Conduction in metals

Question:

Why do certain metals conduct larger currents than others?

Answer:

First of all don't forget also that the shape of your wire also determines what the current through it is. Fat short wires have a low resistance, a property that affects how well they will conduct electricity, and so will conduct large currents while thin long wires have a larger resistance and so will conduct smaller currents.

All materials have a resistance. Even more basic is a quantity known as resistivity which is the resistance between the faces of a 1 metre cube of the material. Metals with a low resistivity conduct electricity better than ones with a high resistivity. For example the resistivity iron is six times that of silver, and so silver will conduct electricity six times better than iron.

However this does not really explain WHY some metals are better conductors than others. To have a go at this we have to look at a more basic level still.

As you probably know all materials contain minute negatively charged particles called electrons. Many of these electrons are fixed to atoms but in conductors (such as all metals) there are lots of electrons that are not held to any particular atom but are free to wander around randomly in the metal. These are called 'free electrons'.

Lets imagine that we have our metal in the form of a wire. When a voltage is connected across the ends of the metal wire these electrons drift in one direction. It is this drift of free electrons that is the electric current.

schoolphysics: Free electron motion animation

To see an animation of the movement of free electrons in an electric current please click on the animation link.

So, why do certain metals conduct larger currents than others?

Some metals contain more free electrons per cubic metre than others. In general the more free electrons the better the metal will conduct (for a certain applied voltage) although this is not always the case. As well as the number of free electrons the structure of the metal is also important. Collisions between the free electrons and between the free electrons and the atoms of the metal also affect their movement.

These collisions mean that they get scattered. It is a combination of the number of free electrons and how much they are scattered that affects how well the metal conducts electricity.

A really good conductor is one that has lots of free electrons that are not scattered much by the crystal lattice.

It all gets really complicated here but if you want to follow it up further try looking up the Band Theory of Solids and read about energy levels!

There are even more things to think about. Any impurities in the metal and also small molecular dislocations in the structure will alter the conductivity of the metal.