Static electricity and electric fields

When two insulating materials are rubbed together, electrons are rubbed off one material and onto the other.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged, by an equal amount.

For example, rubbing a polythene rod with a cloth moves electrons onto the rod, so the rod becomes negative and the cloth positive.

Two charged objects exert a non-contact force on each other:

• Like charges (positive and positive, or negative and negative) repel.
• Unlike charges (positive and negative) attract.

The force gets stronger as the objects get closer together.

You can see this when a charged rod hung from a thread swings away from another rod with the same charge, or towards one with the opposite charge.

A charged object creates an electric field around itself. Any other charged object placed in the field feels a force. The field is strongest close to the charged object and gets weaker further away.

Around an isolated charged sphere, the field lines are straight lines pointing out from the surface in all directions, like the spokes of a wheel, meeting the surface at right angles.

• For a positive sphere, the arrows point away from it.
• For a negative sphere, the arrows point towards it.

The lines are closest together near the sphere, where the field is strongest.

As charge builds up on an object, the pd between it and the earth gets bigger, and so does the strength of the electric field around it.

When the field is strong enough, the air between the object and an earthed conductor starts to conduct. Electrons jump across the gap, and you see a spark.

This is why you sometimes get a small shock when you touch a metal door handle after walking across a nylon carpet. Lightning is the same thing on a huge scale.