Conduction
Heat passed from molecule to molecule through a solid.
Lesson at a glanceShow the key points
- Conduction: energy shared by collisions along a material; no bulk motion.
- Steady state: ΔQ/Δt = KA(T₁ − T₂)/x (28.1); generally −KA dT/dx (28.2).
- K in W m⁻¹ K⁻¹: silver 414, copper 385, aluminium 209, steel 46, glass 0.67, air 0.024.
- Solids > liquids > gases; metals best because of free electrons.
Heat travels by three routes: conduction (usually in solids), convection (in liquids and gases) and radiation (needing no medium at all). First, conduction: why a metal spoon in hot tea soon burns your fingers.
How conduction works
Put one end of a metal rod in a flame. Molecules at that end gain kinetic energy and, colliding with slower neighbours, share it. Those pass it on to their neighbours, and so on along the rod. Energy moves from molecule to molecule while each molecule stays, on average, in place: no matter flows.
Thermal conductivity
A slab of cross-section A and thickness x has its faces held at T₁ and T₂, with its sides insulated. In the steady state, temperatures no longer change, and the same heat crosses every section per second: the heat current ΔQ/Δt. Experiment shows it is proportional to A and to T₁ − T₂, and inversely proportional to x:
For a varying cross-section or before steady state, apply it to a thin layer:
| Material | K (W m⁻¹ K⁻¹) | Material | K |
|---|---|---|---|
| Silver | 414 | Glass | 0.669 |
| Copper | 385 | Brick | 0.711 |
| Aluminium | 209 | Water | 0.585 |
| Brass | 109 | Wood (oak) | 0.146 |
| Steel | 46 | Glass wool | 0.042 |
| Concrete | 1.30 | Air | 0.0238 |
Solids generally conduct better than liquids, and liquids better than gases. Metals are far better still, because their free electrons carry energy rapidly through the metal.
A window pane 1.2 m² in area and 5.0 mm thick separates a room at 20°C from outside air at 5°C (assume the glass surfaces are at these temperatures). K for glass = 0.67 W m⁻¹ K⁻¹. Find the heat current.
- 1Conduction: energy passed by molecular collisions; no bulk movement of matter.
- 2Steady state: ΔQ/Δt = KA(T₁ − T₂)/x; in general −KA dT/dx.
- 3Metals conduct best (free electrons); gases worst.
Check yourself
Keep exploring
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