A body treated as a lumped capacitance object, with a total internal energy of U {\displaystyle U} (in joules), is characterized by a single uniform internal temperature, T ( t ) {\displaystyle T(t)} . The heat capacitance, C {\displaystyle C} , of the body is C = d U / d T {\displaystyle C=dU/dT} (in J/K), for the case of an incompressible material. The internal energy may be written in terms of the temperature of the body, the heat capacitance (taken to be independent of temperature), and a reference temperature at which the internal energy is zero: U = C ( T − T ref ) {\displaystyle U=C(T-T_{\text{ref}})} .