Världens största ånglok, Big Boy, är exempel på en värmemaskin vars effektivitet beror på temperaturskillnaden mellan vattenångan och den omgivande luften.
A spacecraft enters  the atmosphere with a velocity of appr. 40 000 km

The temperature is a measure of the heat content of a body.
Heat or thermal energy is a form of energy that moves from hotter to cooler bodies.

The average kinetic energy of a molecule with an absolute temperature T is given by

E = 3kT/2.

Since temperature is a measure of heat heat is a measure of the kinetic energy of the constituents of the medium of a certain temperature.

If the temperature increases the movements of the individual atoms also gets more violent until the chemical bondings are broken up and the atoms become free.

This is how an element changes from solid to liquid to gaseous form.

Those are the three states of aggregation of matter. In stars a fourth state is possible: the plasma state.

Atoms and molecules never rest (this is the third law of thermodynamics) and when they bounce together electrons in the atoms are excited and within nanoseconds they deexcite by emission of low-energetic electromagnetic radiation so-called Infra-red radiation or heat-radiation.

According to Heisenberg’s uncertainty-principle it’s impossible for an atom to be at rest because this would imply that we have exact knowledge about the speed of the atom. The Heisenberg uncertainty principle δp˙δx>h
then postulates that the uncertainty in the position of the atom must be infinite.

This radiation from e.g. the Sun is perceived as heat and it drives the atoms of the skin into  oscillations whereby the temperature in the skin is increased.

All materials having a temperature in excess of absolute zero emits heatradiation.  This process occurs until the temperature of the body and its surroundings are equal.

Then the body emits and absorbs the same amount of heat energy and an equilibrium has been established.

The first postulate of thermodynamics states that the energy of an isolated system is constant. Energy can not be created or destroyed only transformed between different forms.

kinetic energy

potential energy

nuclear energy

radiation energy

Värmekapaciteten. Kan uttryckas med hjälp av frihetsgraderna med hjälp av ekvipartitionsprincipen. Den anger hur mycket energi som krävs för att öka temperaturen 1C. För ett kg av ämnet.

C=0,5(ntr + nrot + nvib2)R.

Van der Waals gaslag
(P+a/v)(V-b)=kT.

a/V anger växelverkan mellan gasmolekylerna medan b är ett mått på den volym gasmolekylerna upptar.