🍳 Metal pan, plastic handle
The base of a saucepan is metal, a good conductor, so energy passes quickly from the hob to the food. The handle is plastic or wood, an insulator, so very little energy is conducted to your hand.
Energy Transfers » Conduction
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.6
- How thermal energy is transferred through a solid by conduction
- Why metals are good thermal conductors
- Why non-metals, liquids and gases are poor conductors, and why trapped air is so useful
- How to use conduction to explain everyday things, like why tiles feel colder than carpet
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Unlock This CourseWhat conduction is
In Energy Stores and Transfers you met the transfer called 'by heating': energy moves from a hotter place to a cooler place. There are three ways this can happen: conduction, convection and radiation. This lesson is about conduction, the main way that energy travels through a solid. Convection and Thermal Radiation each have a lesson of their own.
Hold one end of a metal skewer in a campfire and, after a short time, the end in your hand gets hot too. The metal has not moved. No hot material has travelled along the skewer. Only energy has.
The spec asks you to describe how this takes place, so you need to learn the story in terms of particles.
Key terms:
- Conduction: the transfer of thermal energy through a material by vibrating particles passing energy on to their neighbours, without the material itself moving.
- Thermal conductor: a material that transfers thermal energy by conduction quickly, such as a metal.
- Thermal insulator: a material that transfers thermal energy by conduction only slowly, such as plastic, wood or air.
How conduction works in a solid
The particles in a solid are packed closely together and held in fixed positions. They cannot move around, but they are always vibrating. The hotter the solid, the more its particles vibrate.
Picture a solid rod with one end in a flame. Conduction along the rod happens like this:
- The particles at the hot end gain energy and vibrate more (faster and further).
- They bump against their neighbouring particles and pass some of their energy on.
- The neighbours now vibrate more, so they pass energy on to their neighbours.
- In this way energy is passed from particle to particle along the rod, from the hot end to the cold end.
It works like a line of people passing buckets along. The buckets (the energy) travel down the line, but each person (each particle) stays in the same place.
Two things are worth noticing:
- Energy is always conducted from the hotter part to the cooler part. A bigger temperature difference means energy is transferred faster.
- Conduction needs particles that are close enough to pass energy to each other. That is why it works best in solids.
Describing it as a diagram
The lesson cannot show a drawing, so here is how to draw one in the exam. Draw a long thin rectangle for the rod. Draw a flame under the left-hand end and label that end 'hot'. Label the right-hand end 'cold'. Inside the rod draw a row of circles that touch each other. Put large double-headed arrows on the circles at the hot end (big vibrations) and smaller ones towards the cold end. Add one long arrow under the rod pointing from hot to cold, labelled 'energy transferred by conduction'.
Why metals are good conductors
All solids conduct by vibrating particles, but that process is quite slow. Metals have a second, much faster way.
Remember from Charge and Current that a metal contains free electrons, which are able to move through the whole piece of metal. In conduction they act like fast messengers:
- At the hot end, the free electrons gain energy and move faster.
- They travel quickly through the metal towards the cooler parts.
- They collide with particles further along and pass energy on to them.
Because the free electrons can travel a long way through the metal, they carry energy along it much more quickly than vibrations passed from neighbour to neighbour. This is why metals such as copper, aluminium and steel are good thermal conductors. It is also why materials that are good conductors of electricity are usually good conductors of thermal energy: the same free electrons do both jobs.
Why non-metals, liquids and gases are poor conductors
🌳 Non-metal solids
Wood, plastic, glass and rubber have no free electrons. Energy can only be passed on by vibrating particles, which is slow. They are insulators.
💧 Liquids
Most liquids, such as water, are poor conductors. They have no free electrons, and their particles are not held in fixed positions, so vibrations are not passed on as well as in a solid.
🌬 Gases
Gases, such as air, are very poor conductors. Their particles are far apart, so they do not collide with each other very often and energy is passed on very slowly.
| Material | Conductor or insulator? | Reason |
|---|---|---|
| Metals (copper, aluminium, steel) | Good conductor | Vibrating particles and free electrons that carry energy quickly |
| Non-metal solids (wood, plastic, glass) | Poor conductor (insulator) | Vibrating particles only, no free electrons |
| Most liquids (water) | Poor conductor (insulator) | No free electrons, particles not in fixed positions |
| Gases (air) | Very poor conductor (very good insulator) | Particles far apart |
Trapped air
Air is one of the best insulators around, but only if it is kept still. Many materials that keep us warm work by trapping air in small pockets:
- A woolly jumper traps air between its fibres.
- A bird fluffs up its feathers on a cold day to trap a thicker layer of air.
- A foam cup is mostly tiny pockets of air, so a hot drink inside does not burn your fingers.
In each case energy has to be conducted through air, which is very slow. The wool, feathers or foam do little of the insulating themselves. Their job is to hold the air in place. How insulation is used to cut wasted energy is covered in Reducing Unwanted Energy Transfers.
Conduction in everyday life
🥄 Metal spoon in hot tea
Leave a metal teaspoon standing in hot tea and the handle soon feels hot. Energy is conducted along the spoon from the hot end in the tea to the cooler end. A plastic spoon in the same tea stays cool at the top.
Worked example: why do tiles feel colder than carpet?
A bathroom has a tiled floor and a carpeted landing outside. Both floors have been in the same house all night, so they are at the same temperature. Explain why the tiles feel colder to bare feet.
Step 1: Your feet are warmer than either floor, so energy is transferred from your feet to the floor.
Step 2: Tiles are a better thermal conductor than carpet. They conduct energy away from your feet quickly.
Step 3: Carpet is an insulator because its fibres trap air. Energy leaves your feet only slowly.
Step 4: Your skin senses how fast it is losing energy, not the actual temperature of the floor. The faster energy leaves, the colder the surface feels. So the tiles feel colder.
The same idea explains why a metal park bench feels much colder than a wooden one on a winter morning, even though both are at the temperature of the air.
You can compare how well different metals conduct with a simple experiment, which you will meet in Practical: Conduction, Convection and Radiation.
Common mistakes
- Saying the particles move along the rod. In a solid the particles stay in their fixed positions and vibrate. Only the energy is passed along.
- Saying 'heat rises' or 'the heat travels'. For conduction, write that energy is passed from particle to particle, from the hot end to the cold end.
- Forgetting the free electrons. 'Metals are good conductors because their particles are close together' is not enough. The particles in glass are close together too. The marks are for the free electrons.
- Saying tiles are colder than carpet. They are at the same temperature. Tiles feel colder because they conduct energy away from your skin faster.
- Saying wool 'makes heat' or 'keeps the cold out'. Wool traps air, and air is a poor conductor, so energy leaves your body more slowly.
Exam-style question
A cook stirs hot soup in a steel saucepan. She has two spoons, one made of steel and one made of wood.
(a) State the name of the process by which thermal energy is transferred through a solid. [1 mark]
(b) After a few minutes in the soup, the handle of the steel spoon is too hot to hold. Describe how energy is transferred along the steel spoon from the soup to the handle. [4 marks]
(c) Explain why the handle of the wooden spoon stays cool. [2 marks]
(d) The cook picks up the saucepan using a thick woollen cloth. Explain how the cloth protects her hand. [2 marks]
Model answer
(a) Conduction (1).
(b) The particles at the hot end of the spoon gain energy and vibrate more (1). They pass energy on to neighbouring particles (1). Steel is a metal, so it contains free electrons, which gain energy and move through the metal (1). The free electrons collide with particles further along and transfer energy to them, so energy travels quickly to the cooler handle (1).
(c) Wood is a (thermal) insulator / poor conductor because it has no free electrons (1). Energy can only be passed on by vibrating particles, which is slow, so little energy reaches the handle (1).
(d) The wool traps air between its fibres (1). Air is a very poor conductor (its particles are far apart), so energy is conducted to her hand only slowly (1).
Exam tip
Part (b) is about a metal, so a full answer needs both parts of the story: vibrating particles passing energy to their neighbours, and free electrons carrying energy quickly through the metal. Many students write only one of the two and lose half the marks. Notice that part (b) says 'describe' and has 4 marks, so aim for four clear steps in order.