Measurement of Temperature (OCR A Level Physics): Revision Note

Scale of Thermodynamic Temperature

• Temperature is a number used to indicate the level of hotness of an object on some scale

• To measure temperature a scale is needed involving two fixed points at known temperatures

  • The temperature of other objects can then be defined as a position on this scale

• The Kelvin scale is known as the thermodynamic scale and was designed to overcome the problem with scales of temperature

• The thermodynamic scale is said to be an absolute scale that is not defined in terms of a property of any particular substance

• This is because thermodynamic temperatures do not depend on the property of any particular substance

• The fixed points on the absolute temperature scale are: 

  • Triple point of pure water 

    • The point where pure ice, pure water and pure water vapour all exist at the same temperature and pressure

• Absolute zero 

  • The lowest possible temperature

Units of Temperature

• As an everyday scale of temperature, Celsius (^oC) is the most familiar

• This scale is based on the properties of water - the freezing point of water was taken as 0 ^oC and the boiling point as 100 ^oC

  • However, there is nothing special about these two temperatures

  • The freezing and boiling point of water will actually change as its pressure changes

• The Celsius scale is used to measure the temperature in a liquid-in-glass thermometer

  • However, the expansion of the liquid might be non-linear

• Other temperature scales include:

  • Fahrenheit, commonly used in the US

  • Kelvin, used in thermodynamics

The Kelvin Scale

• To convert between temperatures θ in the Celsius scale, and T in the Kelvin scale, use the following conversion:

θ / ^oC = T / K − 273.15

T / K = θ / ^oC + 273.15

Conversion chart relating the temperature on the Kelvin and Celsius scales

• The divisions on both scales are equal. This means:

  A change in a temperature of 1 K is equal to a change in temperature of 1 ^oC