Measurement of Temperature (Cambridge (CIE) A Level Physics)

Measurement of temperature

• 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.