Thermal Energy

Thermal conduction is the transfer of thermal energy through a substance without the substance itself moving

• In solids, when one end is heated the vibrational energy of the molecules increases

• This energy is transferred from molecule to molecule along the material

• This is a slow process in most materials 

• But the process in metals is much faster because some of the electrons are free to move and can carry energy through the metal much more quickly

• Use a vacuum: Conduction needs matter; used in vacuum flasks, some types of double glazing

• Use air: Air is a good insulating material. Many materials like wool, feathers trap air so it can't circulate. This works as a very poor conductor of heat. Houses use fibre glass insulation (trap air) and cavity walls are sometimes filled with foam 

• Use water: Wetsuits trap a layer of water aroujnd the body because water is a poor conductor

Thermal convection is the transfer of heat energy through fluids (liquids and gases) by the upward movement of warmer, less dense, regions in the fluid

When a gas or liquid is heated it expands. The expansion means that the density of the fluid gets less so the warmer fluid floats upwards as it is pushed out of the way by colder, and therefore denser, fluid

E.g. Explain why the heating element in a kettle is placed as close to the bottom as possible

Water is a poor thermal condutor. If the heater was at the top, only that layer of water would heat quickly,and, being less dense, would float on top of the colder, denser water at the bottom. 

Placing the heater at the bottom means that as the water in contact with it warms up, the water expands and becomes less dense and it is pushed upwards by colder, denser water which sinks lower from above.This results in a circulating current which heats up all of the water much more quickly.

• Use a Vacuum: Convection needs gases or liquids; used in vacuum flasks, some types of double glazing

• Use Trapped Gas or Liquid: This restricts circulation, which is necessary for convection to occur. The size of the gap between the sheets of glass is a compromise. A narrow gap makes the effect of convection smaller, but it allows a greater amount of heat transfer by conduction

Thermal radiation is the transfer of heat energy in the form of infrared (IR) waves. It is the only method of heat transfer through a vacuum

• IR waves are part of the EM spectrum

• They travel at the speed of light in space and are almost as fast through air

• Radient heaters are often placed high up in a room because heat can travel in any direction

• The heat from radiant heaters is felt almost instantly when the heating element starts to glow

• Use shiny surfaces: Very shiny surfaces reflect infrared radiation well. Fire fighters wear shiny suits to stop heat radiation getting to their bodies. Shiny surfaces are also poor radiators of heat. Space blankets for example retain heat because they have a shiny inner surface to reflects heat back into person and a shiny outer layer which is a poor radiator of infrared