How is thermal energy best defined?

Thermal energy is best defined as energy associated with very small particles called molecules. This definition captures the essence of thermal energy because it is fundamentally linked to the motion and vibration of molecules within a substance. As these molecules move and interact with one another, they generate heat, which is a manifestation of thermal energy.

When temperature increases, it indicates that the average kinetic energy of the molecules is rising, leading to a greater degree of motion. This motion directly translates to thermal energy. Understanding thermal energy through the activity of molecules also highlights the concept that temperature is a measure of the average energy of these particles. In contrast, the other options refer to different forms or sources of energy, which do not encapsulate the unique characteristics of thermal energy that relate specifically to molecular motion and interactions.