Conduction, Convection & Radiation (WJEC GCSE Physics: Combined Science)

• Thermal energy is transferred by the processes of:
  • Conduction
  • Convection
  • Radiation

Conduction, Convection & Radiation in a Mug of Coffee

Energy is transferred away from the coffee, to the surroundings, by the processes of conduction, convection and radiation

• For a mug of hot coffee:
  • Thermal 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 emit infrared radiation, but the hotter an object is, the more IR radiation it emits 

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

 

• Objects will continue to lose thermal energy 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

   

Higher Tier

• Conduction is the main method of energy transfer by heating in solids
  • Metals are extremely good thermal conductors

• Non-metals are poor thermal conductors whilst liquids and gases are extremely poor
  • Poor conductors are called insulators 

Conduction of Thermal Energy for a Person Standing on a Tiled Floor

Thermal energy is transferred  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 a Metal

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

Free Electrons in a Metal

Delocalised electrons in metals speed up thermal conduction

Higher Tier

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

• 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

Convection Current in Air

A convection current caused by heating from the fire