Charge distribution on the surface of a conductor

What does sitting in a car when lightning strikes have in common with a bunch of babies in dirty nappies? Well, both help to explain where electric charges like to go -- when they can choose. Electric charges, as you surely know, come in two flavours: positive and negative. An electron is a particle with negative charge, and a proton is a particle with positive charge. Every 'thing', every object that is electrically charged creates an electric field, that surrounds it.

An electric field is a bit like a magnetic field. The two types of field are related to each other, but they're not the same thing. The electric field that is generated by the charged object, spreads outward in all directions. It is strongest near to the charged object, and gets weaker and weaker further away. When we make a drawing of an electric field surrounding something that is positively charged, we show the field as pointing away from the charged matter.

An electric field surrounding a negatively charged object, is drawn as pointing towards the charged matter. When a charged object enters into another object's electric field, both particles experience a force. If one particle is positive and one is negative, the objects experience a force towards each other. Opposite charges attract. If both particles are negatively charged, or, if both are positively charged, the force they experience will be away from each other.

Like charges repel. Now, let's gather up a bunch of negatively charged particles -- electrons. We'll let them run wild in this steel ball. Steel is a conductor, so the excess electrons that we charge it with can move around freely and place themselves where they want. Each electron wants to get as far away as it can from all the other excess electrons.

Now, the question is: where will they go? Before we answer that question, let's replace the electrons with little babies, in a circular pen. Each baby is wearing a nappy, that really needs a change. The further away you get from a nappy, the less you experience the smell of it. So each baby wants to get as far away as it can from all the other babies.

Where do they go? If there are two babies - they go to opposite sides of the room. Three babies? Four... five...

six babies... In order to get as far away as possible from all the other babies, they end up along the fence. Same thing with electrons. A bunch of excess electrons in a conductor will spread out evenly on the surface of the conductor. Not a single one of them will be found inside the conductor.

Why? Simple. If an electron would try to move inwards, towards the centre of the ball, it would experience a stronger push -- an increased repulsive force -- from all the electrons on the other side of the ball. The electrons repel each other until they get as far away as they can get. At this stage, the electric fields of all the charged particles balance each other out.

The electrons are now in an state of equilibrium. If you were inside the steel sphere, you would experience no electric field at all. Uh? Being inside a charged conductor ... 'When would this ever be useful?' you ask. Well, if you sit inside a box of metal and a bolt of lightning hits, what's going to happen?

You know it already: In a conductor, any excess electrons will only be found on the surface of the conductor. So the electric charges will move along the outer surface of the car, and then make a jump to the ground. It sounds odd, but it's true: You can remain safe from huge electric charges, by being inside them. This is called a Faraday cage, after British scientist Michael Faraday. People say that he was able explain all about where electrical charges end up... ...

without involving any dirty nappies at all.