Thermal Energy

Definition of Thermal Energy

Thermal energy is the energy stored in a system due to the motion of its particles. All matter is made up of tiny particles (atoms or molecules) that are constantly vibrating or moving. The faster these particles move, the more thermal energy the system has.

Thermal energy is closely related to temperature, which measures the average kinetic energy of the particles. However, thermal energy refers to the total energy stored in all the particles, including their kinetic and potential energy within the system.

In solids, thermal energy is stored in the vibrations of particles fixed in place. In liquids and gases, particles move more freely, but the energy is still stored in their motion and interactions.

Thermal Energy Transfer Methods

Conduction is the transfer of thermal energy through a solid material. It happens when vibrating particles pass energy to neighbouring particles without the particles themselves moving from place to place. Metals are good conductors because their free electrons help transfer energy quickly.

Convection occurs in fluids (liquids and gases) where warmer, less dense regions rise and cooler, denser regions sink, creating a circulation pattern that transfers thermal energy. This is why hot air rises above a radiator and cooler air moves down to replace it.

Radiation is the transfer of thermal energy by electromagnetic waves, mainly infrared radiation. Unlike conduction and convection, radiation does not require particles or a medium; it can happen through a vacuum, such as the Sun’s energy reaching Earth.

Thermal Energy and Temperature

Temperature is a measure of the average kinetic energy of the particles in a substance. When temperature increases, particles move faster on average.

Thermal energy depends on both the temperature and the mass of the substance. A larger mass at the same temperature contains more thermal energy because there are more particles storing energy.

It is important to distinguish between heat and temperature:

• Heat is the transfer of thermal energy from a hotter object to a cooler one.
• Temperature is a measure of how hot or cold an object is, related to the average kinetic energy of its particles.

For example, a swimming pool and a cup of boiling water can have the same temperature, but the pool contains much more thermal energy because of its larger mass.

For instance, if a small metal block and a large metal block are both heated to 506C, the larger block stores more thermal energy because it contains more particles moving at that temperature.

A useful formula to calculate the change in thermal energy is Q = mc94T, where Q is the thermal energy transferred, m is the mass, c is the specific heat capacity, and 94T is the temperature change.

Thermal Energy in Systems

When energy is transferred to a system by heating, the thermal energy of the system increases. This causes the temperature to rise as the particles move faster.

Conversely, when a system loses thermal energy, its temperature decreases as the particles slow down.

This is why heating a pan on a stove makes it hotter (thermal energy increases), and why a hot cup of tea cools down over time (thermal energy decreases).

Example: If a metal rod is heated at one end, thermal energy is transferred along the rod by conduction, causing the temperature of the rod to increase gradually along its length.