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Heat, and internal energy

In a recent article, I mentioned in passing that the internal energy of a system is a state function. Just to quickly recap, state functions are properties of a physical system whose values do not depend on how they were arrived at from a prior state of the system. They depend only on the starting and ending states of the system.

I then contrasted state functions with path-dependent functions, which can take on very different values depending on the path by which the system arrived at its current state from its previous state (the history of the system matters).

Perhaps counter-intuitively, while it’s true that internal energy is a state function, the change in a system’s internal energy is the sum of two path-dependent functions. (more…)