The first law of thermodynamics is a restatement of which fundamental principle?

The first law of thermodynamics is a restatement of the conservation of energy. It applies that broad principle to thermodynamic systems, where energy can enter or leave as heat or work and can change the system’s internal energy.

For a closed system, the law is commonly written as ΔU = Q − W when W means work done by the system. Here, ΔU is the change in internal energy, Q is heat supplied, and W is work performed on the surroundings. Different textbooks may use a different sign convention, but the physical idea is unchanged: energy is transferred or transformed, never created or destroyed.

The law emerged from 19th-century work by scientists including Julius Robert von Mayer, James Prescott Joule, and Rudolf Clausius. Joule’s experiments showed that mechanical work and heat are quantitatively interchangeable, helping replace the older caloric theory of heat.

A common mix-up is confusing this law with the second law. The first law tracks the amount of energy; the second law describes direction, irreversibility, and entropy. The first law also explains why a perpetual-motion machine that produces energy without an input is impossible.