Statistical physics Internal energy is the energy contained in the degrees of freedom of a thermodynamic system (apart from the kinetic energy of the system): for a collection of particles (atoms, molecules), it is the sum of the vibrational, translational, rotational and interactional energies.
Thermodynamics. According to the first law of thermodynamics, the variation in internal energy U plus the variation in kinetic energy Ec is equal to the sum of the work W and the amount of heat Q transferred from the exterior to the system. ΔU + ΔEc = Q + W
These variations are independent of the path between the two states. This property is expressed by the fact that U(S,V) is a state function (dU = dW + dQ being a total differential).