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Nowadays we know that the atoms and molecules from which matter is formed are not at rest even though the body they make up is still. In solids the particles vibrate continuously around their point of equilibrium; in fluids they move freely, although they are confined to a determined volume; in gases they move freely, occupying all the space available.
We will call the sum of all the energy of all the particles which make up the body, thermal energy. Temperature is the average value of the kinetic energy of these particles.
We all know that bodies can be heated (increasing their internal energy) or cooled (losing internal energy). The energy gained or lost in these processes is heat.
In the following visual we illustrate the concept of thermal energy, temperature and heat with the example of a gas which we can heat in different ways.
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