Energy transfer by heating

Energy transfer by conduction

Energy transfer through a material by conduction depends on its thermal conductivity.     Good insulators need to have materials with low thermal conductivity, so energy transfer through them is as low as possible.

Energy transfer per second through a layer of insulating material depends on:

  • temperature difference across material
  • thickness of material
  • thermal conductivity of the material

To reduce energy transfer as much as possible:

  • thermal conductivity of insulating material should be as low as possible
  • thickness of insulating layer should be as thick as possible
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Required practical

Use different materials to insulate identical cans of hot water.

When choosing the materials, consider which properties will make the materials good thermal insulators.

The volume and temperature of the water at the start should be the same.

Use the thermometer to measure the water temperature after a fixed time.

The rods need to be the same length and width for a fair test. Each rod is coated with a thin layer of wax near one end and the uncoated parts are heated together.

The wax melts fastest on the rod conducting energy the best

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Infrared radiation

Infrared radiation is part of the electromagnetic spectrum

A perfect black body is an object absorbing all the radiation hitting it. No radiation is reflected or transmitted. They are good emitters and absorbers. Radiation emitted by a perfect black body is called black body radiation.

The hotter the object is, the more infrared radiation is emitted

When an object absorbs radiation faster than it emits it, the temperature increases.

If there was no atmosphere, the surface would not be receiving any radiation from the Sun - it would be emitting radiation into space.

Greenhouse gases absorb longer wavelength infrared from the Earth and prevent its escape into space. They absorb radiation and emit it back to the surface.

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Specific heat capacity

The specific heat capacity of a substance is the energy needed to raise the temperature of 1kg of a substance by 1 degrees Celsius.

energy transferred = mass x temperature change x specific heat capacity

Energy transfer keeps things warm. If an object has a high specific heat capacity, it stores lots of energy.

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Required practical

Heat a metal block of known mass. Use a thermomter and a top-pan balance.

Use the joulemeter to measure energy supplied to the block and the thermometer to measure temperature rise.

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Heating and insulating buildings

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

U-values tell us how much energy per second passes through materials.

ways to reduce the rate of energy transfer4

  • alluminium foil
  • loft insulation
  • cavity wall insulation
  • double glazed windows
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