GCSE AQA Physics 1 Heat Transfer

GCSE AQA Physics 1 Heat Transfer

1. Types of heat transfer

   • Radiation - IR radiation absorbed/emitted by all objects
     • Object hotter than surroundings - emits more radiation than it absorbs (cools down)
       • Object cooler than surroundings - absorbs more radiation than it emits (warms up)
     • Depends on colour & texture of material
       • DARK, MATT surfaces absorb/emit better than LIGHT, SHINY surfaces
         • Solar hot water panels:
           • Water pipes under black surface
             • Radiation absorbed to heat pipes
               • Water can be used for washing/radiators
   • Convection - in liquids/gases
     • More energetic particles move from hotter to cooler region & take heat energy with them
       • Convection currents are about CHANGES in DENSITY
   • Conduction - in solids
     • Vibrating particles pass on extra kinetic energy to neighbouring particles
       • Faster in denser solids, particles closer
         • Metals are good conductors - free electrons transfer energy faster

2. Kinetic Theory

   • SOLIDS - strong forces of attraction, fixed shape, only vibrate in fixed position
   • LIQUIDS - weaker forces, particles close together, irregular, have more energy
     • GASES - almost no forces, more energy, free to move, random directions, high speeds

3. Condensation/Evaporation

   • Condensation - Gas -> Liquid
     • Gas cools, particles slow down, pulls them closer together
     • Rate of condensation faster if...
       • Temperature of gas is lower
         • Temperature of surface gas touches is lower
       • Density is higher
         • Airflow is less
   • Evaporation - Liquid -> Gas
     • Particles escape from liquid - can happen at lower than boiling point if going in the right direction/fast enough
       • Fastest are most likely to evaporate = decrease in average speed (cools down)
     • Rate of evaporation faster if...
       • Temperature is higher
         • Density is lower
           • SA is larger
             • Airflow over liquid is greater

4. Rate of heat transfer

   • Bigger SA - more radiation
   • Vacuum Flasks
     • Vacuum between two walls - stops conduction/convection
       • Silver walls - less radiation
         • Insulating foam - less conduction
           • Stopper is plastic & cork - less conduction
   • Humans & animals
     • Cold - hairs stand up, trap insulating layer of air
       • Less convection
         • Small ears. small SA, minimise heat loss by radiation