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• Objects can become positively charged or negatively charged, usually because of friction between insulators. This is called static electricity.

• Charged objects exert electrostatic forces on each other. These can be attractive forces or repulsive forces.

• Charged objects can attract neutral objects, due to the polarisation of charge.

An electric shock can happen when a person approaches a metal object or another person. This is caused by static electricity.

As well as creating sparks, static electricity can cause other interesting effects like making a balloon stick to a wall or attracting hair. Electrostatic forces can cause objects to levitate.

Have a look at the Electrostatic sparks and shocks guide to find out more.

Experiment 1

Step 1- Rub a balloon on your hair. Your hair will become positively charged and the balloon will become negatively charged.

Step 2- Bring the balloon close to your hair and then move it away. You should find your hair and the balloon are attracted.

Experiment 2

Step 1 - Place an empty tin or can on its side on a table, so it can roll freely.

Step 2- Rub a balloon on your hair or jumper, to charge it.

Step 3 - Bring the balloon close to the can. It should attract the can.

There are two types of electrical charge:

• positive (+)

• negative (-)

Most objects contain equal amounts of positive and negative charges, so they are uncharged or neutral.

Negative charges, known as electrons, can be transferred by friction between insulators. Insulators do not conduct electricity.

When two different insulators are rubbed together, electrons move from one insulator to the other.

The object which gains electrons is now negatively charged. The object which loses electrons is now positively charged.

Have a look through the slide show below to understand more about charging insulators by friction.

To understand why it is the negative charges which are transferred, it is helpful to look at the structure of the atom.

The electrons (negative charges) orbit near the edge of the atom. If the outer electrons have enough energy, they can escape from the atom and be transferred to other objects.

The protons (positive charges) are tightly bound in the nucleus of the atom, so they cannot easily escape.

When an electron escapes from an atom, the atom is no longer neutral because it now has more positive charge than negative charge. We call these charged atoms 'positive ions'.

Negative charges usually move between the insulators when they are rubbed together.

The electrons have a low mass and can escape from the atom easily, so they are mobile.

The positive ions have a much greater mass than the electrons, so it is more difficult for these positive charges to move.

Electrostatic forces between charged objects can cause the objects to attract and repel.

Objects with like charges (both positive or both negative) will repel.

Objects with unlike charges (one is positive and the other is negative) will attract.

Think about the example of rubbing a balloon on a jumper.

The balloon gains electrons and becomes negatively charged.

The jumper loses electrons and becomes positively charged.

Because these objects have opposite charges, there is an attractive force and the balloon and the jumper are attracted to each other.

If two insulators have like charges (the same type of charge), then there is a repulsive force between them.

These balloons are both negatively charged, so they repel each other.

Neutral objects are sometimes attracted by electrostatic forces too.

A charged balloon can stick to a wall, attract a drink can, or deflect a stream of water, even though only the balloon has been electrically charged.

This happens because of a process called polarisation of charge. Since electrons (small, light, negative charges) are mobile, they can move quite easily.

Note that:

• positive ions are much more difficult to move

• when a charged object, like a balloon, is moved close to a neutral object, for example a wall, the electrons are repelled and move further into the object

• this leaves an area of net positive charge on the surface, even though the object is electrically neutral overall

Try these experiments to see some examples of neutral objects being attracted by charged objects. All the experiments involve polarisation of charge.

Experiment 1

Step 1 - Charge a balloon, by rubbing it on your hair or clothing.

Step 2 - Move the rubbed part of the balloon close to a wall. The balloon should be attracted to the neutral wall and stick to it.

Experiment 2

Step 1 - Open a tap very slightly so there is only a thin, gentle stream of water coming from it.

Step 2- Charge a balloon, by rubbing it on your hair or clothing.

Step 3 - Move the balloon close to the stream of water. The water should be attracted towards the charged balloon, so that the stream of water is deflected.

Experiment 3

Step 1- Tear up a sheet of paper into small pieces.

Step 2 - Run a comb through your hair to charge it.

Step 3 - Move the comb close to the pieces of paper. The paper should be attracted to the comb.

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