Why Do Metals Conduct Heat While Wool and Foam Insulate?

• Metal bar
• Free electrons inside the metal
• Foam block
• Small air pockets in the foam
• Heat source

What you will learn

• Explain why heat passes quickly through metal.
• Say why wool and foam slow heat down.
• Pick a sensible material when you want to keep heat in, keep cold in, or move heat away.

Before you start

No prior knowledge is needed. You only need to know that heat moves from a hot place to a colder one.

How does heat move through a metal?

Heat moves fast in metal because of free electrons. According to a physics lecture from Boston University, electrons can also carry heat, which is why metals are generally very good conductors of heat. Metals have many free electrons that move around randomly, and they pass heat from one part of the metal to another.

This is called conduction. Nothing flows away; heat travels inside the material through vibrations of atoms and molecules. In a metal the free electrons do the job much faster.

• Metal bar
• Free electrons
• Hot end
• Direction heat travels

Why do wool and foam slow heat down?

Wool and foam slow heat because they trap many small pockets of air. Natural Resources Canada explains that insulation reduces heat flow through small trapped air pockets.

The reason lies in the air itself. According to Natural Resources Canada, air conducts heat poorly, so trapped air pockets act as insulating barriers that impede heat. The Woolmark Company explains that the natural crimp of wool traps still air and helps provide insulation. The Boston University lecture calls styrofoam a good insulator.

• Small air pockets
• Wool or foam block
• Reduced heat flow through insulation

Real-world example

Think of a cooking pan. The Boston University lecture says copper is a good thermal conductor, which is why some pots and pans have copper bases: heat from the stove passes quickly through the base to the food. A foam box for keeping things cold works the other way round. The air pockets in the foam make heat from outside creep in very slowly.

A wool jumper follows the same idea: its fibres hold air in the fabric, so body heat escapes more slowly.

Common misunderstandings

Insulation stops heat completely. Heat still passes through, only more slowly. Insulation reduces heat flow through the material.

Only metals conduct heat. Every solid conducts heat through vibrating atoms and molecules. Metal is faster because it also has free electrons.

Foam insulates because it is "cold" itself. Foam insulates because of the air trapped inside it, not because of its own temperature.

Frequently asked questions

What is a free electron?

It is an electron that moves around randomly inside a piece of metal. It can carry heat from one part to another.

Why do some pans have copper bases?

Copper is a good thermal conductor, so heat passes quickly through the base to the food.

What does a good insulator do?

It lets heat pass through very slowly, thanks to the small air pockets trapped inside.

Summary

• Metals conduct heat quickly because free electrons carry it along.
• Wool and foam trap many small air pockets, so heat passes slowly.
• Insulation slows heat down; it does not stop it.