Which law of thermodynamics states that total energy in a closed system remains constant?

The First Law of Thermodynamics is fundamentally about the conservation of energy, which states that the total energy within a closed system remains constant over time. It asserts that energy can neither be created nor destroyed; rather, it can only be transformed from one form to another or transferred between systems. This means that when heat is added to the system, it can do work or increase internal energy, but the total energy balance must remain unchanged.

In practical terms, this principle is essential for understanding how machines, engines, and even natural processes work. For instance, when you operate a steam engine, the heat energy derived from burning fuel is converted into kinetic energy to power the engine, but the total amount of energy involved in the system does not increase or decrease—it simply transforms.

The other laws of thermodynamics do have important roles, such as establishing the direction of spontaneous processes (Second Law) and providing insights into the absolute temperature scale (Third Law and Zeroth Law), but they do not specifically address the constancy of total energy in a closed system like the First Law does. Thus, the First Law remains the foundational principle regarding energy conservation.