1.2 Thermal energy can be transferred by conduction - vibration or collision of particles

• Conduction involves heat transfer by particles vibrating against each other in a solid or collisions between particles in a gas or liquid. Conduction is the main mode of heat transfer in a solid.

• Particle theory: In a solid the hotter particles vibrate more strongly, having a greater kinetic energy (KE) store and bang into neighbouring cooler lower KE particles and so transfer KE to them, so heat energy is transferred from a higher temperature region to a cooler region in any solid material. This kinetic energy of vibration is referred to as thermal energy store. In other words the higher the temperature of a material the more 'thermal energy' it contains.

  • There is no effective heat transfer by convection or radiation within a solid material.

• The more dense a material, generally speaking the better the conductor - the faster heat is conducted.

  • In materials where the particles are further apart the rate of heat transfer (rate of conduction) is reduced eg gases like air are much poorer conductors than solids like stone.

• Most non-metallic materials are poor conductors (good heat insulators) eg water, fat (in animals), wood, stone, concrete, plastics - particularly foams where poorly heat conducting gases are trapped giving even better insulation that the bulk solid plastic - and cheaper by using less material.

• Metals are particularly good conductors because of free moving electrons - a different heat transfer mechanism to that described here, which applies to all solids. Because the electrons are free to move in the solid metal, they can rapidly transfer kinetic energy by particle movement. The 'hot' electrons in the higher temperature region collide with neighbouring cooler electrons and so rapidly transferring heat energy (KE) - much faster than vibrating atoms in non-metals which are held in fixed positions.

• Incidentally if you pick up a cold poor conductor like a stone and then pick up an equally cold metal object at the same cool temperature, the metal object feels colder (but it isn't) because it conducts heat from your fingers faster than the stone!

• Water in an electric kettle is a two part system, but even though the kettle contains the water, it is NOT a closed system because electrical energy is coming in, and being changed to thermal energy by the electrical resistance of the heating element.

  • The heat energy conducts through the heating element and into the water whose temperature rises as its thermal energy store increases. However, the heat is then transferred to all the water by convection currents coming from the hottest least dense water by the heating element which then rises and circulates around (see convection below).

  • BUT, you cannot stop some thermal energy escaping from the kettle by conduction through the case, convection through top opening and radiation from the kettle's surface. These heat losses will be minimised by the design of the kettle e.g.

    • Plastic is a poor conductor.

    • A silver surface is a poor radiator.

• In a toasted sandwich maker the heat is transferred to the bread by conduction. In the toaster the electrical energy is converted into heat and the thermal energy stores of the toaster and the sandwich are increased to effect the cooking.

• A measure of good/bad a material is at conducting heat is called its thermal conductivity.

  • A good conductor has a high thermal conductivity and a more insulating material has a low thermal conductivity.

  • For more details on this see More on methods of reducing heat transfer