Unit 2 Thermal physics


Measurement of temperature

  • Temperature is the measure of how hot an object is. The hotness of an object is a measure of the kinetic energy of the molecules that make up that object.
  • A thermometer monitors a suitable physical property. In the case of a liquid-in-glass thermometer, it monitors volume of the liquid. Any device that includes a substance that changes uniformly with temperature can be calibrated and be made into a useful thermometer.
  • To calibrate (create a scale) a thermometer we need known upper and lower fixed points (such as the steam point at 100°C and the ice point at 0°C). The Celsius scale was devised by Anders Celsius.
  • There are several types of thermometer including the mercury-in-glass or the alcohol-in-glass thermometer, shown in the diagram below. This is a very common type of thermometer. The volume of the liquid and hence the length of the liquid column changes uniformly with temperature.
  • Mercury is a liquid metal and as such is useful for measuring higher temperatures. Its boiling point is 357°C and it freezes at –39°C.
  • Alcohol has a very low freezing point and as such is useful for measuring low temperatures. It has a boiling point of 78°C and freezes at –114°C.
  • Other types of thermometers include:
    • The liquid crystal thermometer – it contains heat-sensitive liquid crystals that change colour to indicate different temperatures.
    • The constant-volume gas thermometer – it contains a gas; when the temperature rises, the pressure increases. The change in pressure is used to indicate a change in temperature.
  • The range of a thermometer is the difference between the maximum and minimum temperatures that the thermometer can read. A common range for liquid-in-glass thermometers is from –10°C to 110°C.
  • The range of a liquid-in-glass thermometer is limited by the length of the thermometer and can be increased by:
    • increasing the diameter of the capillary – this means that the liquid will not expand as far along the tube per degree rise in temperature
    • decreasing the volume of the bulb – this means there is less liquid and so it will not travel as far along the capillary tube as it is heated.
  • A liquid-in-glass thermometer is said to be linear if the liquid expands by the same amount for every degree Celsius rise in temperature. This means that the scale will be marked in degrees of equal size (as below).
    linear non linear
  • If the liquid did not expand uniformly, the scale would be non-linear. It would have to be marked with degrees of differing sizes (as above). It would be very difficult to calibrate and to use.

Top Tip

Characteristics that make a substance suitable for use in a liquid-in-glass thermometer include:

  • expanding uniformly over a large temperature range
  • a low specific heat capacity for quick response
  • a low freezing and high boiling point so that the substance remains liquid over a good range of temperatures.
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