Describe conduction, convection and radiation, and explain everyday ways to control heat transfer

What this dot point is asking

SEAB wants you to describe the three ways thermal energy is transferred (conduction, convection and radiation), to know which method works in solids, liquids, gases and a vacuum, and to explain everyday examples and ways to control heat transfer. The big idea is that thermal energy can travel through materials and across empty space, by different processes.

The answer

Conduction

Conduction is the transfer of thermal energy through a material without the material itself moving. When one end of a solid is heated, its particles vibrate more and pass the energy on to neighbouring particles.

• Metals are good conductors, because they have free electrons that carry energy quickly. This is why a metal spoon in hot soup soon feels hot at the handle.
• Non-metals such as wood, plastic and air are poor conductors (good insulators), which is why pan handles are made of plastic or wood.

Conduction is the main way energy travels through solids.

Convection

Convection is the transfer of thermal energy by the movement of a heated fluid (a liquid or a gas). When a fluid is heated:

1. It expands, becomes less dense, and rises.
2. Cooler, denser fluid sinks to take its place.
3. This circulation is called a convection current.

Convection heats a room from a radiator and a pot of water on a stove. It cannot happen in a solid, because the particles cannot flow.

Radiation

Radiation (thermal radiation, a form of infrared) is the transfer of thermal energy as waves. It needs no particles, so it is the only way energy reaches us from the Sun across the vacuum of space.

• Dark, dull surfaces are good emitters and good absorbers of radiation.
• Light, shiny surfaces are poor emitters and poor absorbers; they reflect radiation well.

This is why people wear light clothes in hot weather and why shiny foil is used to reflect heat.

Controlling heat transfer

To keep something warm or cool, we reduce all three transfers. A vacuum flask is the classic example, reducing every method at once.

Examples in context

Example 1. Sea breezes. During the day the land heats faster than the sea, so air above the land rises by convection, and cooler air flows in from the sea as a sea breeze. At night the land cools faster, so the flow reverses. These breezes are large-scale convection currents driven by temperature differences.

Example 2. Loft insulation. A layer of fibre insulation in a roof traps pockets of air. Air is a poor conductor, and trapping it stops it forming convection currents, so much less thermal energy escapes from the house. This is why insulation saves heating energy in winter.

Try this

• Cue. Name the method of heat transfer that works through a vacuum and say why. [2 marks] Radiation, because it travels as waves and does not need particles to carry the energy.

• Cue. Explain why metal feels colder than wood at the same room temperature. [2 marks] Metal is a good conductor, so it conducts thermal energy away from your hand quickly, making it feel cold; wood is a poor conductor and does not.

• Cue. Explain why a hot water tank is often given a shiny outer surface. [2 marks] A shiny surface is a poor emitter of radiation, so it radiates less thermal energy away and the water stays hot longer.