How is thermal energy defined in terms of the particles in a system?

Thermal energy is essentially the internal energy of a system that arises from the random motion of its particles. In this context, the kinetic energy of the particles plays a crucial role. As particles vibrate, rotate, or translate, they possess kinetic energy that contributes to the total internal energy of the system. This collective energy correlates to the temperature of the substance; higher temperatures indicate faster moving particles, which translates to greater thermal energy.

The concept encapsulates how thermal energy is not just about the motion of individual particles but their overall collective motion within the system. Therefore, the understanding of thermal energy in terms of particle motion is foundational in physics, particularly when discussing heat transfer and temperature variations in different materials.