Heat transfer: conduction, convection and radiationIB MYP Physics: Revision notes
Section 1
Conduction
Conduction is the transfer of energy through a substance by vibrating particles passing on energy to their neighbours, without the substance itself moving. It is the main way energy travels through solids.
When one end of a solid is heated, the particles there vibrate more and collide with their neighbours, which then vibrate more, and so on along the solid.
In metals there is a second way: free electrons can move through the whole metal and carry energy quickly, so metals are very good conductors. Particles in liquids and gases are far apart, so conduction in them is very slow.
In conduction the particles do not travel from the hot end to the cold end. They only vibrate and pass on energy.
Section 2
Conductors and insulators
A conductor transfers energy well, for example metals such as copper and aluminium. An insulator transfers energy poorly, for example plastic, wood, glass, and trapped air. Non-metals have no free electrons, so energy can only be passed on by vibrating particles.
We choose conductors for saucepan bases and radiators, and insulators for handles, oven gloves and house walls.
Section 3
Convection
Convection is the transfer of energy by the movement of a fluid (a liquid or gas). It cannot happen in solids.
When part of a fluid is heated, it expands and becomes less dense, so it rises. Cooler, denser fluid sinks to take its place. This continuous circulation is a convection current, and it carries energy through the fluid.
Examples: warm air rising from a radiator and circulating around a room, and water circulating in a kettle or a pan on the hob.
Do not say 'heat rises'. Say that the warmed air or water expands, becomes less dense and so rises.
Section 4
Infrared radiation
Radiation is the transfer of energy by infrared waves, which are part of the electromagnetic spectrum. It needs no medium, so it can travel through a vacuum. This is how the Sun's energy reaches the Earth.
All objects emit infrared radiation, and the hotter the object, the more it emits.
Surface matters:
- Dull black surfaces are the best emitters and the best absorbers of infrared radiation.
- Shiny, light surfaces are the worst emitters and the worst absorbers: they reflect infrared radiation.
Dull black = best at emitting AND absorbing. Shiny = best at reflecting.
Section 5
Factors affecting the rate of transfer
The rate at which a hot object loses energy depends on:
- temperature difference: the bigger the difference between the object and its surroundings, the faster the energy transfer
- surface area: a larger surface area loses energy faster
- material: conductors transfer energy faster than insulators
- thickness: thicker insulation slows the transfer
- surface type: dull black surfaces radiate and absorb faster than shiny ones
Section 6
Everyday applications
Vacuum flask: the vacuum between the double walls stops conduction and convection, shiny surfaces reduce radiation, and the stopper is an insulator.
House insulation: loft insulation and cavity walls trap air, which is a poor conductor and cannot circulate, so less energy is lost.
Radiators: placed low so the air they warm sets up a convection current around the room. Painting them dull black helps radiation, but most of their heating is by convection.
Solar panels: a dull black surface absorbs infrared radiation from the Sun well, which heats water in the pipes behind it.
In a 'how does it work' question, name each of conduction, convection and radiation and say how the design reduces (or uses) each one.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Heat transfer: conduction, convection and radiation
- A chef in Istanbul cooks soup on a hob using a stainless steel pan with a plastic handle. The base of the pan heats up very quickly, but the plastic handle stays cool enough to hold.Explain why the metal base heats up quickly but the plastic handle stays cool.2 marks
- A radiator is fixed to one wall of a classroom in Oslo, close to the floor. When it is switched on, the whole room soon warms up, even the corner furthest from the radiator.Explain how the energy from the radiator spreads through the whole room.2 marks
- A student investigates how the surface of a can affects how quickly it cools. She uses three identical metal cans with lids: one painted dull black, one painted white and one with a shiny silver surface. She fills each can with 200 cm³ of water at 80 °C and leaves them in the same room at 20 °C. After 10 minutes the water temperatures are 55 °C (dull black can), 63 °C (white can) and 68 °C (shiny silver can). Her hypothesis was that dull black surfaces lose energy fastest.Identify the independent variable and the dependent variable in the investigation, and state one control variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).