Reducing Energy Loss (Edexcel IGCSE Physics)
Revision Note
Reducing energy loss
There are many situations where energy transfers are actually unwanted:
Keeping a house warm
Keeping a drink hot or cold
Dressing to stay warm in cold weather
Insulated mugs are used to maintain the temperature of hot or cold drinks
When an appliance is used for heating something (a kettle, a heater, a tumble dryer, a central heating system, etc.), the appliance requires a lot of energy
It can become expensive for a household to run such appliances
The production of electricity using fossil fuels produces greenhouse gases, which contribute to global warming
The combustion of (methane) gas produces greenhouse gases, which contribute to global warming
Therefore, it is often useful to explore ways of reducing unwanted energy transfers
Energy that is dissipated to the surroundings is often the main source of wasted energy transfers
If these unwanted energy transfers can be prevented, or reduced, the useful energy transfers can be made more efficient
Reducing conduction
Energy transfers by heating due to conduction are one of the most common sources of dissipated energy
To reduce energy transfers by conduction, materials with a low thermal conductivity should be used
Materials with low thermal conduction are called insulators
Reducing convection
Convection can also be a source of dissipated energy
To reduce energy transfers by convection, convection currents must be prevented from forming
Therefore, the fluid (liquid or gas) that forms the currents must be prevented from moving
Insulation
Insulation reduces energy transfers from both conduction and convection
The effectiveness of an insulator is dependent upon:
The thermal conductivity of the material
The lower the conductivity, the less energy is transferred
The density of the material
The more dense the insulator, the more conduction can occur
In a denser material, the particles are closer together so they can transfer energy to one another more easily
The thickness of the material
The thicker the material, the better it will insulate
Insulating the loft of a house lowers its rate of cooling, meaning less energy is transferred to the surroundings (outside)
The insulation is often made from fibreglass (or glass fibre)
This is a reinforced plastic material composed of woven material with glass fibres laid across and held together
The air trapped between the fibres makes it a good insulator
The gaps or cavities between external walls are often filled with insulation
This is called cavity wall insulation
This is often done by drilling a hole through the external wall to reach the cavity and filling it with a special type of foam which is made from blown mineral fibre filled with gas
This lowers the conduction of heat through the walls from the inside to the outside
Cavity insulation
Less energy is transferred by conduction and convection if the cavity is insulated
Examiner Tips and Tricks
A common mistake when explaining how an insulator keeps something warm is to state something along the lines of “The object warms up the insulator which then warms the object up”.
Avoid giving this kind of answer!
The real explanation is:
The insulator contains trapped air, which is a poor thermal conductor
Trapping the air also prevents it from transferring energy by convection
This reduces the rate of energy transfer from the object, meaning that it will stay warmer for longer
Other things to watch out for:
Heat does not rise (only hot gases or liquids rise)
Shiny things do not reflect heat (they reflect thermal radiation)
Black things do not absorb heat (they absorb thermal radiation)
And remember, a good answer will often include references to more than one method of thermal energy transfer.
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