Temperature (5.1.1)

Temperature is a measure of the hotness of an object. On any temperature scale, a higher temperature means the object is hotter.

When two objects at different temperatures are in thermal contact, thermal energy flows from the hotter to the cooler object until their temperatures are equal.

It is useful to note that thermal contact does not necessarily require direct physical contact.

When two objects reach the same temperature, there is no net flow of thermal energy; they are in thermal equilibrium.

The zeroth law of thermodynamics states that if objects A and B are both in thermal equilibrium with object C, then A and B are in thermal equilibrium with each other, providing a physical definition of temperature.

Heat transfer through conduction occurs when the particles of a hotter object collide with those of a cooler object, resulting in the transfer of thermal energy.

Particles in the hotter object have higher average kinetic energy. When objects are in contact, collisions transfer kinetic energy from the hotter object to the colder one.

Over time, the hotter object loses kinetic energy, and the cooler object gains kinetic energy, until both have the same average kinetic energy.

It is important to note that particles are not exchanged between two bodies or systems through conduction.

As a substance is cooled, the average kinetic energy of its particles decreases. The temperature at which the average kinetic energy is zero is known as absolute zero, the lowest theoretical temperature.

The absolute (thermodynamic) scale starts at this point, so it has no negative values. The standard unit of temperature is the kelvin , one of the seven SI base units. The increment is the same as Celsius.

On the absolute scale, temperature is directly proportional to the average kinetic energy of the particles in a substance. Therefore, doubling the absolute temperature doubles the average kinetic energy, regardless of the material. This is why the scale is independent of any particular substance.

The Kelvin temperature scale is defined by two specific points:

• Absolute zero () where particle kinetic energy is zero.
• The triple point of water, which is the unique temperature ( ~) and pressure () where ice, liquid water, and water vapour coexist in equilibrium.

This defines the scale and sets the offset from Celsius:

This means that a change of corresponds to a change of

The Kelvin scale is used in scientific calculations, extreme temperatures, and experiments because it is absolute and proportional to particle kinetic energy.

The Kelvin temperature scale is defined in relation to absolute zero and the triple point of pure water.

The triple point occurs at on the Celsius scale and on the kelvin scale. So, the offset between the scales is 273.15, because:

Therefore, the relationship is:

Where:

• is the temperature in Kelvin, and
• is the temperature in degrees Celsius.

It is important to note that the values and are the true, precise definitions; however, for A-level calculations, temperatures are usually rounded to the nearest kelvin, so 273 is sufficient.