According to the first law of thermodynamics, heat minus work equals the change in the internal form of this quantity.

• A. Energy
• B. Temperature
• C. Pressure
• D. Entropy

Answer: (A)

The key idea is energy conservation for a system, captured by the first law of thermodynamics. It says that the change in the system’s internal energy equals the heat added to the system minus the work the system does on its surroundings: ΔU = Q − W. The quantity that changes with heat and work is the internal energy, which reflects the microscopic energy stored in the system due to particle motion and interactions. So heat minus work corresponds to the change in internal energy. Temperature is related to average kinetic energy but isn’t the energy quantity itself, while pressure and entropy are separate properties with different roles in thermodynamics.