In thermodynamics, when  is exact, the function  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 , Entropy , Enthalpy , Helmholtz free energy , and Gibbs free energy  are state functions. Generally, neither work  nor heat  is a state function. (Note:  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.)