Temperature

The temperature of an object is a property of the object that determines how hot or cold the body is. It depends on the amount of thermal energy absorbed by the object and also the nature of the object and its mass.

The temperature of two objects determines the direction of the transfer of thermal energy between them. Although both objects will give out heat energy there will be a net transfer of thermal energy from the hot object to the cold object. This transfer may be by radiation or by conduction through the intervening material between them.

The temperature of body A is greater than that of body B. T₁ is greater than T₂ and so there is a net flow of thermal energy from A to B. This fits in with the second law of thermodynamics which in one of its forms says that thermal energy cannot pass from one object to another object which is at a higher temperature without some work being done on the system.

Temperature scales

Temperature is measured with a thermometer but before we can take any readings we have to set up a temperature scale.

Thermodynamic or Kelvin scale

The thermodynamic scale is the one that is used for scientific measurement. It is measured in units called kelvins (K), the temperature itself being given the letter T. It is defined using one fixed point - the triple point of water. This is the temperature where saturated water vapour, pure water and ice are all in equilibrium at a temperature of 273.16 K.

Absolute zero

This is defined as 0 K, or –273.15 ^oC. This is the lowest temperature possible. In fact the third law of thermodynamics states that it is impossible to actually reach this temperature. When an object is cooled its internal energy is reduced and the temperature approaches absolute zero it becomes more and more difficult to lower the temperature further. You can think of the energy being reduced in smaller and smaller steps – the steps being rather like those of a stationary escalator as you near the bottom – they get smaller and smaller. We are always left with what is known as 'zero point energy' and so we can define absolute zero as the temperature at which substances have a minimum internal energy.

The temperature in deep space is about 3 K above absolute zero and temperatures as low as 10^–6 K have been achieved in the laboratory by sophisticated means.

Celsius scale

The Celsius scale is now defined by θ = T - 273.15.
The two fixed points on this scale are the ice point (0^oC) and the steam point (100^oC). The ice point and the triple point differ by 0.01 K.

General definition of a temperature scale

A temperature scale depends on the particular property on which it is based. In setting up a scale of temperature we must:
(a) choose a property that varies with temperature,
(b) assume that it varies uniformly with temperature.
If we denote the property by F, then on the Celsius scale:

θ/100 = [F_θ – F₀]/[F₁₀₀ – F₀]

where θ is the temperature to be measured and F_θ its value at θ ^oC, F₀ its value at 0 ^oC and F₁₀₀ its value at 100 ^oC.

Standard temperature and pressure (STP on NTP).

This set of conditions which is usually applied to gases is defined as a temperature of 273 K and a pressure of 760 mm of mercury (1.013x10⁵ Pa). At STP 1 mole of any gas has a volume of 22.4x10^-3 m³.