The First Law

The first law of thermodynamics states the principle of energy conservation for systems in which heat and work interactions can take place and the internal energy of the system can change. Using unit mass or molar quantities the First Law has the form:

du = d'q - d'w

The heat interaction, d'q, is positive if heat is added to the system. The symbol, d' ,is used to indicate that the process is path dependent (d'q is an inexact differential).

The work interaction, d'w, is positive if work is done by the system. The work interaction is path dependent and d'w is an inexact differential.

The change in the internal energy, du, depends only upon the initial and final state of the system and NOT on the path taken between the end points. du is an exact differential.