CIE IGCSE Physics

Revision Notes

2.3.4 Consequences of Energy Transfer

Conduction

  • The main means of thermal energy transfer in solids
  • When heated, atoms vibrate more, knocking into each other and transferring energy from atom to atom as a result
  • Metals are excellent conductors; Non-metals are poor; Liquids and gases are very poor
  • If a question mentions metals, the answer will probably have something to do with conduction
  • Trapped air is a very good insulator of heat. Air is a gas and so is a poor conductor. Trapping it prevents it from circulating and forming a convection current

 

Convection

  • The means of thermal energy transfer in liquids and gases
  • When heated, a gas will expand and become less dense. This causes it to rise (a convection current). Cooler (denser) gas falls, replacing the hot gas
  • If a question refers to a liquid or gas (that isn’t trapped) then convection currents will probably form
  • Heat sources placed at the bottom of things will generally create convection currents. Likewise, cooling units placed high up will cool any rising air, causing it to sink again

 

Thermal Radiation

  • The only way in which heat can travel through a vacuum
  • Thermal radiation is heat transfer by infrared (part of the electromagnetic spectrum)
  • Black objects are good at emitting and absorbing thermal radiation; shiny objects are poor at emitting and absorbing it
  • If a question refers to the colour of something (black, white or shiny) then the answer will probably have something to do with thermal radiation
  • If a piece of apparatus contains a vacuum then radiation will be the only way heat can travel through that part of the apparatus

 

Consequences Example Question

In many hot countries it is common for houses to produce hot water using solar panels

Solar panel worked example, IGCSE & GCSE Physics revision notes

Diagram showing a section through a solar panel

Explain the features of the solar panel that help it heat the water efficiently

Answer:

  • The thermal radiation (infrared) is able to pass through the glass sheet
  • The black metal backing sheet absorbs the thermal radiation (sunlight)
  • Being metal (an excellent conductor) it then conducts it into the copper pipes
  • The copper pipes (also metal) then conduct the heat into the water
  • The insulated material reduces the conduction of heat through the back of the panel, decreasing heat loss
  • The glass also traps air which is a good insulator, preventing heat loss due to conduction from the front of the panel and preventing heat loss by convection (due to the air being trapped)

 

Exam Tip

A common mistake made by candidates 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 conductor of heat
  • Trapping the air also prevents it from transferring heat by convection
  • This reduces the rate of heat loss 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.

 

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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