Conduction, Convection & Radiation (Edexcel IGCSE Physics)
Revision Note
Did this video help you?
Conduction
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
Examiner Tips and Tricks
Make sure you learn the key terms in this topic and are comfortable using them. You may be asked to explain how conduction, convection or radiation transfers energy.
Did this video help you?
Convection
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
Examiner Tips and Tricks
If a question refers to thermal energy transfers and a liquid or gas (that isn’t trapped) then make sure your answer mentions that convection currents will probably form!
Thermal radiation
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
Examiner Tips and Tricks
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 the question involves a vacuum (empty space), then remember to mention radiation! Because conduction and convection require particles to transfer energy!
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?