How does the first law of thermodynamics express conservation of energy?

The first law of thermodynamics serves as a formal expression of the law of conservation of energy, asserting that energy cannot be created or destroyed, only converted from one form to another.123

Within an isolated system, the total energy remains constant regardless of internal conversions. For non-isolated systems, the law provides a strict energy accounting system by tracking the flow of energy across the system's boundaries. This is expressed by the equation:12

$$\Delta U=Q-W$$

In this relation:
* $\Delta U$ represents the change in the system's internal energy.12
* $Q$ is the heat added to the system from its surroundings.14
* $W$ is the work done by the system on its surroundings.14

Essentially, any change in the internal energy of a system must be exactly balanced by the difference between the energy deposited as heat and the energy withdrawn as work. While internal energy is a state function determined by the system's equilibrium state, heat and work are path-dependent quantities that describe the process of energy transfer.15