In thermodynamics, when d Q is exact, the function Q is a state function of the system: a mathematical function which depends solely on the current equilibrium state, not on the path taken to reach that state. Internal energy U , Entropy S , Enthalpy H , Helmholtz free energy A , and Gibbs free energy G are state functions. Generally, neither work W nor heat Q is a state function. (Note: Q is commonly used to represent heat in physics. It should not be confused with the use earlier in this article as the parameter of an exact differential.)