Which principle explains that energy in a closed system remains constant?

The First Law of Thermodynamics is the principle that asserts energy in a closed system remains constant. This law highlights the concept of conservation of energy, indicating that energy can neither be created nor destroyed; it can only change forms. For example, when energy is transferred within a system—like when heat is applied to a substance—the total amount of energy before and after the transfer remains unchanged.

In contrast, the Laws of Motion pertain to the relationships between the forces acting on an object and the motion of that object, not to the conservation of energy. The Principle of Relativity deals with the physics of moving bodies and the effects of gravity on time and space, rather than energy conservation. Universal Gravitation describes the attractive force between masses but does not involve the concept of energy remaining constant in the context of a closed system. Therefore, the First Law of Thermodynamics is the principle that specifically addresses energy conservation in a closed system.