Thermal conductivity of metals

These two saucepans have the same temperature. One of them has a plastic handle, and the other a metal handle. You can hold the plastic handle, but - ouch! - you’ll burn your hand on the metal. The metal is good at conducting heat. As one end is heated by the saucepan, the heat spreads into the entire handle.

Metals are good thermal conductors. The plastic handle is heated the same amount by the saucepan at this end. But the heat does not spread as easily through the plastic. Plastic is a thermal insulator. Most materials are thermal insulators, for instance glass, rock, wood, and cloth.

But all metals are good thermal conductors. Why is that? To answer that, we need to look at what heat is – at the atomic level. All substances are made of atoms. In solids, the atoms stay in place, but they aren’t completely still: they are moving constantly back and forth.

They vibrate. The hotter the material is, the more strongly the atoms vibrate. And that’s what heat is: movement at the atomic level. So how does heat spread? This is a simplified representation of a non-metal.

The atoms are attached to each other with chemical bonds. The bonds consist of electrons located between the atoms. Let’s heat one side of this material. The atoms that are heated start moving around more forcefully. This movement is transferred through the chemical bonds, to the neighboring atoms.

Which in turn begin to vibrate more. Then the movement is passed on to the next atoms, and the next. In this material, each atom can only affect its closest neighbors. And with each step, the vibrations get a little weaker. Heat is conducted poorly in this non-metal.

So what about the metals, then? Metal atoms also arrange themselves in fixed places. And there are electrons between the atoms. In metals however, the electrons are free to move. These free electrons aren’t bound to certain atoms.

They can move around anywhere in the entire material. And this really improves thermal conductivity. Look what happens when we heat one side of the metal: These atoms start vibrating more. A vibrating atom can push a free electron. This makes the electron move faster.

The fast-moving electron hits another atom, and makes that vibrate more. And the electron can continue, hitting even more atoms before it slows down. The electrons hit each other as well. This means we get more and more fast moving electrons throughout the material heating more of the atoms at the same time. It’s these free electrons that make all metals such good thermal conductors.

Heat is movement at the atomic level. Heat is spread as vibrations are passed through the material. What’s special with metals is that they have free electrons. These help carry the heat – not just to the nearby atoms, but also further away. This makes thermal conductivity much higher in metals.

So that’s why you burn your hand on the metal handle. Unless you use a pot-holder of course – that doesn’t have any free electrons.