The important consequence of the first law is that energy is ways conserved. This law governs the transfer of energy from one place to another, in one form or another a heat energy, mechanical energy, electrical energy, radiation energy etc. The energy contained within a thermodynamic system is termed the internal energy or simply the energy of the system, U. In all processes. reversible or irreversible the change in internal energy must be in accord with the first law of thermodynamics.