HEAT TRANSFER Energy transfer (3 TYPES) Conduction Convection Radiation
Conduction Heat transfer 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
Conduction cont… Occurs when particles hitting each other and so energy is transferred. Can happen in solids, liquids and gases Happens best in solids-particles very close together
Conductors Conductors: Materials that conduct heat quickly All metals are good conductors of heat –Pans for cooking are usually made with a copper or aluminium bottom and plastic handles (plastic is a poor conductor…you won’t get burned)
Insulators/poor conductors Insulators: materials that conduct heat slowly or poorly –Glass, wood, plastic and rubber are poor conductors (good insulators)
Convection Convection: Heat transfer by movement of fluids Convection Currents: heat carried by the particles.
Convection Currents Hot liquids and gases expand and rise while the cooler liquid or gas falls (all caused by a difference in densities!) 1. Hot air rises 2. Goes across 3. Then down 4. And across
Radiation Radiation: Transfer of heat directly from the source to the object by a wave, travelling as rays. –Heat radiation is also known as Heat transfer by radiation does not need particles to occur and is the only way energy can be transferred across empty space INFRA-RED RADIATION
Emission of Radiation (canned heat lab 2) 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 –Survival kits have shiny heat blankets in them, covering in a shiny blanket reduces radiation and therefore heat loss.
Emitters of heat Bright shiny can Poor radiator Dull black can Good Radiator
Absorption of Heat (canned heat lab 1) 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. Gas/Oil storage tanks are sprayed silver to reflect sun’s rays
Absorbers Shiny, bright can Poor absorber Dull black can Good absorber
Evaporation is Cooling!! Sweating is cooling… dogs have no sweat glands, so they pant
Chapter 10 bonus questions Why is boiling a cooling process??