Final answer:
Thermal energy is the kinetic energy of particles in a substance, dictating its temperature. It is the microscopic motion and vibrations of atoms and molecules, with higher thermal energy leading to a higher temperature.
Step-by-step explanation:
Thermal energy is the kinetic energy associated with the random motion of atoms and molecules. It can be understood as the energy of heat or the kinetic energy resulting from the microscopic movements and vibrations of particles in a substance. The greater the movement of these particles, the higher the temperature of the substance, since temperature is essentially a measure of the average kinetic energy of the molecules within a material. When a substance has more thermal energy, it is considered hotter, and conversely, with lower thermal energy, the substance is colder.
Among the four physical states of matter, a solid has the lowest thermal energy due to strong intermolecular forces that tightly hold the molecules in place, allowing only vibrational motion. No matter the state of matter, as the thermal energy increases, the temperature likewise increases, assuming no phase change or chemical reaction occurs. Conversely, as thermal energy decreases, so does the temperature.