GCSE AQA Physics 1 Heat Transfer

Types of heat transfer
1. Radiation - IR radiation absorbed/emitted by all objects
  1. Object hotter than surroundings - emits more radiation than it absorbs (cools down)
    1. Object cooler than surroundings - absorbs more radiation than it emits (warms up)
  2. Depends on colour & texture of material
    1. DARK, MATT surfaces absorb/emit better than LIGHT, SHINY surfaces
      1. Solar hot water panels:
        Water pipes under black surface
        Radiation absorbed to heat pipes
        Water can be used for washing/radiators
2. Convection - in liquids/gases
  1. More energetic particles move from hotter to cooler region & take heat energy with them
    1. Convection currents are about CHANGES in DENSITY
3. Conduction - in solids
  1. Vibrating particles pass on extra kinetic energy to neighbouring particles
    1. Faster in denser solids, particles closer
      1. Metals are good conductors - free electrons transfer energy faster
Kinetic Theory
1. SOLIDS - strong forces of attraction, fixed shape, only vibrate in fixed position
2. LIQUIDS - weaker forces, particles close together, irregular, have more energy
  1. GASES - almost no forces, more energy, free to move, random directions, high speeds
Condensation/Evaporation
1. Condensation - Gas -> Liquid
  1. Gas cools, particles slow down, pulls them closer together
  2. Rate of condensation faster if...
    1. Temperature of gas is lower
      1. Temperature of surface gas touches is lower
    2. Density is higher
      1. Airflow is less
2. Evaporation - Liquid -> Gas
  1. Particles escape from liquid - can happen at lower than boiling point if going in the right direction/fast enough
    1. Fastest are most likely to evaporate = decrease in average speed (cools down)
  2. Rate of evaporation faster if...
    1. Temperature is higher
      1. Density is lower
        SA is larger
        Airflow over liquid is greater
Rate of heat transfer
1. Bigger SA - more radiation
2. Vacuum Flasks
  1. Vacuum between two walls - stops conduction/convection
    1. Silver walls - less radiation
      1. Insulating foam - less conduction
        Stopper is plastic & cork - less conduction
3. Humans & animals
  1. Cold - hairs stand up, trap insulating layer of air
    1. Less convection
      1. Small ears. small SA, minimise heat loss by radiation

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GCSE AQA Physics 1 Heat Transfer

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