Transfer of thermal Energy (O-Levels)

What these notes cover

These notes cover the transfer of thermal energy for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): temperature and thermal equilibrium, conduction and why metals are good conductors, convection currents, infrared radiation and how surface colour and texture affect absorption and emission, and everyday applications such as the vacuum flask, double glazing and insulating a home. They include experiments and past-paper questions.

Key formulas and definitions

• Thermal energy is transferred from a region of higher temperature to a region of lower temperature until both reach the same temperature (thermal equilibrium).
• Conduction is the transfer of thermal energy through a material without the material itself moving: vibrating particles pass energy to their neighbours. Metals are the best conductors because their free (delocalised) electrons also carry energy quickly through the metal.
• Conduction is fastest in solids, slower in liquids and poorest in gases. Poor conductors (insulators) such as plastic, wood and trapped air are used for saucepan handles and insulation.
• Convection happens only in fluids (liquids and gases). Heated fluid expands, becomes less dense and rises, while cooler, denser fluid sinks, setting up a convection current.
• Radiation is the transfer of thermal energy by infrared electromagnetic waves. It needs no medium, so it can travel through a vacuum.
• Dull black surfaces are good absorbers and good emitters of infrared radiation. Shiny white or silvered surfaces are poor absorbers and poor emitters, because they reflect infrared.
• Vacuum flask: the stopper reduces convection and conduction, the vacuum between the double walls stops conduction and convection, and the silvered surfaces reduce radiation.
• Heat loss from buildings is reduced by carpets, curtains, loft insulation and double glazing, which traps a layer of air (a poor conductor) between two panes of glass.