What principle explains that temperature is directly proportional to the kinetic energy of molecules?

The correct answer is the Kinetic Molecular Theory, which states that temperature is a measure of the average kinetic energy of the molecules in a substance. As the temperature increases, the molecules move more vigorously, indicating that their kinetic energy has increased. Conversely, a decrease in temperature means that the molecular motion decreases, leading to lower kinetic energy.

This theory bridges the macroscopic observations of temperature and heat with the microscopic behavior of particles. It explains various physical properties, including how gases expand when heated, how they exert pressure, and how they behave in different temperature conditions. Thus, the direct relationship between temperature and the kinetic energy of molecules is a foundational concept in understanding gas behavior and thermodynamics as a whole.

Other principles mentioned do not specifically address the relationship between temperature and molecular kinetic energy. The First Law of Thermodynamics focuses on the conservation of energy and the transformation between energy forms, while the Law of Conservation of Energy deals with the total energy being constant in an isolated system. The Principle of Thermal Equilibrium pertains to the state when two bodies or systems reach the same temperature and no longer exchange heat, which does not elaborate on the role of kinetic energy in relation to temperature changes.