Thermal Energy Transfer



Conduction occurs mainly in solids. Most liquids and all gases are poor conductors. If one end of a solid object is heated, the particles at that end gain kinetic energy and vibrate more. This energy is passed on through the particles and transferred up the object.

This process occurs in metals. When metals are heated their free electrons gain kinetic energy and move through the metal, transferring energy by colliding with other particles. All metals are good conductors. Poor conductors are known as insulators. Materials such as wool and fibreglass are good insulators because they contain trapped air.

The U-Value tells you how much heat energy will pass through the material.

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Convection and Radiation

Convection occurs in liquids and gases.

When a fluid is heated it expands. It becomes less dense and rises. The warm fluid becomes less dense and rises. It is then replaced by cooler, more dense fluid. The resulting convection current transfers energy throughout the fluid.

Convection could happen on a small scale e.g. a boiling pot of water. Or could happen on a large scale e.g. heating the air above land and sea.

Dark and matt surfaces are good absorbers of infrared radiation. An object painted dull black and left in the sun will be hotter than a shiny white surface.

Dark and matt surfaces are also good emitters of infrared radiation. So the dark surface will cool down quicker than the white surface. 

Light, shiny surfaces are good reflectors of infrared radiation.

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Specific Heat Capacity

The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1 kilogram of the substance by 1 degree celcius.

Different substances have different specific heat capacities. The greater the specific heat capacity, the more energy required for each degree temperature change.

For example the specific heat capacity of aluminimum is 900J/kg C and of copper is 490J/kg C. If we wanted to raise the temperature of 1kg of aluminimum, we would need to transfer almost twice the energy needed to raise the temperature of 1kg of copper by the same amount.

The greater the mass of a substance the more energy required for each degree temperature change.

The equation for specific heat capacity is:

E = m x c x (theta)

c is specific heat capacity, (theta) is temp change, E is energy transfered and m is mass.

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Vacuum Flask

A vacuum flask, or thermos, does not allow heat transfer by any of the three ways that heat can travel. The silver coating on the inner bottle prevents heat transfer by radiation, and the vacuum between its double wall prevents heat movingby convection. The thinness of the glass walls stops heat entering or leaving the flask by conduction. The case surrounding the flask provides additional insulation .                                 (                                                 

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U- Values

Most people want to reduce their fuels bills by minimising the rate of energy transfer out of their homes. This can be done by fitting:

  • Fibreglass loft insulation to reduce energy transfer by conduction.
  • Cavity wall insulation that traps air in small pockets to reduce energy transfer by convection.
  • Double glazing to reduce energy transfer by conduction through windows.
  • draught proofing to reduce energy transfer by covection.
  • Aluminimum foil behind radiators to reflect infrared radiation back into the room

The u-value of a material tells us how much energy per second passes through it. For good insulation, we choose materials with low u-values. 

Solar heating panels do not use fuel to heat water but they are expensive to buy and install.

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Evaporation and Condensation

Evaporation is when a liquid turns into a gas. It occurs because the most energetic particles escape from the liquids surfec and enter the air.

The rate of evapouration is increased by:

  • increasing the surface area of the liquid.
  • increasing the temperature of the liquid.
  • creating a draught of air across the liquids surface.

Condensation is when a gas turns into a liquid. This often takes place on cold surfaces such a windows and mirrors.

The rate of condensation is increased by:

  • increasing the surface area.
  • reducing the surafce temperature.
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