Internal energy (U) is a state function representing the total energy contained in a system, determined entirely by the system's current macrostate (e.g., pressure, volume, temperature) regardless of history. The first law of thermodynamics states that changes in internal energy arise only from heat and work exchanged with the surroundings, expressed as ΔU = Q + W (or equivalently ΔU = Q − W depending on sign convention for work done by versus on the system). Unlike internal energy, heat and work are not state functions but transfer mechanisms—processes by which energy moves into or out of a system—so a system cannot be said to "contain" a fixed quantity of heat or work.
Internal energy (U) is a state function representing the total energy contained in a system, determined entirely by the system's current macrostate (e.g., pressure, volume, temperature) regardless of…