Thermal Conduction

Thermal conduction is the transfer of thermal energy through a material without the material moving as a whole. Energy flows from the hotter region to the colder region when they are in contact.

How conduction works in solids

In a solid, particles are packed closely in a fixed arrangement (a lattice). Heating makes these particles vibrate faster. They pass on energy to neighbouring particles, like a row of touching balls where one shakes and the shake spreads along the line. This is called transfer by lattice vibrations.

Metals: the role of free electrons

Metals contain free (delocalised) electrons that move through the lattice. These electrons pick up energy in the hot region and carry it quickly to cooler parts. This is why metals such as copper and aluminium are very good conductors.

Gases and most liquids: why conduction is poor

In gases (and most liquids), particles are farther apart and collide less often. Because energy is passed on mainly during collisions, fewer collisions mean slower conduction. In fluids, convection is usually more important than conduction.

Good vs bad conductors

• Good conductors: most metals (copper, aluminium, steel) – fast energy transfer.
• Thermal insulators (bad conductors): air, wood, plastic, rubber, foam – slow energy transfer.
• In-between conductors: glass, stone, water – better than insulators but much worse than good metals.

Everyday applications

• Saucepans: metal bases conduct heat to food; plastic or wooden handles reduce heat reaching your hand.
• Cooking utensils: a metal spoon feels cold because it conducts heat away from your hand quickly.
• Home insulation: foam and double glazing trap air to reduce conduction through walls and windows.
• Clothing: fluffy fabrics trap air, reducing conduction from your body to the outside.
• Heat sinks: metal fins spread and remove heat from electronics efficiently.

Simple experiments to demonstrate conduction

• Pin-and-wax test: Attach small pins with wax along a heated metal rod. Pins drop off fastest near the flame, showing energy spreading along the rod.
• Metal vs wooden spoon: Place each handle in hot water. The metal handle warms quickly (good conductor); the wooden handle warms slowly (insulator).

What affects the rate of conduction

• Bigger temperature difference (larger $\Delta T$) gives faster transfer.
• Material: higher thermal conductivity (often written as $k$) conducts faster.
• Thickness and length: thicker or longer paths slow conduction; larger contact area speeds it up.

Common misconceptions

• Metals are not “cold”; they just conduct heat away from your skin quickly.
• In conduction, particles vibrate or electrons move, but the solid does not flow.
• Air is a poor conductor, but in real life, convection in air can still carry heat unless air is trapped.

Key points

• Conduction transfers energy through contact, fastest in metals.
• Solids conduct via lattice vibrations; metals also use free electrons.
• Gases and most liquids are poor conductors because particles are far apart.
• Designs use good conductors to spread heat and insulators to reduce unwanted heat loss or gain.