The first law of thermodynamics, often called the law of energy conservation, states that energy cannot be created or destroyed, only transformed from one form to another. In simpler terms, the total amount of energy in a closed system remains constant. If energy enters or leaves a system, it must be accounted for by changes in the system’s internal energy, work done by the system, or heat transferred to or from the system. For example, when you heat a pot of water on a stove, the heat energy from the stove is transferred to the water, increasing its internal energy and raising its temperature.