3.1c Heat transfer

Conduction takes place within a solid object, between solid objects or at a boundary between a solid and a liquid or a gas. The molecules of a solid are held in place by intermolecular forces, but the molecules are able to vibrate and hence have kinetic energy. The temperature of a body is a measurement of the average kinetic energy of the body. If part of an object is heated then the temperature of that part will increase, i.e. the kinetic energy / movement of those molecules will increase. Due to the intermolecular forces acting as flexible connections between the molecules this increased movement will be passed on to the neighbouring molecules. Unless there is a continuous external source of energy, the energy will be dissipated until the object is in thermal equilibrium.

Metals are good thermal conductors due to the free-electrons present in metallic bonding. These electrons can pass the energy more rapidly and effectively to neighbouring molecules.

Convection occurs in fluids (liquids and gases). When part of a fluid is heated, the molecules are able to move rapidly. Since they are moving more they exert a greater pressure on the surrounding volume of molecules and push outwards, decreasing the density of that volume of molecules. They can then rise through the medium, carrying their energy with them.

As the hot fluid rises it will become cooler as it moves way from the heat source and may sink back down again. As it rose it was replaced by cooler surrounding volumes of fluid which will become heated and rise. This repeated process gives rise to convection currents.

All object / materials emit electromagnetic radiation as a function of their temperature. This radiation can pass through all transparent media, including empty space. When the radiation is incident on an opaque surface then it will be reflected and/or absorbed by that surface, heating it up. For objects less than a few hundred kelvin in temperature, this radiation is mostly in the form of infra-red.

How effective a surface is at radiation / absorbing heat will depend on its nature and colour (a characteristic known as emissivity).

In an isolated (closed) system then heat transfer will take place between all parts of the system until thermal equilibrium is reached, i.e. all part sof the system are the same temperature.