Heat Transfer

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  • Created by: Clare
  • Created on: 19-11-12 16:42
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  • Heat Transfer
    • Conduction
      • Kinetic energy is transferred to cooler parts  by free electrons defusing through it
        • Metals have free electrons= good conductors
      • Poor conductor= insulator
      • Solids
      • Particles gain heat energy from a asource
        • They vibrate more (kinetic energy)
          • Collisions with asses energy on to other particles
    • Radiation
      • All objects give off INFRA RED radiation. Hotter= more infra red is given out
      • Heat travels as IR waves known as thermal radiation
        • Waves act like light waves
        • Can be REFLECTED and ABSORBED
          • Heats objects that absorb it
      • Can travel through a vacuum (don't need particles)
      • Emitt thermal radiation-MATT BLACK
        • So shiny silver keeps heat in
      • Absorb thermal radiation- MATT BLACK
        • Shiny silver reflects heat
    • Covection
      • In fluids (liquids and gas)
      • Particles are heated and gain kinetic energy
        • They expand to become less dense (increased collisions= spread out)
          • At top it is cooler and particles contract to become more dense so they fall
      • At top= cooler= less dense= falls
      • Convection currents, cycle to transfer heat
    • Evapouration
      • At the surface of a liquid particles have enough kinetic energy to change state and escape to the air around it
      • Increase evapouration: increase temperature, increase surface area
      • Sweat transfers heat away from the body
    • Prevention
      • House
        • Double glazing- creates a vacuum (no convection or conduction)
          • Cavity Wall
        • Insulation- heat trapped in layers (no convection or conduction)
          • Carpets and draft excluders
        • Shiny surface- behind radiator to reflect heat back into a room
        • Pay back time- Cost of installation/ money saved per year
      • Heat Loss
        • Radiation
          • Silver surface- doesn't emit or absorb
        • Convection/ Conduction
          • No particles no air (conv) or solid (cond)
    • Energy can be transferred not created or destroyed
      • It is either useful or wasted
      • Wasted energy is transferred to the surroundings which warm up
      • Most substances expand when heated
    • Efficiency
      • Useful/ Total x100= %
      • Rate of heat transfer depends on: material, thickness, shape and size, temperature difference
      • Inefficient- energy lost from the start to the end
    • Energy diagrams
      • Energy transfer diagrams
        • Shows stages   of useful energy transfer
          • E.g.: Car Engine= Chemical energy in fuel goes IN, Kinetic energy in fuel comes OUT
      • Sanky diagrams
        • All energy  transfers (breaks down useful and wasted)
        • Thicker arrow= more energy
    • U Values
      • Low U Value= good insulator= efficient
      • Rate of heat transfer through a part of a building (walls, roof)

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