# Energy and temperature

A2 Physics

Energy can be stored in an object as heat. This is different to temperature, which is a measure of the hotness of the object.

The internal energy of an object is the sum of the random distributions of kinetic and potential energies of all the molecules in a body. The thermal energy is the internal energy due to the temperature of an object.

Heat transfer takes place between adjacent objects. Net heat transfer is always from a hotter to a colder body. When the two objects are the same temperature heat is still transferred, but the net heat transfer is zero. If two objects are both in thermal equilibrium with a third object, then the two objects are in thermal equilibrium with each other (this is the zeroth law of thermodynamics).

Thermal equilibrium is used to measure temperature. The mercury in a thermometer reaches thermal equilibrium with the glass of the thermometer, which reaches thermal equilibrium with the object whose temperature you’re measuring. By measuring the temperature of the mercury by its expansion, you then know the temperature of your object.

Temperature is often measured in °C. In Physics, it is sometimes more useful to have temperature in a unit called Kelvin, K. Temperature in K = temperature in °C + 273.15

When the temperature is 0 K this is called absolute zero. The object has minimum internal energy.