Conduction, convection and radiation

The handle of a metal spoon in hot soup gets hot. Warm air rises above a fire. The Sun warms the Earth across 150 million kilometres of empty space. These are the three ways heat travels: conduction, convection and radiation.

Learning objectives

By the end of this lesson you should be able to:

• describe and explain conduction, convection and radiation
• compare good and bad conductors and explain their uses
• describe experiments that demonstrate each method of heat transfer
• state the factors that affect the emission and absorption of radiation

Conduction

How it works: particles at the hot end vibrate more vigorously and pass energy to their neighbours by collisions. In metals, free electrons also carry energy quickly through the material, which is why metals are the best conductors.

Uses of good conductors: cooking pots and kettles (aluminium, steel), soldering-iron bits (copper), heat sinks in electronics.

Uses of bad conductors: wooden or plastic pot handles, oven gloves, lagging round hot-water pipes and tanks, woollen clothes and blankets (trapped air), polystyrene cool boxes, thatched roofs.

Experiment — comparing conductors: rods of copper, aluminium, iron and glass of equal length and thickness are coated with wax and fixed through holes in the side of a trough of hot water. The wax melts furthest along the best conductor (copper) in a given time.

Water is a poor conductor: ice wrapped in gauze and held at the bottom of a tube of water does not melt while the water at the top is boiled.

Convection

Convection cannot happen in solids (the particles cannot flow) or in a vacuum.

Experiment: drop a crystal of potassium permanganate into a beaker of water near one side and heat gently beneath it. Purple streaks rise above the crystal, move across the top and sink on the other side, showing the convection current.

Examples:

• a pot of soup heats throughout because heating at the bottom sets up convection currents
• the heating element of an electric kettle is at the bottom; the freezer compartment of a fridge is at the top
• smoke rising from a chimney or a cooking fire
• sea and land breezes (next lesson)
• ventilation: warm, stale air leaves through high windows, cool air enters lower down

Radiation

This is how the Sun's heat reaches the Earth. All bodies emit radiation; hotter bodies emit more.

Good and bad emitters and absorbers

Leslie's cube experiment: a metal cube with four different faces (dull black, shiny silver, white, grey) is filled with boiling water. A thermopile (or a hand) held at the same distance from each face shows that the dull black face radiates most and the shiny face least.

Absorption experiment: two metal plates, one dull black and one shiny, each have a coin stuck to the back with wax and are placed equal distances from a heater. The wax behind the black plate melts first, and its coin falls first.

Applications:

• people wear white or light-coloured clothes in the hot sun to reflect radiation
• the inside of a vacuum flask is silvered to reduce radiation
• solar water heaters and the collectors of solar cookers are painted dull black
• fuel storage tanks are painted white or silver
• cooling fins at the back of a fridge are painted black to radiate heat well

Comparing the three methods

Key points

• Conduction: particle to particle, mainly in solids; metals are the best conductors.
• Convection: fluid currents; hot fluid rises, cold fluid sinks.
• Radiation: infrared waves, no medium needed; dull black surfaces are the best emitters and absorbers, shiny ones the worst.

WASSCE examination focus

Quick check

1. Which method of heat transfer can take place through a vacuum?
2. Why is the heating element of a kettle placed at the bottom?
3. Which surface is the best absorber of radiation?

Answers: 1. Radiation. 2. Heated water at the bottom rises and cold water sinks to be heated, so convection currents heat all the water. 3. A dull black surface.