# Heat transfer

A2

Heat is transferred via conduction, convection and radiation.

Conduction occurs when particles in a material vibrate due to their heat energy, and collide with other particles, making them move and thereby passing energy to them. This is possible in solids and gases.

Convection occurs in fluids. As fluids heat up the particles move more and spread out more, so the fluid becomes less dense. This means it rises above the more dense fluid, and the colder fluid sinks to the heat source (represented by an arrow) so it too can be heated up.

Radiation is the transfer of heat by infra red waves. This radiation carries the heat energy to the surroundings, even when there are no particles (ie in a vacuum).

University

Convection becomes very complicated because it’s hard to predict the flow of fluids, as the flow becomes turbulent:

Radiation, H, is proportional to temperature4 (T4):

A is surface area, e is the emissivity of the material, σ is the Stefan-Boltzmann constant, 5.67 x 108 Wm-2K-4. H is the rate of heat transfer (heat current) (Q is heat energy):

For conduction, H has a different equation:

k is the thermal conductivity of the material, TH is the hotter temperature and TC is the colder temperature, and L and x are lengths along the material.

L / k is called the thermal resistance, R, so the equation can also be written: