KS3 · Physics

Heating and thermal equilibrium

Forget your tea for an hour and it goes cold, but never colder than the room. Why does it stop there, and why does it cool fastest at first?

A cup of tea cooling in a room at 20 °C

Time (minutes): 30. Temperature of the tea (°C): 22.7

Drag the dot along the curve. How far does the temperature drop between 0 and 4 minutes? Now try 30 to 34 minutes. (Example numbers, to show the shape.)

Watch out: The tea never drops below 20 °C. Once it is at the same temperature as the room there is no temperature difference left, so it stops cooling.

What the particles are doing

Reason it through

Why does a hot drink cool down, and where does its energy go?

Link 1 of 4

First link · your turn

The tea is hot and the air around it is cooler. What is different about their particles?

Locked — reveal the link above first

Predict, then check

Commit to an answer before you look.

You pour a big jug of hot water at 60 °C into a small cup of cold water at 20 °C and stir. What will the temperature of the mixture be?

Physics · Conductors and insulators

Fast passer-on or slow passer-on?

Sort each material. Is it a thermal conductor or a thermal insulator?

Still to sort

Thermal conductor (0)

Thermal conduction happens quickly through it.

Where the line is: A conductor passes particle motion on quickly; an insulator passes it on slowly. It is a question of how fast, not whether it happens at all.

Thermal insulator (0)

Thermal conduction happens slowly through it.

Where the line is: Poor thermal conductors are good thermal insulators: they are the same materials, described from opposite sides.

6 of 6 still to sort.

Thermal conduction is particles passing on their motion to their neighbours, through the forces between them or by colliding. Some materials do this quickly and some do it slowly.

What do you think?

The woolly can

On a hot day you take two cold cans of drink out of the fridge. You wrap one in a wool sock and leave the other unwrapped, side by side in the warm air.

What happens to the wrapped can, and why? Pick the idea closest to what you think.

The wrapped can stays cold for longer, because the wool gives out cold.The wrapped can warms up faster, because wool is warm.The wrapped can stays cold for longer, because wool passes particle vibrations on slowly.

How sure are you?

I'm sureI think soI'm guessing

The wrapped can stays cold for longer, because the wool gives out cold.

This is a common misconception

Cold is not a thing that flows out of anything. Only energy is transferred, and it goes from the hotter thing to the cooler thing. Here the warm air is hotter than the can, so energy flows from the air into the can. The wool does not add cold; it slows that flow down.

The wrapped can warms up faster, because wool is warm.

This is a common misconception

We call wool warm because wrapping it round something hot, like you, keeps that thing warm. It does not make any energy itself. It passes particle vibrations on only slowly, so it slows down whichever way the energy is going, into a cold thing or out of a hot one.

The wrapped can stays cold for longer, because wool passes particle vibrations on slowly.

This matches the evidence

Yes. The warm air is hotter than the can, so energy is transferred into the can. Wool is a thermal insulator with air trapped in it, so it slows that transfer down. The can still warms up in the end, just more slowly.

An insulator slows energy transfer in both directions: it keeps a hot thing warm and a cold thing cool.

Spot the unfair test

Which insulator is better?

Priya wants to find out which of two materials, X and Y, is the better thermal insulator. Here is her plan. Find the one line that stops it being a fair test.

Priya's plan — which line goes wrong?

Put the same volume of hot water into two identical cans.
Wrap can A in a thin layer of material X and can B in a thick layer of material Y.
Start timing when both cans are at the same starting temperature.
Measure how quickly the temperature of each can falls.
The can whose temperature falls more slowly is wrapped in the better insulator.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a hot drink cools fast at first, slows right down, and never ends up colder than the room.

What you need to know

A temperature difference between two objects leads to energy transfer from the hotter one to the cooler one, through contact (conduction) or by radiation.
The hotter something is, the faster its particles move. In a solid the particles vibrate about fixed positions.
Collisions make faster particles slow down and slower particles speed up, so the temperature difference shrinks.
Mixing hot and cold water gives a temperature in between, closer to that of the water there was more of. Temperatures are never just added or subtracted.
The greater the temperature difference, the faster something cools. So a cooling curve starts steep and flattens as it nears room temperature.
Energy is not destroyed when things cool: it is transferred to the surroundings and spread out (dissipated).
Thermal conductors (metals) pass particle motion on quickly; thermal insulators (wood, plastics, wool, air) pass it on slowly.

The big picture

When two things are at different temperatures, energy is transferred from the hotter one to the cooler one, by contact (conduction) or by radiation. The transfer shrinks the temperature difference, so a hot drink cools quickly at first and then more and more slowly, until it reaches room temperature. The energy is not destroyed; it spreads out into the surroundings. Insulators slow this transfer down.

Key points

1Energy is transferred from hotter to cooler, and the transfer makes the temperatures move towards each other.

2A hot object left alone cools until it reaches room temperature, and no further.

3Cooling rate means how quickly the temperature falls. It gets lower as the temperature difference gets smaller.

4A cold object in hot water warms up while the water cools; a hot object in cold water cools while the water warms.

5The energy a hot drink loses equals the energy its surroundings gain, but in a big room the rise in temperature is usually too small to measure.

6Materials with trapped air, such as wool, wood and double glazing, insulate well because gases are very poor thermal conductors.

7An insulator keeps hot things warm and cold things cool, because it passes particle vibrations on only slowly.

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Worked example

Problem

A cold spoon is dropped into a mug of hot water. Explain what happens to the particles of the spoon and the water, and what happens to their temperatures.

⚠ Watch out

Saying 'the cold from the ice goes into my drink'. Cold is not a substance that flows. Energy is what is transferred, from the hotter object to the cooler one: the drink gets colder because it loses energy to the ice.

🧠

Memory hook

Energy runs downhill, from hot to cold, and a smaller hill means slower running. When the hill is flat, the running stops. That flat bit is thermal equilibrium.

✓

Check yourself

Can you explain, using particles, why a hot drink cools quickly at first, then slowly, and stops at room temperature? And can you say why a woolly wrap keeps a cold drink cold?

Flashcards

(14)

What makes energy transfer from one object to another?

A temperature difference. Energy is transferred from the hotter object to the cooler one.

Name the two routes this energy can take between objects.

Through contact (conduction) or by radiation.

How does particle speed link to temperature?

The higher the temperature, the faster the particles move. In a solid they vibrate about fixed positions.

Hot water is mixed with cold water. What temperature does the mixture reach?

Somewhere in between: cooler than the hot water and warmer than the cold water.

When hot and cold water mix, which starting temperature is the result closer to?

The temperature of the water there was more of. Temperatures are not simply added or subtracted.

What happens to the particles when hot and cold water mix?

Fast particles from the hot water collide with slow ones from the cold water: the fast ones slow down and the slow ones speed up.

What is the cooling rate?

How quickly the temperature of an object falls.

Why does a hot drink cool more slowly as time goes on?

Its temperature difference from the air keeps shrinking, and the smaller the difference, the lower the cooling rate.

What is a cooling curve, and what happens to its slope?

A line graph of temperature against time. Its slope decreases over time as the object cools.

Is energy destroyed when a hot drink cools?

No. It is transferred to the surroundings and dissipated. The drink's loss equals the surroundings' gain.

What is thermal conduction?

A change in temperature caused by particles passing on their motion, through the forces between them or by colliding.

Thermal conductor vs thermal insulator?

Conduction happens quickly through a conductor (metals) and slowly through an insulator (wood, plastics, wool, air).

Why do materials with trapped air insulate well?

Gases are very poor thermal conductors, so the trapped air passes particle vibrations on very slowly.

How do you make a fair test of two insulators?

Change only the type of insulation. Keep the volume of water, the thickness of the layer and the starting temperature the same. The slower the temperature falls, the better the insulator.

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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A cup of tea cooling in a room at 20 °C

Fast passer-on or slow passer-on?

The woolly can

Which insulator is better?

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