Why can energy not be created or destroyed under the first law of thermodynamics?

The first law of thermodynamics asserts that energy cannot be created or destroyed because it is a fundamental conservation of energy principle. In any system, energy is simply converted from one form to another—such as from potential to kinetic or thermal energy—meaning the total energy of an isolated system remains constant.12

When a system is not isolated, the change in its internal energy ($\Delta U$) is determined by the energy exchanged with its surroundings through heat ($Q$) and work ($W$), expressed by the equation:

$$\Delta U=Q-W$$

This acts as a strict accounting system where any energy added to or removed from a system is accounted for by a corresponding change in the system's internal state. Ultimately, this principle holds because the laws of physics are uniform over time; if energy were not conserved, identical physical phenomena would require different amounts of energy at different moments, which contradicts the consistency of physical laws.31