In mechanics, the second law in conjunction with the fundamental thermodynamic relation places limits on a system's ability to do useful work. The entropy change of a system at temperature T absorbing an infinitesimal amount of heat δ δ q in a reversible way, is given by δ δ q / T . More explicitly, an energy T R S is not available to do useful work, where T R is the temperature of the coldest accessible reservoir or heat sink external to the system. For further discussion, see Exergy.