09-11-2012, 06:15 PM
HEAT ENERGY
HEAT ENERGY.ppt (Size: 173 KB / Downloads: 32)
Heat and Temperature
The temperature of an object tells us how HOT it is
Measured in degrees Celsius - °C
It is NOT the same as heat energy although the two quantities are related. e.g. a beaker of water at 60 °C is hotter than a bath of water at 40 °C BUT the bath contains more joules of heat energy
Conduction
Heat is transferred through a material by being passed from one particle to the next
Particles at the warm end move faster and this then causes the next particles to move faster and so on.
In this way heat in an object travels from:the HOT end the cold end
Occurs by the particles hitting each other and so energy is transferred.
Can happen in solids, liquids and gases,
Happens best in solids-particles very close together
Conduction does not occur very quickly in liquids or gases
Insulators/poor conductors
Materials that conduct heat slowly or poorly are called insulators
Glass, wood, plastic and rubber are poor conductors (good insulators)
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
Radiation
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
Emitters
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.
Absorbers
Cooler objects absorb (take in) heat
Substances absorb heat at different speeds
Dull, black surfaces absorb heat quickly
Bright, shiny surfaces absorb heat slowly
In hot countries, people wear bright white clothes and paint their houses white to reduce absorption of energy from the sun.
Petrol storage tanks sprayed silver to reflect sun’s rays