Page 346 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
334
Temperature / °C
Resistance / Ω
10
3120
50
3600
75
3900
100
4200
150
4800
220
5640
260
6120
The thermodynamic scale of temperature is designed to overcome a problem with scales of temperature such as the Celsius scale, which depends on the melting point and boiling point of pure water. To measure a temperature on this scale, you might use a liquid-in-glass thermometer. However, the expansion of a liquid may be non-linear.
This means that if you compare the readings from two different types of liquid-in-glass thermometer, for example a mercury thermometer and an alcohol thermometer, you can only be sure that they will agree at the two fixed points on the Celsius scale. At other temperatures, their readings may differ.
The thermodynamic scale is said to be an absolute scale as it is not defined in terms of a property of any particular substance. It is based on the idea that the average kinetic energy of the particles of a substance increases with temperature. The average kinetic energy is the same for all substances at a particular thermodynamic temperature;
it does not depend on the material itself. In fact, as you will see in Chapter 22, the average kinetic energy of a
gas molecule is proportional to the thermodynamic temperature. So, if we can measure the average kinetic energy of the particles of a substance, we can deduce the temperature of that substance.
The thermodynamic scale has two fixed points:
■■ absolute zero, which is defined as 0 K
■■ the triple point of water, the temperature at which ice, water
and water vapour can co-exist, which is defined as 273.16 K (equal to 0.01 °C).
QUESTIONS
So the gap between absolute zero and the triple point of water is divided into 273.16 equal divisions. Each division is 1 K. The scale is defined in this slightly odd way so
that the scale divisions on the thermodynamic scale are equal in size to the divisions on the Celsius scale, making conversions between the two scales relatively easy.
A change in temperature of 1 K is thus equal to a change in temperature of 1 °C.
Thermometers
A thermometer is any device which can be used to measure temperature. Each type of thermometer makes use of some physical property of a material which changes with temperature. The most familiar is the length of
a column of liquid in a tube, which gets longer as the temperature increases because the liquid expands – this
is how a liquid-in-glass thermometer works. Other properties which can be used as the basis of thermometers include:
■■ the resistance of an electrical resistor or thermistor
■■ the voltage produced by a thermocouple
■■ the colour of an electrically heated wire
■■ the volume of a fixed mass of gas at constant pressure.
In each case, the thermometer must be calibrated at two
or more known temperatures (such as the melting and boiling points of water, which correspond to 0 °C and
100 °C), and the scale between divided into equal divisions. There is no guarantee that two thermometers will agree with each other except at these fixed points. Now we will look in detail at two types of electrical thermometer.
4 a
Convert each of the following temperatures from the Celsius scale to the thermodynamic scale: 0 °C, 20 °C, 120 °C, 500 °C, −23 °C, −200 °C.
b Convert each of the following temperatures from the thermodynamic scale to the Celsius scale: 0 K, 20K, 100K, 300K, 373K, 500K.
5 The electrical resistance of a pure copper wire is mostly due to the vibrations of the copper atoms. Table 21.1 shows how the resistance of a length
of copper wire is found to change as it is heated. Copy the table and add a column showing the temperatures in K. Draw a graph to show these data. (Start the temperature scale of your graph at 0 K.) Explain why you might expect the resistance of copper to be zero at this temperature.
Table 21.1 The variation of resistance with temperature for a length of copper wire.