What must happen to energy that enters a system, according to the first law of thermodynamics?

The first law of thermodynamics, which is often summarized as the principle of conservation of energy, states that energy cannot be created or destroyed, but can only change forms or be transferred. When energy enters a system, it must either leave the system or be stored in some form. This means that any energy introduced will eventually lead to either work being done by the system, or result in an increase in internal energy, which can manifest as thermal energy, kinetic energy, or potential energy, among others.

The key idea is that while the energy can be stored as potential energy or internal energy, it cannot just remain in its incoming form indefinitely without causing any changes in the system. Thus, the requirement that energy entering a system must either leave or be stored aligns directly with the core principles laid out in the first law of thermodynamics.