Which of the following describes a first law of thermodynamics?

The first law of thermodynamics, also known as the law of energy conservation, states that energy can neither be created nor destroyed, only transformed from one form to another. This foundational principle highlights that the total amount of energy in a closed system remains constant, although it can change forms—such as from potential energy to kinetic energy, or from thermal energy to work done.

This law is crucial in the analysis of energy systems because it allows engineers to account for energy inputs and outputs in any thermodynamic process. For instance, when analyzing a boiler or engine, understanding that the energy supplied must equal the energy used for work and losses helps ensure proper system design and optimization.

The other options represent different concepts in thermodynamics: the second option addresses the natural direction of heat flow, the third speaks to the nature of thermal equilibrium, and the fourth concerns entropy in isolated systems, collectively illustrating different aspects of thermodynamic principles but not the first law specifically.