KS3 · Physics

Thermal conductors and insulators

Put one end of a metal rod in a flame and soon the far end is hot too. Yet not a single particle has made the journey. So what has?

Heat one end. Watch the vibration get passed along.

Section of the rod: 1. State: Section 1 · in the flame. static

Section of the rod1

In the flame, the particles vibrate hardest and fastest. But look closely: each one is still jiggling about its own fixed spot in the pattern. Forces of attraction hold every particle in place.

A zoomed-in look at the particles inside a metal rod whose left end is in a flame. Slide from section 1, in the flame, to section 5, the far end.

Watch out: The particles never travel along the rod — every one stays in its own place. What gets passed along is their vibration. So heat isn't a substance flowing through the metal.

Predict, then check

Commit to an answer before you peek.

A metal pan with a wooden handle has been on a hot hob for a few minutes. You need to move it. Which part can you safely hold, and why?

Which conducts best?

How quickly thermal conduction happens in the material

1 · Best thermal conductor4 · Poorest thermal conductor

Glass
Diamond
Plastic, paper or wood
A metal, such as steel or copper

Why trapped air insulates

Reason it through

Why does a woolly jumper keep you warm — and why is double glazing better than a single sheet of glass?

Link 1 of 4

First link · your turn

Start with liquids. Why are they generally poorer thermal conductors than solids?

Locked — reveal the link above first

What does the wool actually do?

Two drinks are left on a table in a room: a mug of hot chocolate and a glass of ice-cold juice. Each is wrapped in a thick woolly sock.

Which is closest to what you think the wool does?

Wool is warm, so it keeps the hot chocolate hot and makes the juice warm up faster.The wool keeps the juice cold by holding the coldness in.The wool slows energy transfer: the hot chocolate cools more slowly and the juice warms up more slowly.Unwrapped, the hot chocolate would keep cooling until it's colder than the room.

How sure are you?

I'm sureI think soI'm guessing

Wool is warm, so it keeps the hot chocolate hot and makes the juice warm up faster.

This is a common misconception

Wool feels cosy, but it has no warmth of its own to hand out — insulators don't warm things up. It slows energy transfer. So the wrapped juice actually warms up MORE slowly than an unwrapped one, not faster.

The wool keeps the juice cold by holding the coldness in.

This is a common misconception

There's no 'coldness' to hold in. The juice warms up because energy is transferred to it from the warmer room around it. The wool slows that transfer down, so the juice heats up more slowly.

The wool slows energy transfer: the hot chocolate cools more slowly and the juice warms up more slowly.

This matches the evidence

Exactly right. Wrapped around a hot object, an insulator reduces cooling. Wrapped around a cold object, it reduces heating. Same job both ways: it slows the transfer down.

Unwrapped, the hot chocolate would keep cooling until it's colder than the room.

This is a common misconception

A hot drink cools because it's warming up its surroundings. Energy transfer tends to shrink the temperature difference, so once the drink reaches room temperature there's no difference left — it cools to room temperature, but not below it. It would get there faster standing on something that's a good thermal conductor.

Insulators don't warm things up or add coldness. They slow energy transfer — keeping hot things hot and cold things cold for longer.

Design a fair test

For each factor, decide: is it the one thing we change, the thing we measure, or something we keep the same?

Still to sort

The one thing we change (0)

The material we're comparing.

Where the line is: Only one thing changes. If two things changed at once, you couldn't tell which one made the difference.

What we measure (0)

The result that tells us which material is best.

Kept the same (control variables) (0)

Everything else, so that the test is fair.

10 of 10 still to sort.

Two experiments. In the first, rods of different metals each have one end in the same flame and a drawing pin stuck on with wax at the other end. In the second, cans of hot water are wrapped in different materials.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Thermal conduction: particles pass their vibrations on to each other, through the forces between them and through collisions.
In a solid, particles vibrate about fixed positions. During conduction they don't move along the material — only the vibration is passed on.
Thermal conductors (like metals) conduct very quickly; thermal insulators (like plastic, paper and wood) conduct very slowly.
Materials that trap air insulate well, because gases are very poor thermal conductors.
Insulators slow energy transfer: they keep hot things hot and cold things cold.

The big picture

Thermal conduction is how energy is passed through a material by its particles. Heat one part and its particles vibrate more; they pass that extra vibration on to their neighbours, through the forces between them and through collisions. The particles stay in place — only the vibration is passed along. In thermal conductors, such as metals, this happens very quickly. In thermal insulators, such as plastic, paper, wood and trapped air, it happens very slowly.

Key points

1Heat one part of a solid and the part nearest the heat warms first; the temperature then rises gradually along the material.

2What we feel as 'heat' is particles in an object vibrating against our skin. Heat is not a substance that flows.

3Good conductors are poor insulators, and good insulators are poor conductors.

4From poorest to best conductor: plastic, paper and wood; then glass; then metals such as steel and copper; then diamond, the best of all.

5Stronger forces between particles make it easier to pass vibrations on — that's why diamond conducts best.

6Liquids are generally poorer conductors than solids; gases, with no forces of attraction between particles, are very poor conductors.

7A hot object cools by warming its surroundings. It cools to room temperature but not below, and cools faster touching a good conductor.

8To compare materials fairly, change only the material, measure one result, and keep everything else the same.

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Worked example

Problem

A class tests three rods, A, B and C, with the wax-and-pin method. Each rod is tested three times. Made-up results for the time taken for the pin to fall: Rod A — 42 s, 38 s, 40 s. Rod B — 65 s, 71 s, 68 s. Rod C — 29 s, 33 s, 31 s. Which rod is the best thermal conductor?

⚠ Watch out

Saying that heat 'flows' through a metal like water, or that the hot particles travel from one end to the other. They don't. Each particle stays in its place and passes its extra vibration on to its neighbours.

🧠

Memory hook

Think of a stadium wave: every fan stays in their seat, yet the wave races round the ground. Conduction works the same way — each particle stays put and passes its wobble on. (It's only a picture: a crowd wave carries no heat.)

✓

Check yourself

Cover the page. In two or three sentences, explain how energy gets from the hot end of a metal spoon to your end — and why no particle has to make that journey.

Flashcards

(13)

What is thermal conduction?

Particles passing their motion (their vibrations) on to each other, through the forces between them and through collisions.

Thermal conductor or thermal insulator: what's the difference?

In a conductor, thermal conduction happens very quickly; in an insulator, very slowly. So a good insulator is a poor conductor, and a good conductor is a poor insulator.

When a metal rod conducts, do its particles move along the rod?

No. Each particle vibrates about a fixed position. Only the vibration is passed along.

Is heat a substance that flows through a conductor?

No. What we feel as 'heat' is particles in an object vibrating against our skin.

One end of a solid is heated. Which part warms up fastest?

The part nearest the heat source. The temperature then rises gradually along the material.

Why is diamond the best thermal conductor?

It has the strongest forces of attraction between its particles, so vibrations are passed on most easily.

Why are liquids generally poorer thermal conductors than solids?

Their particles are a little further apart and the forces of attraction are weaker, so vibrations aren't passed on as easily.

Why are gases very poor thermal conductors?

There are no forces of attraction between gas particles, so energy is passed on only when particles directly collide.

Why is wool a good thermal insulator?

Its fibres trap air. Air is a gas, and gases are very poor thermal conductors.

How does double glazing keep a house warmer than one sheet of glass?

Its two sheets of glass have trapped air between them. Glass conducts better than trapped air, so the air reduces the passing on of vibrations.

Does wrapping something in an insulator warm it up?

No. An insulator slows energy transfer: a hot object cools more slowly, and a cold object heats up more slowly.

A hot drink is left in a room. How far does it cool?

Down to room temperature — but not below it.

Wax-and-pin test: which result shows the best conductor?

The rod with the smallest mean time for its pin to fall.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Heat one end. Watch the vibration get passed along.

Which conducts best?

What does the wool actually do?

Design a fair test

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