If the gas inside the syringe is heated, it will expand. In order to keep the volume constant, we have to apply more pressure (by adding weights) to the plunger to keep it from moving out. We can do this by placing the syringe vertically with the plunger at the top.
As the air inside the syringe is heated, more and more weights are needed to keep the plunger static and therefore the volume of gas constant. There is a proportional relationship between the amount of pressure applied by the weights and the temperature of the gas.
Volume at constant pressure (Fig. 10.4b)
When we start, the gas in the syringe is at atmospheric pressure. As we heat it up, it will expand in order to stay at atmospheric pressure. If we allow the plunger to move out under normal atmospheric pressure, we can read the volume of the gas as temperature changes. This allows volume to be a thermometric property.
Colour of crystals
Certain crystals change colour depending on their temperature. These can be used in ‘fever strips’ (Fig. 10.5). A strip of plastic containing these crystals is placed on a person’s forehead and the colour quickly tells you if the person has a fever.
NOTE
Electromotive force is essentially the same as voltage but emf is seen as the original supply and voltage is the difference in electrical potential levels within the circuit; see Chapter 21.
98 FUSION Electrical resistance
Like electromotive force (emf), resistance changes measurably with temperature (see Chapter 24).
• As a metallic conductor increases in temperature, its resistance increases.
• As a semiconductor increases in temperature, its resistance decreases.
These facts can be used to design a thermometric scale. A digital thermometer based on resistance is shown in Fig. 10.6.
FIG. 10.6 A digital thermometer is based on resistance
FIG. 10.5 The crystals in a fever strip will change colour if a person has a fever
b
FIG. 10.4 Measuring a the pressure of a gas at constant volume and b the volume of a gas at constant pressure
