Measurement of Temperature (CIE A Level Physics)

• Temperature is measured using a physical property that varies with temperature
  • A thermometer is any device used to measure temperature
• Each type of thermometer uses the physical property of a material that varies with temperature, for example:
  • The density of a liquid
  • The volume of a gas at constant pressure
  • Resistance of a metal
  • E.m.f. of a thermocouple
• Each type of thermometer is calibrated at two or more known temperatures (commonly the boiling and melting points of water, 0 ^oC and 100 ^oC respectively) and the scale is divided into equal divisions

The density of a liquid

• A liquid-in-glass thermometer depends on the density change of a liquid (commonly mercury)
  • The thin glass capillary tube contains a liquid that expands with temperature
  • The scale on the side of the tube is measured based on the length of liquid within the tube

A liquid-in-glass thermometer

As the bulb is heated, the liquid expands and moves along the capillary tube

Volume of a gas at constant pressure

• The volume of an ideal gas is directly proportional to its temperature when at constant pressure (Charles’s law)

V ∝ T

• As the temperature of the gas increases, its volume increases and vice versa
• A gas thermometer must be calibrated when the temperature of a gas at a certain volume is known
  • A temperature scale can be determined depending on how quickly the gas expands with temperature

Resistance of a metal

• Recall that electrical resistance changes with temperature e.g. the resistance of a filament lamp increases when the current passing through it also increases
  • For metals: resistance increases with temperature at a steady rate
  • For thermistors: resistance changes rapidly over a narrow range of temperatures
• As a thermistor gets hotter, its resistance decreases
  • A thermistor can be used to measure a range of temperatures
• The relationship between the resistance and temperature of a thermistor is non-linear
  • The graph of temperature against resistance is a curved line, so the thermistor has to be calibrated

Graph of resistance against temperature

As the temperature of a thermistor increases, its resistance decreases.

E.m.f. of a thermocouple

• A thermocouple is an electrical device used as the sensor of a thermometer
• A thermocouple has two wires of different, or dissimilar, metals attached to each other to create a junction at one end
  • The other ends of the wires are connected to a voltmeter
• When this junction is heated, an e.m.f. is produced between the two wires and measured by the voltmeter
  • The greater the difference in temperature between the wires, the greater the e.m.f

Diagram showing a thermocouple

A thermocouple consists of two dissimilar wires connected together

• A thermocouple requires calibration since the e.m.f. does not vary linearly with temperature
• The graph against e.m.f. and temperature is a positive, curved line

Graph showing e.m.f against temperature

The e.m.f. and temperature are not directly proportional in a thermocouple.