Conduction, Convection & Radiation (Edexcel IGCSE Physics)

Conduction, convection and radiation

• Energy is transferred by heating and radiation via the processes of:
  • Conduction
  • Convection
  • Radiation

Conduction, conductors and insulators

• Conduction is the main method of energy transfer by heating in solids
  • Metals are extremely good thermal conductors
  • A material is a good conductor if it transfers energy by heating 

• Non-metals are poor thermal conductors whilst liquids and gases are extremely poor thermal conductors
  • Poor conductors are called insulators 
  • A material is a good insulator if it does not transfer energy by heating 
  • Insulators are used to prevent energy transfer by conduction

 

Example of a conductor and an insulator

Energy is transferred by heating from the hotter foot to the cooler tiles by conduction

 

• Materials containing small pockets of trapped air are especially good at insulating because air is a gas and hence a poor conductor
  • The air is trapped, so it cannot move and form a convection current, therefore energy transfer by conduction occurs, but it happens very slowly since air is a gas

  

 

• When a substance is heated, the atoms start to move around (vibrate) more
  • As they do so they bump into each other, transferring energy from atom to atom

Conduction in solids

Conduction: the atoms in a solid vibrate and bump into each other

• Metals are especially good at conducting heat as the delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material and hence transfer heat better

Conduction in metals

Delocalised electrons in metals speed up thermal conduction in metals

• Convection is the main way that thermal energy is transferred through liquids and gases
  • Convection cannot occur in solids

Convection currents

• When a fluid (a liquid or a gas) is heated:
  • The molecules push each other apart, making the fluid expand
  • This makes the hot fluid less dense than the surroundings
  • The hot fluid rises, and the cooler (surrounding) fluid moves in to take its place
  • Eventually, the hot fluid cools, contracts and sinks back down again
  • The resulting motion is called a convection current

A convection current caused by heating from the fire

• All bodies (objects), no matter what temperature, emit infrared radiation
• The hotter object, the more infrared radiation it radiates in a given time

The infrared emitted from a hot object can be detected using a special camera

• The colour of an object affects how well it emits and absorbs thermal radiation
  • Black objects are the best at emitting and absorbing thermal radiation
  • Shiny objects are the worst at emitting and absorbing thermal radiation

• The table below summarises the absorbing and emitting abilities of different colours:

Table of the effect of coloured surfaces on absorbing and emitting ability

Colour	Absorbing	Emitting
Black	Good absorber	Good emitter
Dull/dark	Reasonable absorber	Reasonable emitter
White	Poor absorber	Poor emitter
Shiny	Very poor absorber (it reflects)	Very poor emitter

Conduction, convection and radiation in a mug of coffee

• For a mug of hot coffee:
  • Energy is transferred by radiation from the surface to the mug to the surroundings
    • Due to the infrared radiation being emitted from its surface
    • All objects (above 0 K) emit infrared radiation, but the hotter an object is, the more IR radiation it emits 

• • Energy is transferred by heating from the surface of the coffee to the surroundings
    • The most energetic particles of the coffee evaporate setting up a convection current

• • Energy is transferred by heating from the bottom of the mug to any surface it is in contact with, such as a table
    • This energy transfer happens by conduction

Energy is transferred by conduction, convection and radiation in a mug of hot coffee

 

• Objects will continue to lose heat until they reach thermal equilibrium (equal temperature) with their surroundings
  • For example, a mug of hot coffee will cool down until it reaches room temperature