First law of thermodynamics

The first law of thermodynamics states that the change in internal energy of a system ([math]\Delta U[/math]) is equal to the work done by or to the system ([math]W[/math]) and the heat that flows in or out of it ([math]Q[/math]).

The equation for this relationship is: [math]\Delta U = W + Q[/math][1]

This means that if energy is input to a system by doing work [math]W[/math] on it, then it will get hotter. If the system is to stay the same temperature, then you must remove heat from the system - [math]Q[/math].

The first law of thermodynamics is a generalization of the law of conservation of energy, which states that energy can neither be created nor destroyed.[1]

This is a particularly important law when discussing heat engines (like car engines), because it helps to understand that heat must go somewhere—the basis for cogeneration. As an engine creates heat, if the heat is not vented, the engine will become hotter and hotter, which will eventually lead to melting. Therefore, because it is impossible to achieve 100% efficiency (see carnot efficiency), heat must be vented. This can either be treated as waste heat, or used for cogeneration, which is applying the waste heat to another cause, like heating a car, or heating houses.

To learn more about the first law of thermodynamics, please check out hyperphysics.

For Further Reading


  1. 1.0 1.1 R. Knight, Physics for scientists and engineers. Boston, Mass.: Addison-Wesley, 2012, p. 478.