Consequences of Thermal Energy Transfer (CIE IGCSE Physics)

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

 

Thermal energy is transferred from the hot coffee to the mug and to the cold hands

 

• The mechanism by which the thermal energy is transferred is by either conduction, convection or radiation
  • In this case the diagram focuses on conduction

 

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 energy is transferred from the hot coffee to the air by convection currents rising from the surface

 

• The mechanism by which the thermal energy is transferred is by either conduction, convection or radiation
  • In this case the diagram focuses on convection

 

EXTENDED

• In real situations there is very rarely only one form of energy transfer
  • Usually all three happen at once

• In the diagram below a more complex - and more 'real' - version of the situation above is shown
• Thermal energy is transferred from hotter areas (the tea) to cooler areas (the cup, hands and air) by the processes of:
  • Conduction; by direct contact between the tea and the solid sides of the cup and also by direct contact from the cup to the surface it is sitting on 
  • Convection; from the surface of the coffee to the air directly above it
  • Radiation; from the sides of the hot cup in all directions to the surrounding air

• Objects will always lose heat until they are in thermal equilibrium (same temperature) with their surroundings
  • For example, a mug of hot tea will cool down until it reaches room temperature
  • Eventually the room, tea and cup will all be at the same temperature

   

     

Solar Panels & Heat Transfer

 

EXTENDED

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

Diagram showing a section through a solar panel

 

Step 1: Describe how thermal radiation arrives at the water:

• • 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

   

Step 2: Describe how the heat is trapped inside the solar panel (making it more efficient):

• • 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
  • This prevents heat loss by conduction from the front of the panel
  • It also prevents heat loss by convection (due to the air being trapped)