The first law of thermodynamics is concerned with the conservation of energy.

It states that energy cannot be created or destroyed, but can be changed from one form to another. In other words, the total energy of an isolated system remains constant.

More formally, the first law of thermodynamics is often expressed as:

ΔU = Q - W

• Where:
• ΔU is the change in internal energy of the system
• Q is the heat added to the system
• W is the work done by the system

Source: Khan Academy - First law of thermodynamics