[عاطف خليفة]

The first law of thermodynamics is a consequence of conservation of energy and requires that a system may exchange energy with the surroundings strictly by heat flow or work. Therefore, for change in energy dE, heat change , work done dW,

For a reversible process in which only expansive work is considered, the first law takes the form

where T is the temperature, dS the entropy change, P the pressure, and dV volume change.