The particles in a system have various types of energy, and the sum of them all is what we call* Internal energy of a system.*

For this sum to be calculated, the kinetic energies of agitation, aggregation potential, binding and nuclear between particles are considered.

Not all of these energies considered are thermal. Being supplied to a body with thermal energy causes a variation in the internal energy of this body. This variation is what the thermodynamic principles are based on.

If the system in which the internal energy is varying is a perfect gas, the internal energy will be summarized in the translational energy of its particles and is calculated by *Joule's Law:*

Where:

U: internal energy of gas;

n: mol number of gas;

R: universal constant of perfect gases;

T: absolute temperature (kelvin).

As for a given mass of gas, **no** and **R** constant, the change in internal energy will depend on the change in the absolute temperature of the gas, ie

- When the absolute temperature increases, there will be a positive variation of the internal energy. .
- When the absolute temperature decreases, there is a negative variation of internal energy..
- And when there is no change in gas temperature, the internal energy change will be zero.

Knowing the Clepeyron equation, it is possible to compare it to the equation described in Joule's Law, and thus we will obtain: