Static electricity – GCSE Combined Science Physics

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Rub a balloon on your hair and your hair stands up. Hold the balloon near a wall and it sticks without any glue. Walk across a carpet and touch a metal door handle, and you might feel a small zap.

All of these are caused by static electricity. Static means staying still, so this is electric charge that builds up on an object and stays there instead of flowing round a circuit. It is different from the current in a wire.

To see how it works, we need to look inside an atom. An atom has a nucleus in the middle, made of protons, which are positive, and neutrons, which have no charge. Electrons, which are negative, move around the outside.

Normally an atom has the same number of protons and electrons, so the positive and negative charges cancel out and the atom is neutral. Neutral means no overall charge. Think of two equal teams in a tug of war, where neither side wins, so the atom as a whole has no pull.

By the end of this lesson you will be able to do three things. You will explain, using electrons, how rubbing gives an object a charge, and say what happens between charges. You will also describe an electric field, and explain how sparks, earthing and paint spraying work.

Charging by rubbing

Insulators are materials that do not let charge flow through them easily, such as plastic, rubber and dry cloth. When you rub two insulators together, the surfaces drag against each other. This is friction, and it pulls electrons off one surface and onto the other.

Only electrons move. Protons are stuck inside the nucleus, and a proton is about 1800 times heavier than an electron, so the light electrons are the ones that get pulled away. This is why we only ever talk about electrons moving, and never about protons.

The material that gains electrons now has more negative charge than positive, so it becomes negatively charged. The material that loses electrons has more positive than negative, so it becomes positively charged. The two objects end up with equal and opposite charges.

Picture rubbing a plastic rod with a dry cloth. Electrons move from the cloth to the rod, so the rod is negative and the cloth is positive. With a different pair of materials, the electrons might move the other way.

No charge is made or destroyed, it is only moved from one place to another. Count the electrons that left the cloth and you have counted the electrons that arrived on the rod. That is why the two charges are equal in size.

Forces between charges

Charged objects push or pull on each other without touching. This is a non-contact force. Like charges repel, which means push apart, and unlike charges attract, which means pull together.

Like charges means two of the same sign, such as two negative balloons. Unlike charges means one of each, such as a positive rod and a negative ball. Think of magnets, where the same ends push apart and opposite ends pull together.

The size of the force depends on the distance between the charges. The closer the charges are, the stronger the force. Move them further apart and the force gets weaker.

A charged object can also attract a neutral one. A charged balloon pulls the charge in a wall slightly out of position, so the side of the wall nearest the balloon has the opposite charge. The nearer charges pull harder than the ones further away, so the balloon sticks.

The same idea explains why a rubbed ruler picks up tiny bits of paper. The paper stays neutral overall. Its charge just shifts a little, and that is enough to make a small pull towards the ruler.

Electric fields

An electric field is the region around a charged object where another charged object feels a force. You cannot see it, just as you cannot see gravity. A field can act across empty space, so the objects do not have to touch.

Scientists draw field lines to show a field. The arrow on a line shows the direction of the force on a small positive charge placed there. The lines point away from a positive charge and towards a negative charge.

The spacing of the lines shows the strength of the field. Where the lines are close together the field is strong, so the force is large. Where they are far apart the field is weak.

Near the surface of a charged sphere the lines are close together, so the field is strongest there. Further away the lines spread out and the field gets weaker. So the force on a charge is greatest when it is close to the charged object.

Think of people leaving a stadium through a gate. Right at the gate they are packed close together, and further away they spread out. Field lines are packed close together near a charge and spread out as you move away.

Sparks

Rubbing can build up a lot of charge on an insulated object. Insulated means the charge cannot escape, because the object is not connected to anything that conducts. As the charge grows, the potential difference between the object and its surroundings gets bigger.

Potential difference is the push that makes charge move. If it gets large enough, the air between the object and something nearby is ionised. Ionised means electrons are pulled off the air particles, so the air can conduct.

Then charge jumps across the gap as a spark. You see a flash and hear a crack, because the moving charge heats the air very quickly. The small zap from a door handle is a tiny spark that jumps through the air to your hand.

Lightning is a giant spark. Charge builds up in a storm cloud, and when the potential difference between the cloud and the ground is big enough, the air ionises and a huge current flows.

A spark in a classroom is usually harmless, but in the wrong place it can be very dangerous. A spark can set fire to a flammable gas or fuel vapour. That is why charge has to be controlled in some places.

Earthing

Earthing means connecting an object to the ground with a conductor, such as a copper wire. The Earth is so big that it can take or give as many electrons as needed. So charge can flow away safely, and the object cannot hold on to a large charge.

If an object has extra electrons, they flow along the wire into the ground. If it is short of electrons, electrons flow up from the ground to fill the gap. Either way the object becomes neutral, so no large potential difference can build up.

Take a fuel tanker. As fuel flows through the pipe, friction builds up charge on the tanker. A spark could set fire to the fuel vapour, which is the gas given off by the fuel.

To stop this, the tanker is connected to the ground with an earth wire before refuelling. The charge flows away through the wire instead of building up. Aircraft are earthed in the same way.

In an exam, say that the wire is a conductor and that charge flows to earth, so there is no big potential difference and no spark. Do not just say that earthing stops static. Say why it works.

Uses and nuisances of static

Static electricity can be useful as well as dangerous. One good use is electrostatic paint spraying, which is used on cars and bikes. The spray gives every paint droplet the same charge.

Because the droplets all have the same charge, they repel each other. This makes them spread out into a fine, even mist. The object being painted is given the opposite charge, so it attracts the droplets.

The droplets are pulled towards the object from all directions, even round the back. This gives an even coat with less paint wasted, because the droplets are pulled onto the object and not off into the air. Think of a magnet pulling iron filings onto itself.

Static can also be a nuisance. You can get a small shock from a car door handle after sliding across a seat, and clothes can cling together after a tumble dryer. These are small effects, but the same build-up of charge is dangerous near fuel, which is why the rules for refuelling are strict.

Two safe habits follow. Earth anything that could build up charge near a fuel or a flammable gas. In an exam, always give the reason in terms of electrons or charge, and not just the word static.

Recap

Rubbing two insulators moves electrons from one to the other. The one that gains electrons is negative, and the one that loses them is positive. Count the electrons that left one object and you have counted the electrons that arrived on the other.

Like charges repel and unlike charges attract, and the force is stronger when the charges are closer together. A charged object can also attract a neutral one, because it shifts the charge inside it.

Field lines show the direction of the force on a small positive charge, and they are close together where the field is strong. A spark happens when the air ionises, and earthing lets charge flow away safely, so that no big potential difference can build up.

How this is asked in an exam: explain questions give a short story, such as a rod rubbed with a cloth, and ask what moved and why. Use the words electrons, gain or lose, and repel or attract. For earthing, say that the wire conducts the charge away so a spark cannot ignite the fuel.