Thermal energy is the internal energy of a system due to the motion of its particles. Atoms and molecules are always moving, vibrating, and colliding. The faster they move, the more thermal energy the system contains. Temperature is a measure of the average kinetic energy of those particles.

Heat is not the same as thermal energy. Heat is the transfer of thermal energy from a hotter object to a cooler one. A cup of coffee has thermal energy. When you hold it, heat flows to your hand. The coffee cools, your hand warms, and the transfer stops when temperatures equalize.

Thermal energy drives engines, warms homes, and shapes weather. Power plants convert heat into electricity. Heat pumps move it from one place to another. Insulation slows its escape. Understanding thermal energy is essential for building efficiency, industrial processes, and climate science.

Key concepts

• Temperature vs. heat
• Specific heat capacity
• Conduction, convection, radiation
• Thermal expansion
• Latent heat