Energy can be converted from one form to another as the systems and surroundings transfer energy back and forth. The Law of Conservation of Energy states that the energy of the universe is constant.

The First Law of Thermodynamics states that energy cannot be created or destroyed. The first law can be expressed mathematically using the following equation.

The energy change only depends on the initial and final states of the system and not on how the change occurs. This is called a state function.