The important consequence of the first law is that energy is always conserved.This law governs the transfer of energy from one place to another ,in one form or another as heat energy ,mechanical energy ,electrical energy ,radiation energy ,etc . The energy contrained within a thermodynamic system is the internal energy or simply 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 thermodymics.