Which law states that energy cannot be created or destroyed, only interconverted between heat and work?

• A. Zeroth Law
• B. Second Law
• C. Third Law
• D. First Law of Thermodynamics

Answer: (D)

Energy cannot be created or destroyed; it can only be transformed from one form to another, and the total energy in a closed system stays the same. This is captured by the First Law of Thermodynamics, which says the change in a system’s internal energy equals the heat added plus the work done on the system (energy balance). In practical terms, heat flow and work are two ways energy can cross the system’s boundary, but the overall amount of energy remains constant. For example, compressing a gas increases its internal energy and may store energy as work on the surroundings, while heating the gas raises its internal energy that can later be used to do work. The Zeroth Law deals with temperature and thermal equilibrium, the Second Law introduces entropy and the direction of processes, and the Third Law concerns entropy at absolute zero, none of which state the conservation of energy.