2.3.1 Conduction
- Syllabus
- 0625–2026–2027
- Topic
- 2.3.1
- Level
- —
Learning objectives
2.3.1.1Experiments to demonstrate the• Describe experiments to demonstrate the properties of good thermal conductors and bad thermal conductors (thermal insulators)2.3.1.2Thermal conduction in all solids in• Describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations and also in terms of the movement of free (delocalised) electrons in metallic conductors2.3.1.3Describe, in terms of particles, why• Describe, in terms of particles, why thermal conduction is bad in gases and most liquids2.3.1.4There are many solids that conduct• Know: there are many solids that conduct thermal energy better than thermal insulators but do so less well than good thermal conductors
Compare thermal conductors with a fair experiment
A good thermal conductor transfers energy quickly through itself; a poor conductor, or thermal insulator, transfers energy slowly.
| Method | Set-up and observation | Conclusion |
|---|---|---|
| wax-pin comparison | use rods of equal length and diameter; attach identical pins with equal wax blobs at the same distance from one end; heat the ends equally and record when each pin falls | the rod whose wax melts first is the better conductor |
| temperature comparison | place equal lengths of different rods equally deep in hot water; measure temperature at points the same distance from the water at equal times | the rod whose measured point warms faster is the better conductor |
Keep rod dimensions, distance from the heated end, initial temperature, heating time or water temperature, wax amount and thermometer position the same. Repeat readings so the comparison depends on material rather than geometry or heating.
Feeling metal and plastic at the same room temperature shows how quickly they transfer energy to or from the hand, not that the metal began colder.
Explain conduction in non-metals and metals
Thermal conduction in every solid occurs through lattice vibrations. Metals have an additional, faster route: mobile delocalised electrons.
| Solid | How energy moves from hotter to cooler regions |
|---|---|
| all solids | particles at the hot end vibrate more energetically and transfer energy to neighbouring lattice particles, so the vibration spreads through the solid |
| metals | delocalised electrons gain energy in the hot region, move through the lattice and transfer energy in collisions with ions; lattice vibration also occurs |
The atoms, molecules or ions remain around fixed lattice positions; they do not travel from the hot end to the cold end. Energy is passed through the solid down the temperature difference.
Positive ions do not move freely through a metal. Metals usually conduct better than non-metals because mobile electrons provide an extra energy-transfer route, not because metal atoms migrate along the object.
Explain why gases and most liquids conduct poorly
Gases and most liquids are poor thermal conductors because their particles are not held in a closely connected, fixed lattice.
| Material | Particle arrangement | Consequence for conduction |
|---|---|---|
| non-metal solid | particles are close in a lattice | vibrations pass energy repeatedly to neighbouring particles |
| liquid | particles are close but not fixed in a lattice | collisions can transfer energy, but there is no continuous lattice-vibration pathway |
| gas | particles are far apart | collisions are much less frequent, so conduction is especially slow |
Poor conduction does not mean no energy transfer. Liquids and gases can still transfer energy slowly by particle collisions, and bulk movement may also transfer energy by convection; this card is comparing conduction only.
Thermal conductivity is a continuum
Solids do not fall into only two absolute groups. Many solids conduct thermal energy better than thermal insulators but less well than good thermal conductors.
| Position on the conduction scale | Behaviour for the same shape and temperature difference |
|---|---|
| good thermal conductor | transfers energy rapidly |
| intermediate solid | transfers energy at a moderate rate |
| thermal insulator | transfers energy slowly |
‘Conductor’ and ‘insulator’ are comparative descriptions. Choose a material by comparing transfer rates under the same conditions; a solid called an insulator still conducts some energy.
Do not infer that every non-metal is equally insulating or that every metal has one identical conductivity. Material, thickness, area and temperature difference all matter in a real comparison.