What is the first law of thermodynamics?

The first law states that energy cannot be created or destroyed — only transformed: ΔU = Q - W, where ΔU = change in internal energy, Q = heat added to the system, W = work done BY the system. If Q > 0: heat flows in. If W > 0: system does work. Example: A gas absorbs 500J of heat and does 200J of work → ΔU = 500 - 200 = 300J increase in internal energy. This is essentially conservation of energy applied to thermal systems.