The second property to be considered in the overall balances on a control volume is energy. We shall apply the principle of the conservation of energy to a control volume fixed in space in much the same manner as the principle of conservation of mass was used to obtain the overall mass balance. The energy-conservation equation will then be combined with the first law of thermodynamics to obtain the final overall energy-balance equation.
We can write the first law of thermodynamics as
where E is the total energy per unit mass of fluid, Q is the heat absorbed per unit mass of fluid, ...