Energy and Heat · Grade 8

Why does a metal spoon in soup get hot while a wooden one does not?

Because of conduction. In a metal, free electrons carry the particle motion quickly along the spoon, so the heat reaches your fingers within seconds. Wood has no such carriers, the motion passes slowly from particle to particle, and the handle stays cool. It is a difference in thermal conductivity, not in the temperature of the soup.

Learning objectives

Conduction: direct contact

In conduction, heat travels through the material itself, from one particle to its neighbour, without the material going anywhere. A fast-vibrating particle jostles the one beside it, and the motion spreads along the object.

Metals contain free electrons that move throughout the whole body, and they carry energy very quickly. That makes metal an excellent thermal conductor — and it is also why a metal handle feels cold to the touch: it is drawing heat out of your hand fast.

Wood, plastic and glass have no such carriers, and the transfer is slow. Materials like these are called insulators, and a wooden spoon in a pot is the most useful demonstration of it in any kitchen.

Convection: the material itself moves

In convection the hot material moves and carries the energy with it. This can only happen in liquids and gases, because a solid cannot flow.

Air or water that is heated expands slightly, becomes less dense, and rises. Cooler material sinks to take its place, warms in turn, and a steady circular flow forms — a convection current.

This is why the air near the ceiling is warmer than the air near the floor, why an air conditioner is mounted high and a heater low, and why a coast has a sea breeze by day and a land breeze by night.

Radiation: it crosses a vacuum too

Radiation is energy carried by electromagnetic waves, mostly in the infrared. It needs no material at all, and that is the property which separates it from the other two routes.

It is why we feel the Sun. Between the Sun and us there is almost perfect vacuum, so conduction and convection are impossible — radiation alone crosses that distance.

A dark, rough surface absorbs radiation better than a light, shiny one, and emits it better too. Hence pale clothes in summer and the mirrored lining inside a vacuum flask.

Worked examples

  1. Classify it: a room warmed by a heater standing in one corner.

    1. The heater warms the air right next to it
    2. That warm air rises and cooler air sinks to replace it
    3. A circular flow develops across the room

    Answer: Convection, plus direct radiation onto anyone sitting facing it

  2. How does a vacuum flask keep a drink hot?

    1. A double wall with vacuum between blocks conduction and convection
    2. A mirrored coating reflects radiation back inside
    3. A tight stopper stops hot vapour escaping

    Answer: It blocks all three routes at the same time

  3. Why does a wool jumper keep you warm, when wool makes no heat of its own?

    1. The fibres trap many small pockets of air
    2. Trapped air cannot flow, so there is no convection
    3. Air is a poor conductor, so conduction is slow

    Answer: The trapped air slows the loss of heat from your body

Common mistakes

Thinking a metal handle is colder than a wooden one beside it
Both are at room temperature. Metal conducts heat away from your hand quickly and therefore feels cold — the sensation reports a rate of heat transfer, not a temperature.
Assuming convection happens in solids too
Convection requires the material itself to flow. In a solid the particles are locked in place, so only conduction is available.
Explaining the Sun's warmth as conduction or convection
There is almost no matter between the Sun and the Earth. Only radiation crosses a vacuum, and that is exactly why it is counted as a separate route.

What to remember

More in Energy and Heat