Conduction passes thermal energy through a material by particle vibrations and free electrons. Convection carries it by the movement of a hot fluid. Radiation carries it as infrared waves that need no material at all. Exam questions ask you to name the process, then explain it with particles.
This lesson follows expansion using a particle model, because convection depends on hot fluids becoming less dense.
How does each process work?
Conduction. Particles at the hot end vibrate more strongly and pass energy to their neighbours through collisions. In metals, free electrons also drift through and carry energy much faster. Metals are good conductors. Wood, plastic, wool and air are poor conductors, also called insulators.
Convection. This happens only in liquids and gases. A heated fluid expands, so its density falls, and it rises. Cooler, denser fluid sinks to take its place. The result is a circulating current that moves energy around.
Radiation. Every object gives out infrared radiation, and hotter objects give out more. It travels as waves at the speed of light and needs no medium. Dark, matt surfaces are good absorbers and good emitters. Light, shiny surfaces are poor absorbers and poor emitters, because they reflect most radiation.
How to decide which process a question wants
- Is there a solid and contact? Think conduction.
- Is there a fluid moving because one part is hotter? Think convection.
- Is energy crossing empty space or air without contact? Think radiation.
- Several may act at once, so name each and say where it acts.
Worked example
A small electric heater is placed on the floor of a room. A student sits two metres away and feels warm.
The heater casing is hot to the touch. Name the processes involved in (a) the casing, (b) the warm feeling at two metres, (c) the warm air reaching the ceiling. (Invented scenario.)
(a) Energy passes from the hot element through the metal casing by conduction. Metal conducts well, so the outer case becomes hot.
(b) The student feels warm through radiation, because energy is carried as infrared waves through the air. The air between them is a poor conductor, and convection moves air upwards, not sideways to the student.
(c) Air next to the heater is warmed, expands and becomes less dense. The less dense air rises, and cooler, denser air moves in along the floor to replace it. This is a convection current.
The mistake to watch for
Mistaken answer: “Heat rises, so the hot air goes to the ceiling.”
“Heat” is not a substance that rises. The air rises because of its lower density.
The correction is to write that the warm air expands, becomes less dense than the surrounding air and rises. Cooler air sinks because it is denser. Always include the density step, since that is the mark-scoring idea.
Check yourself
1. Why is a metal spoon left in a hot cup of soup uncomfortable to hold but a wooden spoon is not?
Show answer
Metal is a good conductor, because its free electrons and vibrating particles pass energy along quickly to your hand. Wood is a poor conductor, so energy travels along it slowly.
2. Why is an air-conditioning unit usually fitted high on a wall?
Show answer
The unit cools the air near it. Cool air is denser and sinks, so warmer air rises to take its place and is cooled in turn. A high position sets up a convection current that circulates air through the whole room.
3. Which process carries energy from the Sun to a satellite in space?
Show answer
Radiation. Space is almost a vacuum, so conduction and convection cannot occur without particles.
Where this leads next
To see how a graph shows energy being gained or lost, move on to distinguishing temperature from thermal energy. The three processes return in explaining insulation from a supplied structure, where you decide which one each design feature reduces.
Some students get the naming right but lose marks on the particle explanation. Our teachers practise that wording with you in online one-to-one Physics tuition.