Thermal Conductivity

Introduction

On a cold morning, a metal door handle feels much colder than a wooden door, even though both are at the same temperature. The difference is thermal conductivity — how easily heat flows through a material. The metal draws heat out of your hand quickly; the wood does not. This property guides the design of saucepans, oven gloves, houses, flasks and computer processors.

Core explanation

Conductors and insulators. A thermal conductor lets heat pass through it easily. A thermal insulator conducts heat poorly. Metals are generally good conductors; non-metals such as wood, plastic, glass, rubber and especially air are poor conductors (insulators).

Approximate thermal conductivities (W per metre per kelvin):

Material Thermal conductivity --- --- Copper about 400 Aluminium about 240 Iron about 80 Glass about 1 Water about 0.6 Wood about 0.1–0.2 Air about 0.026 Expanded polystyrene about 0.03

Copper conducts heat roughly ten thousand times better than air.

How conduction works. When one end of a solid is heated, its particles vibrate more vigorously and pass energy to their neighbours, which pass it on in turn. In metals, heat is also carried very efficiently by free electrons , which move quickly through the structure and transfer energy over long distances. This is why metals conduct heat so well.

Why "cold" metal feels cold. Touch detects the rate at which heat leaves your skin, not temperature directly. Metal at 15 °C conducts heat away from your 33 °C skin rapidly, so it feels cold. Wood at 15 °C conducts heat away slowly, so it feels less cold.

Trapped air makes good insulators. Air is a very poor conductor, but it can transfer heat by moving (convection). Materials that trap air in tiny pockets — wool, feathers, foam, fibreglass — stop the air from moving and make excellent insulators.

Step-by-step reasoning

To explain why saucepans have metal bases and plastic handles:

1. The base must transfer heat from the flame to the food quickly — it needs a good thermal conductor (metal). 2. The handle must stay cool enough to hold — it needs a thermal insulator (plastic or wood). 3. Choosing materials by thermal conductivity makes the pan both effective and safe.

Visual explanation

Picture rods of copper, iron, glass and wood of equal size, each with a blob of wax stuck at the far end, with the near ends placed in hot water. The wax melts first on copper, then iron, while the glass and wood rods barely warm up. A particle diagram shows vibrating atoms passing energy along a chain, with free electrons racing through the metal rod.

Real-world analogy

Conduction through a non-metal is like passing a bucket along a line of people: each person hands it to the next, and progress is slow. In a metal, free electrons act like runners who grab the energy and sprint along the line, so heat travels much faster.

Real-world example

Double-glazed windows have two panes of glass with a gap of still air or argon between them. Because gases are very poor conductors and the gap is narrow enough to limit convection, double glazing greatly reduces the heat lost from homes in winter and the heat gained in summer, cutting energy use.

Why?

Why are metals good conductors of both heat and electricity? Both properties come from the same feature: free electrons that can move through the metal. Moving electrons carry electric charge (electric current) and also carry kinetic energy (heat), so metals that conduct electricity well usually conduct heat well too.

Common misconception

"Insulators keep things warm by producing heat" or "a coat makes you warm". Insulators do not produce heat; they slow down heat transfer. A good flask keeps cold drinks cold for the same reason it keeps hot drinks hot — it slows heat flow in either direction.

Worked example

Question: Explain why a metal spoon left in a cup of hot tea becomes hot at the handle, but a plastic spoon does not.

Reasoning: The metal is a good thermal conductor; heat is carried from the tea along the spoon by vibrating particles and free electrons. Plastic is a thermal insulator with no free electrons, so heat travels along it very slowly.

Answer: Metal conducts heat well, so the handle heats up; plastic conducts heat poorly, so its handle stays cool.

Quick check

1. Why are oven gloves made from thick fabric containing trapped air? Answer: Fabric and trapped air are poor conductors of heat, so they slow heat transfer from the hot dish to the hand.

Exam focus

Classify materials correctly and explain metal conduction using free electrons as well as vibrating particles. Everyday-use questions (pans, windows, clothing, cool boxes) expect you to link the use to high or low thermal conductivity. Remember that insulators slow heat transfer in both directions.

Advanced insight

Diamond, although a non-metal with no free electrons, is an even better thermal conductor than copper (about 2000 W/m K). Its rigid network of strong bonds carries vibrations extremely efficiently. This is why synthetic diamond is used as a heat spreader for high-power electronic devices.

Summary

Thermal conductivity describes how well heat passes through a material. Metals are good thermal conductors because vibrations and free electrons transfer energy quickly; non-metals and gases are insulators. Materials that trap still air are especially good insulators. Engineers use conductors where heat must flow and insulators where it must be kept in or out.

Practice questions

1. Classify as thermal conductor or insulator: copper, wool, aluminium, polystyrene. Answer: Conductors: copper, aluminium. Insulators: wool, polystyrene. 2. Why does a tiled floor feel colder to bare feet than a carpet in the same room? Answer: Tiles conduct heat away from the feet faster than carpet, which traps air and is an insulator. 3. Why are cooking pans often made with copper bases? Answer: Copper is an excellent thermal conductor, so heat spreads quickly and evenly to the food. 4. How does a layer of feathers help a bird stay warm? Answer: Feathers trap still air, a very poor conductor, which reduces heat loss from the bird's body.