The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only converted from one form to another. In a thermodynamic system, the total internal energy change is equal to the heat added to the system minus the work done by the system on its surroundings.

Mathematically, it is expressed as:
ΔU = Q − W,
where:

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

This law implies that all energy transfers and transformations—whether in engines, living organisms, or chemical reactions—must account for every unit of energy involved. No energy is lost; it simply changes form (e.g., from chemical energy to thermal energy or mechanical work).

The first law forms the foundation of all energy-related science and engineering, ensuring energy balance in systems from power plants to cells in the human body.