Internal energy is energy stored in a body in the form of kinetic energy of molecules and atoms or potential energy of its molecules and atoms due to their mutual interactions. Internal energy, U, is the sum of the kinetic and potential energies of all the particles making up a body but does not include the kinetic and potential energies of the system as a whole.

The value of the absolute energy internal energy of a system in any particular state is not a measurable quantity. However, the change in internal energy ΔU can be measured. For a closed system, ΔU is equal to the heat absorbed by the system from its surroundings (Q), less the work done by the system on its surrounding (W), i.e. ΔU = Q – W.