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2.1 Energy Transfer by Conduction

GCSE Physics · AQA · Chapter 2: Energy Transfer by Heating

A free GCSE Physics lesson for AQA (8463) from OBA School: conductors and insulators, thermal conductivity, and how thickness and temperature difference affect the rate of energy transfer. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Name materials that are good thermal conductors and good thermal insulators
  • Explain conduction in terms of particles, and why metals conduct best
  • Describe how the thermal conductivity of a material affects the rate of energy transfer through it
  • Describe how the thickness of a layer and the temperature difference across it affect the rate of energy transfer
  • Explain why materials that trap air make good insulators

Key words

  • Conduction: Energy transfer through a material by particles passing energy to their neighbours. It happens in solids (and slowly in liquids and gases).
  • Thermal conductor: A material that lets energy pass through it quickly by conduction, e.g. copper.
  • Thermal insulator: A material that lets energy pass through it only slowly, e.g. wool, foam or still air.
  • Thermal conductivity: A measure of how well a material conducts energy. The higher it is, the higher the rate of energy transfer through the material.
  • Rate of energy transfer: The energy transferred each second, in joules per second (watts, W).
  • Free electrons: Electrons in a metal that can move about between the atoms. They carry energy through the metal quickly.

Conductors and insulators

When one end of a solid is heated, energy is transferred along it to the cooler end. This is conduction. Some materials do this quickly (conductors), others very slowly (insulators).

A fair way to compare materials is to use rods of the same length and diameter, heat one end of each equally, and time how long it takes for wax at the same distance along each rod to melt.

  • Metals are the best conductors. Copper and aluminium are better than steel.
  • Non-metals such as glass, wood and plastic are poor conductors. Glass conducts better than wood.
  • The best insulators are materials that trap air, such as wool, fibreglass, feathers and plastic foam.

Fair test: In the rod test, the independent variable is the material; keep the length, diameter, starting temperature, heat source and distance of the wax from the hot end the same.

How conduction works

In a solid, the particles are held in place but they vibrate. Heating one end makes the particles there vibrate more. They knock into their neighbours and pass energy along, so the energy spreads towards the cold end.

Metals contain free electrons that move through the whole metal. They gain energy at the hot end and move quickly to cooler parts, where they pass the energy on. That is why metals conduct so much better than non-metals.

In liquids and gases the particles are far apart or move about freely, so they pass energy on by collisions much less effectively. Gases are very poor conductors.

Thermal conductivity

The thermal conductivity of a material tells you how well it conducts. If two layers have the same size, thickness and temperature difference across them, the one with the higher thermal conductivity transfers more energy per second.

  • copper; Thermal conductivity in W/m °C (approximate) 400; Good conductor or insulator? very good conductor
  • aluminium; Thermal conductivity in W/m °C (approximate) 240; Good conductor or insulator? very good conductor
  • steel; Thermal conductivity in W/m °C (approximate) 50; Good conductor or insulator? good conductor
  • glass; Thermal conductivity in W/m °C (approximate) 1.0; Good conductor or insulator? poor conductor
  • brick; Thermal conductivity in W/m °C (approximate) 0.7; Good conductor or insulator? poor conductor
  • wood; Thermal conductivity in W/m °C (approximate) 0.15; Good conductor or insulator? insulator
  • wool; Thermal conductivity in W/m °C (approximate) 0.04; Good conductor or insulator? good insulator
  • still air; Thermal conductivity in W/m °C (approximate) 0.025; Good conductor or insulator? very good insulator

Worked example: choosing materials for a frying pan

The base should transfer energy quickly from the hob to the food: choose a material with a high thermal conductivity, such as aluminium or copper.
The handle should transfer energy slowly to your hand: choose a material with a low thermal conductivity, such as wood or a hard plastic.

Answer: metal base (high conductivity), wooden or plastic handle (low conductivity)

Exam tip: You do not need to remember thermal conductivity values. You do need to use them: higher value = better conductor = faster energy transfer.

What affects the rate of energy transfer through a layer?

The energy transferred each second through a layer of material (a wall, a window, a coat) depends on three things:

  • the temperature difference across the layer – a bigger difference gives a higher rate
  • the thickness of the layer – a thicker layer gives a lower rate
  • the thermal conductivity of the material – a lower conductivity gives a lower rate.

So to cut energy transfer as much as possible, use a material with the lowest possible thermal conductivity, made as thick as is practical.

Worked example: energy transferred through a layer

From the graph, 4 cm of foam transfers 15 W. How much energy passes through it in 5 minutes?
time t = 5 × 60 = 300 s
E = P t = 15 × 300 = 4500 J

Answer: 4500 J (4.5 kJ)

Why trapped air makes a good insulator

Still air has a very low thermal conductivity. Materials such as fibreglass, wool, feathers and polystyrene foam are full of tiny pockets of trapped air. The air cannot move about, so it cannot carry energy away by convection either, and the material as a whole is a very good insulator.

  • Loft insulation: thick layers of fibreglass on the loft floor greatly reduce energy transfer through the roof.
  • Clothing: several thin layers trap more air than one thick layer.
  • Animals: fur and feathers trap air next to the skin.

Exam tip: In an exam answer, say why: “the material traps air, which is a poor conductor, so the rate of energy transfer by conduction is reduced”.

Common mistakes

  • Saying insulators “keep the cold out”. Energy is transferred from the warmer place to the colder place; insulators slow down that transfer.
  • Thinking a metal object at room temperature is colder than a wooden one. They are at the same temperature; the metal only feels colder because it conducts energy away from your hand faster.
  • Mixing up thickness and conductivity: a thicker layer reduces the rate; a higher conductivity increases it.
  • Forgetting free electrons when explaining why metals are good conductors.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these materials is the best thermal conductor? (Choose from: copper, glass, wood, wool.)

Answer: copper

Working: Copper is a metal. Metals are the best thermal conductors because free electrons carry energy through them quickly.

Question 2. True or false? The thicker a layer of insulating material, the higher the rate of energy transfer through it.

Answer: False

Working: False. A thicker layer gives a lower rate of energy transfer.

Question 3. Name the particles in a metal that move through it and carry energy quickly from the hot end to the cold end.

Answer: free electrons

Working: Free (delocalised) electrons gain energy at the hot end and move quickly to cooler parts, passing the energy on.

Related lessons

Continue in the full course

This lesson is section 2.1 (AGCPH0201) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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