All revision notes topics

ConductionCambridge IGCSE Physics: Revision notes

Section 1

What Is Thermal Conduction?

Thermal conduction is the transfer of thermal energy through a substance without the substance itself moving from place to place — energy passes from particle to particle.

Materials differ hugely in how well they conduct thermal energy:

  • Good thermal conductors (mostly metals) allow energy to pass through quickly.
  • Bad thermal conductors, also called thermal insulators (such as wood, plastic, air and most non-metals), allow energy to pass through only slowly.
  • Some solids fall in between: they conduct better than good insulators, but not as well as good conductors.
Key termsthermal conductionthermal insulator
Exam tip

Not every solid is either 'a good conductor' or 'a good insulator' — many, such as glass or ceramics, are somewhere in between.

Section 2

How Can You Demonstrate Good and Bad Thermal Conductors?

A typical experiment compares rods of different materials (for example, copper, aluminium, glass and wood) of the same size:

  1. Coat one end of each rod with wax, or attach a small object with wax, or fix a temperature sensor along each rod.
  2. Heat the opposite end of each rod at the same time using an identical heat source.
  3. Observe which rod's wax melts first, or compare the temperature readings along each rod over time.
  4. The rod along which the wax melts fastest (or where heat spreads fastest) is the best thermal conductor.

This shows metals such as copper are much better thermal conductors than materials such as glass or wood.

Example

Copper conducts heat far faster than glass, so wax on a copper rod melts noticeably sooner than wax at the same distance along a glass rod, given the same heat source.

Section 3

Why Do Metals Conduct Thermal Energy So Well?

Conduction happens in ALL solids through the vibration of the particles in the lattice: a heated particle vibrates more vigorously and passes this vibration on to neighbouring particles.

However, metals conduct thermal energy much better than non-metals because metals also contain free (delocalised) electrons that can move throughout the metal:

  • These free electrons gain kinetic energy at the hot end.
  • They move quickly through the metal, carrying this energy with them.
  • They transfer energy to particles throughout the metal through collisions.

This electron movement is a much faster way of transferring energy than lattice vibration alone, which is why metals are such good thermal conductors.

Key termsfree electronslattice vibration
Common mistake

Don't say metals conduct heat 'because they are shiny' or 'because they are dense' — the correct reason is the presence of free (delocalised) electrons that carry energy quickly through the material.

Section 4

Why Is Thermal Conduction Poor in Gases and Most Liquids?

In gases and most liquids, the particles are far apart (gases) or more loosely arranged (liquids) compared with solids, and they are not held in a fixed lattice. This means:

  • There is much less direct, regular contact between neighbouring particles.
  • Vibrations cannot pass efficiently from particle to particle, as they do in a solid's fixed lattice.
  • As a result, conduction through gases and most liquids is slow, and they are generally poor thermal conductors.

This is why gases such as air are often used as insulating layers (for example, trapped air in double-glazed windows or in a woolly jumper).

Think of it like this

Passing a vibration through a solid is like a row of people standing shoulder to shoulder passing a nudge along instantly; in a gas, the people are spread far apart, so a nudge cannot pass along nearly as easily.

Must Know

  • Thermal conduction: transfer of thermal energy through a substance, from particle to particle, without the substance moving overall.
  • Good thermal conductors (mainly metals) transfer energy quickly; bad thermal conductors (thermal insulators) transfer it slowly; some solids sit in between.
  • Experiments comparing rods of different materials with wax or temperature sensors demonstrate which materials conduct best.
  • Conduction in all solids involves the vibration of particles in the lattice, passing energy to neighbouring particles.
  • Metals conduct especially well because free (delocalised) electrons move quickly through the metal, carrying energy with them.
  • Conduction is poor in gases and most liquids because particles are not held in a fixed, closely packed lattice, so vibrations cannot pass efficiently between them.

That's the notes covered.

Carry on to the next subtopic.