it works
https://youtu.be/eiPWPyU-KgI
• As temperature increases, the mercury’s
volume increases. As it expands, the
mercury moves up the capillary.
• Range is the lowest temperature to the
highest temperature it can record.
Thermometers
Sensitivity is how much a thermometer responds for
each 1 °C change in temperature.
Task: How can you make a liquid-in-glass thermometer
as sensitive as possible?
Linearity: shows the same increase for each degree.
E.g. the mercury moves up the capillary by 1 mm for
every 1 °C of temperature increase.
Task: Why would a thermometer be difficult to use if it
was non-linear?
Thermometers
Read p 90-91 in the textbook and answer q 1-2 on p 91.
Thermistor
Thermistors are resistors that can be used to measure
temperature because their resistance depends on the
temperature.
Is the relationship
between a
thermistor’s resistance
and temperature
linear or not?
Resistance (Ω)
2000
1500
1000
500
0
0
50
100
Temperature (°C)
150
No, but the electronics
in a digital
thermometer will
compensate for that
A thermocouple.
Video explaining how it works
https://youtu.be/882BqC1mYyc
This is a diagram of
a thermocouple.
It has two different
metals with two
junctions and a
milli-voltmeter.
There is a voltage between the copper and the iron that depends
on the temperature difference between the two junctions.
The cold junction is at 0 °C and the voltage is kept at 0 V here.
The voltameter measures the difference in temperature between
the two junctions
Thermometers
A thermocouple can:
Measure high temperatures; the metals have high melting point.
Measure temperatures that vary rapidly; the mass of the metals in
each junction is small.
Thermometers
Answer the questions about Thermometers in your
booklet.