We discuss the role of the internal forces and how their work changes the energy of a system. We
illustrate the contribution of the internal work to the variation of the system’s energy, using a
pure mechanical example, a thermodynamical system and an example from electromagnetism. We emphasize
that internal energy variations related to the work of the internal forces should be pinpointed in
the classroom and placed on the same footing as other internal energy variations such as those
caused by temperature changes or by chemical reactions.