1Convection currents
In a liquid or gas, warm fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place. This flow is a convection current.
convection current
Lesson 15.3 notes
Convection and radiation
Convection currents in liquids and gases, infrared radiation, and black and shiny surfaces.
2Why warm fluid rises
When a fluid is heated, its particles move faster and spread further apart. The same particles take up more space, so the warm fluid is less dense and rises.
dense
3Convection in a room
A radiator warms the air next to it. The warm air rises, spreads across the ceiling, cools and sinks again: a convection current carries energy round the room.
4Infrared radiation
All objects give out infrared radiation, and hotter objects give out more. It travels as a wave and needs no particles, so it can cross the vacuum of space.
infrared radiation
5Black and shiny surfaces
Matt black surfaces are good absorbers of infrared. Shiny silver surfaces are poor absorbers: they reflect most of it. A black can in the sun warms up faster.
absorbreflect
6Good emitters
Matt black surfaces are also good emitters: a black can of hot water cools faster than a shiny silver one. Shiny surfaces are poor emitters.
emit
7Three ways at once
A pan on a hob shows all three: conduction through the metal base, convection in the water, and infrared radiation you can feel from the side.
Explore: Radiation lab
Choose a surface for each can, then warm them with a lamp or let them cool. See which surface absorbs or gives out infrared best.
Common mistake: Heat doesn't rise; warm air does
It is warm air or water that rises, because it is less dense. Energy itself can travel in any direction: downwards by conduction through a solid, or sideways and downwards by radiation.
Remember
- Convection: warm fluid expands, becomes less dense and rises; cooler fluid sinks. It only happens in liquids and gases.
- Radiation: infrared waves carry energy. They need no particles, so they can cross a vacuum.
- Matt black surfaces are good absorbers and emitters of infrared; shiny silver surfaces are poor absorbers and emitters.
Going further at GCSE