Nearly all liquids including water are poor conductors (good insulators)
Gases, including air are poor conductors,e.g., wool feels warm because it traps a lot of air
A fridge has insulation material round it to keep it cold – reduces amount of heat conducted to inside from the warmer room
Takes place in material where particles can move around inside the material, i.e. liquid or gas
The heat is carried by the particles themselves moving Convection currents
Occur because an area with warm particles expands and becomes less dense than the cooler areas nearby. The warm area rises. Cooler particles fall into the space left by the warm particles and convection current is set up
Hot liquids and gases expand and rise while the cooler liquid or gas falls
1. Hot air rises
2. Goes across
3. Then down
4. And across
The sun can cause large convection currents - WINDS
During daytime the land warms up more than the sea. The warm air rises over the land and cool air falls over the sea. So we feel a sea breeze.
Rising convection currents can be uses by glider pilots to keep their planes in the air and by birds to stay aloft.
Transfer of heat directly form the source to the object by a wave, travelling as rays.
Heat radiation is also known as
All objects that are hotter than their surroundings give out heat as infra-red radiation
Heat transfer by radiation does not need particles to occur and is the only way energy can be transferred across empty space
Hotter objects emit (give out) heat
Different surfaces emit heat at different speeds
A dull black surfaces loses energy more quickly – it is a good radiator
A bright shiny or white surface is a poor radiator
Marathon runners need to keep warm at the end of races, covering in shiny blankets reduces radiation and therefore heat loss.
Emitters of heat
Bright shiny can Poor radiator
Dull black can Good Radiator
Cooler objects absorb (take in) heat
Substances absorb heat at different speeds
Dull, black surfaces absorb heat quickly
Bright, shiny surfaces absorb heat slowly