In thermodynamics, which principle explains the conservation of energy?

The first law of thermodynamics is fundamentally concerned with the concept of energy conservation. It states that energy cannot be created or destroyed, only transformed from one form to another. This principle can be articulated through the equation ΔU = Q - W, where ΔU represents the change in internal energy of a system, Q is the heat added to the system, and W is the work done by the system. This formula highlights how the energy within a closed system is preserved, emphasizing that all energy changes can be accounted for within the system's internal energy and the work and heat interactions with its surroundings.

In contrast, the second law of thermodynamics deals with the direction of energy transfer and the increase of entropy in a closed system, which does not directly address the conservation of energy itself. The ideal gas law, while a useful equation in understanding the behavior of gases, is not focused on energy conservation but rather on the relationships between pressure, volume, and temperature of an ideal gas. Similarly, the law of thermal expansion addresses the expansion of materials as temperature increases, lacking the foundational principle of energy conservation found in the first law.

Thus, the first law clearly and specifically encapsulates the conservation of energy, making it the correct choice.