Which principle states energy cannot be created or destroyed, only transformed?

• A. Law of Conservation of Mass
• B. Law of Conservation of Energy
• C. Second Law of Thermodynamics
• D. Law of Entropy

Answer: (B)

Energy conservation is being tested here: energy cannot be created or destroyed, only transformed from one form to another. In any process, the total amount of energy in a closed system stays the same, even as it shifts between forms such as kinetic, potential, thermal, chemical, electrical, or light. For example, a swinging pendulum continually trades potential energy for kinetic energy and back, with the total energy remaining constant if you ignore air resistance. A light bulb converts electrical energy into light and heat, and the overall energy input equals the energy that appears as these outputs plus small losses.

The other options describe different ideas. Conservation of mass refers to matter not appearing or disappearing in ordinary reactions, which is a separate principle. The Second Law of Thermodynamics deals with entropy and the direction of energy transfer, often noting that some energy becomes less useful for doing work, but it doesn’t state that energy itself cannot be created or destroyed. The Law of Entropy is not a separate standard in basic physics; it’s encompassed by the Second Law.