Physics Chapter 1.1 - Thermal Radiation
Thermal or heat radiation is a transfer of energy by infra-red waves, these are part of the electromagnetic spectrum.
- All objects emit heat radiation.
- The hotter the object is the more heat radiation it emits.
- Heat radiation can travel through a vaccum, like space. This is how heat travels from the Sun to us.
Remeber: transfer of heat energy is by infra-red radiation and it donesn't involve particals.
Physics Chapter 1.2 - Surfaces and Radiation
Dark matt surfaces are good aborbers of radiation. An object painted dull black and left in the sun will become hotter than the same object painted shiny white.
Dark, matt surfaces are also good emitters of radiation. So an object that is painted dull black will lose heat and cool down quicker that the same object would in white.
Don't be fooled by most radiators that are painted white, the best radiators of heat are dark, matt surfaces.
An object that is warmer than its surrondings will lose heat energy and cool down. Likewise a object colder than its surroundings will gain heat energy and warm up.
Physics Chapter 1.1 - Conduction
Conduction mainly occurs in solids. Most liquids and gases are poor gases.
- If one end of a solid is heated, the particals at that end gain kinetic energy and vibrate more. This energy is passed to neighbouring particals and in this way the heat is transferred though the solid.
This process occurs in metals too:
- In addition, when metals are heated their free electrons gain kinetic energy and move though the metal transferring energy by colliding with other particals. Hence all metals transferring are good heat conductors.
Poor conductors are called insulators. Fibre glass is a good insulator as it contains pockets of trapped air.
Convection occurs in liquids and gases, fluids.
When a fluid is heated it expands and becomes less dense and rises. This warmer fluid is replaced by cooler, denser fluid. The resulting convection current transfers heat through a fluid.
Convection currents can be on a very small scale, such as heating water in a beaker or on a very large scale such as heating the air above land and sea. Convections currents are reponsible for onshore and offshore breezes.
Convection doesn't happen in solids and the type of surface makes no difference to the amount of conduction or convection from an object - it only affects the radiated energy.
Heat Transfer By Design
In many situations we want to minimise heat loss. We do this by reducing the losses due to conduction, convection and radiation.
- We can reduce heat loss by conduction by using insulators, eg trapping a thin layer of air.
- Heat loss by convection can be reduced by preventing convection currents being set up, eg trapping small pockets of air.
- We can reduce heat loss by radiation by using light, shiny surfaces, which are poor emitters.
Sometimes we need to maximise heat loss to keep things cool. To do this we may use things that are:
- Good conductors.
- Painted matt black.
- Have the air flow around them maximised.
Heat Transfer by Design Examples
- A radiator has a large surface area so it looses heat easily.
- Small objects lose heat more easily than larger objects.
- Heat loss from buildings can be reduced by using, aluminium foil behind radiators, cavity wall insulation, double grazing and loft insulation.
Loft insulation - such as fibergalss reduces heat loss by having pockets of air in the fibre glass to stop conduction.
Cavity wall insulation - reduces heat loss through the walls.
Aluminium foil behind a radiator helps reflect the heat off the wall and back into the room.
Double glazed windows - the two windows have a vaccum inbetween so cuts down the heat conduction and convection cannot occur in a vaccum.