Thermal conduction

Thermal conduction is the transfer of energy through a material from a hotter region to a cooler region. Energy passes between neighbouring particles, but the material as a whole does not move from one region to the other.

In a kitchen pan, energy passes from the heat source into the pan and then through its material. The pan transfers energy to its contents, heating them.

Metals are good thermal conductors, so energy spreads through a metal pan. A handle made from a poor thermal conductor reduces the rate at which energy passes from the pan towards the hand. It does not necessarily prevent the handle from becoming hot.

Heating a room by convection

A room heater warms the air close to it. As this air warms, it expands: the same amount of air occupies more space, so its density decreases. The warmer air is then less dense than the cooler air around it and rises.

Cooler, denser air moves towards the heater to replace the rising air. It is warmed in turn, expands and rises. Meanwhile, air elsewhere in the room cools and sinks. This continuous circulation is called a convection current.

The moving air carries thermal energy away from the heater and distributes it around the room. This transfer of thermal energy by the bulk movement of a fluid, such as air, is called convection.

Thermal radiation

Objects transfer energy by emitting electromagnetic waves. This transfer is called thermal radiation and includes infrared radiation.

All objects emit thermal radiation. For objects with similar surface areas and properties, the hotter object emits more energy by radiation each second.

When an object absorbs thermal radiation, its thermal energy increases and it may warm up. A hot source can therefore warm its surroundings directly: radiation carries energy from the source to objects that absorb it.

Thermal radiation does not require contact or a material medium. It can travel through empty space, so energy from the Sun can reach and warm Earth.

Thermal energy transfer in a fire

A wood or coal fire transfers thermal energy to its surroundings in three ways: conduction, convection and radiation. These transfers can also heat nearby fuel until it is hot enough to burn.

Conduction transfers energy through solid fuel from hotter regions to cooler regions. Particles in the hotter region transfer energy to neighbouring particles, passing energy through the material without the fuel moving as a whole.

Convection transfers energy through the air and combustion gases around the fire. As these gases are heated, they expand, become less dense and rise. Cooler, denser air moves in to replace them. This movement forms convection currents that carry thermal energy away from the fire.

Thermal radiation is emitted by the flames and hot fuel. It can travel directly to surrounding objects and warm them without needing air or another material in between.

Thermal energy transfer in a car radiator

A car radiator removes thermal energy from the engine by transferring it to the surroundings. A liquid called coolant circulates between the engine and the radiator, carrying thermal energy as it moves.

Hot coolant flows from the engine through tubes in the radiator. Thermal energy passes from the coolant by conduction through the metal walls of the tubes and the metal fins attached to them, reaching their outer surfaces.

Air flowing over these warm surfaces is heated and carries energy away by convection. The car’s motion or a fan can help maintain the airflow. The radiator also emits thermal radiation, transferring energy to its surroundings.

After passing through the radiator, the cooled coolant returns to the engine, where it can absorb more thermal energy.