Transfer of thermal energy · revision map · IGCSE 0625, Topic 2.3

In liquids and gases, heated fluid expands, becomes less dense, and rises.

• Cooler, denser fluid sinks to take its place.
• This circulation is a convection current.
• It carries energy through the whole fluid.
• It cannot happen in a solid or a vacuum, because the particles cannot flow.
• Explains sea breezes, heating a room, and water heated in a pan.
• Heaters are placed low and freezer shelves high to set up useful currents.
• The denser, colder fluid is pulled down by gravity, which keeps the current flowing.
• Convection moves energy through the bulk of a fluid, unlike conduction.

All objects emit infrared radiation, part of the electromagnetic spectrum. It needs no medium, so it works through a vacuum and reaches us from the Sun.

• Black, dull, rough surfaces are the best emitters and absorbers.
• Shiny, white, smooth surfaces are poor emitters and good reflectors.
• A hotter object radiates more energy each second.

A vacuum flask is designed to reduce all three ways energy is transferred.

• Vacuum gap: no particles, so no conduction or convection.
• Silvered walls reflect radiation back in.
• Stopper blocks convection and evaporation at the top.

The same ideas explain home insulation, heating systems, and the Earth's energy balance.