Heat Transfer

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Heat Transfer

• Conduction, Convection and Radiation

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Convection

• This is heat transfer through a solid
• Metals make good conductors
• Non-metals and gases are poor conductors
• Heat energy moves from hot area to cold
• Particles in solids are closely packed but
  vibrate
• The hotter an area, the more kinetic energy the
  particles have
• Vibrating particles pass the thermal energy onto
  neighbouring particles

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First the metal is heated
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The particles gain kinetic energy
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Vibrations spread through the material
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Convection

• This is heat transfer through liquids and gases
• It is a flowing heat transfer
• When these particles gain thermal energy they
  also gain kinetic energy
• The more they have the less dense they get
• So heat rises, and cold particles sink
• This sets up a Convection Current
• Convection currents are everywhere a saucepan
  of soup, the earths mantle, this classroom!

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A convection current in a gas
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Radiation

• All objects give out and take in Thermal
  Radiation. Also known as Infra-red Radiation.
• The hotter an object is the more Radiation it
  emits
• No particles are involved, so it can take place
  in space
• This is how we get heat from the Sun

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Some surfaces Radiate better than others

• Light coloured objects reflect heat energy, so
  they stay cooler.
• Shiny objects do too.
• Dark coloured objects absorb heat energy, they
  feel hotter.
• Dull/matt objects absorb heat too.
• So dark, matt objects will radiate heat better
  than light, shiny objects.

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Everyday Radiation

• What colour t-shirts are most comfortable to wear
  in the summer?
• What colour sand would be uncomfortable to walk
  on barefoot?
• Soyour radiators at home are they painted the
  right colour?!

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Glowing heat

• Radiation is also a glowing heat.
• When you blush or go red after exercise, you are
  radiating thermal energy

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Ways to prevent heat loss

• Insulation

13
Heat loss in the home

• Heat energy is transferred from homes by
  conduction through the walls, floor, roof and
  windows.
• It is also transferred from homes by convection.
  For example, cold air can enter the house through
  gaps in doors and windows, and convection
  currents can transfer heat energy in the loft to
  the roof tiles.
• Heat energy also leaves the house by radiation
  through the walls, roof and windows.

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Infra-red image of heat loss from a house
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Where is it going?
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Task

• For each area of heat loss
• Roof
• Floors
• Windows
• Draughts
• Walls
• Think of ways the home owner can reduce this
  energy loss.