ELECTROSTATICS

All matter contains small negatively charged particles called electrons and when these electrons are taken from one surface to another and stay there, they produce what is known as a static charge. The movement of the electrons can occur by rubbing.

When you rub these objects, you are giving them a static charge and this charge is stored on the plastic. As static charge contains electrical energy, it can make things move.

Notice these objects are plastics. Metals cannot be used since they are conductors of charge and the electrons flow through them rather than stay on them.

POSITIVE AND NEGATIVE CHARGES

If you rub a polythene strip with a cloth, electrons leave the cloth making it positive and stick to the polythene making it negative

If you rub an acetate strip with a cloth, electrons leave the acetate making it positive and stick to the cloth making the cloth negative.

Charging objects is all about moving electrons. If an object gains electrons it becomes negative but if it loses electrons it becomes positive.

If you rub a piece of dry polythene (like a comb) on your jumper and bring it near to paper, the paper jumps up and sticks to the polythene.

If you bring the polythene next to a stream of water, the water is attracted to the polythene.

If you rub a balloon on your jumper it will stick to your jumper.

ACETATEPOLYTHENE

BRINGING CHARGES TOGETHER

The rules are:

POSITIVE AND POSITIVE REPEL

NEGATIVE AND NEGATIVE REPEL

POSITIVE AND NEGATIVE ATTRACT

UNCHARGED OBJECTS ANDCHARGED OBJECTS ATTRACT

WHY DO CHARGED AND UNCHARGED OBJECTS ATTRACT?

POSITIVELY UNCHARGEDCHARGED

If you rub a polythene strip with a wool cloth and hang it up on a thread and bring another charged polythene strip next to it, the two strips repel each other.

If you do the same thing with two acetate strips you also get repulsion.

If you bring a charged polythene strip near a charged acetate strip, you get an attraction.

POLYTHENE

If you bring a charged strip near an uncharged strip then you also get attraction

When you bring a positively charged ball near an uncharged ball, the electrons on the uncharged ball collect on the side facing the positive ball. This causes the positive ball and uncharged ball to attract.

HOW TO TELL WHAT CHARGE IS ON AN OBJECT

We must use repelling as a test for charge.

CHARGING BY INDUCTIONThis involves separating charges.

In this example the negatively charged balloon is brought near a wall that has no charge. The negative balloon repels any electrons on the wall surface into the wall so the wall surface is now positively charged. The negative balloon now sticks to the positive wall surface.

REPEL

This object must be positive

This object must be negative

REPEL

If you hang a light positively charged balloon near to an object and the balloon repels then the object must be positive.

If you hang a light negatively charged balloon near to an object and the balloon repels then the object must be negative.

WALL

In the example below a negatively charged strip is brought near an uncharged sphere. The electrons build up on the right hand side of the sphere but when you bring your finger to touch the right hand side of the sphere, your finger acts as a conductor and the electrons flow to the ground. This leaves the sphere with a positive charge.

THE ELECTRIC FIELDThis is a region around an electric charge in which another charge experiences a force.

An electric field is from a positive region to a negative region.

Here are the field patterns between two electrodes.