GCSE · Physics · AQA · Spec 8463

Static electricity and charging by friction (physics only)

Pull a jumper off in the dark and you might see tiny sparks. Nothing is plugged in, so where did that electricity come from?

Follow the electrons

Stage 1 of 5: Two neutral insulators. electrons moved, cloth → rod: 0. paused

Two neutral insulators · 1/5scrub the rubbing →

Meet a rod and a cloth. Both are insulators, and both are made of atoms holding equal amounts of positive and negative charge. The positives and negatives cancel out, so each object starts neutral. Tally: rod 0, cloth 0.

Keep an eye on the tally. Whatever the cloth loses, the rod gains, so the two charges always match.

So what happened to the cloth?

A cloth is rubbed on a rod. Afterwards, the cloth is positively charged.

Which is closest to what you think happened to the cloth?

Positive charges were rubbed on to the cloth from the rod.The rubbing made new charge on the cloth. Friction creates it.Only the cloth got charged. The rod is still neutral.Electrons were rubbed off the cloth on to the rod, so the cloth was left with an equal positive charge.

How sure are you?

I'm sureI think soI'm guessing

Positive charges were rubbed on to the cloth from the rod.

This is a common misconception

It's a natural guess: the cloth is positive, so something positive must have arrived. But the positive charges are protons, held tight in the centre of each atom, and rubbing can't shift them. The only particles that move are electrons. The cloth is positive because it lost some of its electrons, so its positive charge is no longer cancelled out.

The rubbing made new charge on the cloth. Friction creates it.

This is a common misconception

Rubbing doesn't create charge. It separates it. Here's the giveaway: the cloth's positive charge always matches the rod's negative charge exactly. If friction were making charge, there'd be no reason for the two to match. They match because every electron that left the cloth ended up on the rod.

Only the cloth got charged. The rod is still neutral.

This is a common misconception

Both objects get charged, every time. The electrons that left the cloth had to go somewhere, and they went on to the rod. So the rod is negative by exactly the amount the cloth is positive. Charging by rubbing always gives you a pair.

Electrons were rubbed off the cloth on to the rod, so the cloth was left with an equal positive charge.

This matches the evidence

Exactly right. The cloth kept all its protons but lost some electrons, so its positive charge now outweighs its negative. And the rod gained exactly those electrons, which is why the two charges are equal and opposite.

Only electrons move. Whatever one object loses, the other gains, so you always end up with one negative object and one equally positive one.

Push apart or pull together?

The rule: the same type of charge repels, different types attract. The skill is working out each charge first. Gained electrons means negative; lost electrons means positive.

Still to sort

Attract (0)

Different types of charge: one negative, one positive.

Where the line is: Look for one object that gained electrons and one that lost them.

Repel (0)

The same type of charge: both negative, or both positive.

Where the line is: Look for two objects with the same electron history: both gained, or both lost.

5 of 5 still to sort.

Each pair has been charged by rubbing. You're only told what happened to their electrons, so that's where to start.

Watch out: Two positives repel too. 'Same type' means both negative or both positive.

Predict, then check

The two rods never touch. Decide what happens before you look.

A rod gained electrons when it was rubbed. It hangs from a thread so it can turn freely. A second rod, which also gained electrons, is brought close to one end of it, but the rods don't touch. What does the hanging rod do?

Now you explain it

A rod is rubbed with a cloth. Afterwards, the rod is negatively charged. Explain how the rod and the cloth became charged. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Rubbing transfers electrons (not 'charge' in general, and not positive charges). (1 mark)
The electrons move from the cloth on to the rod, so the rod gains electrons. (1 mark)
The cloth loses electrons, so it is left with a positive charge equal in size to the rod's negative charge. (1 mark)

Model answer

Rubbing transfers electrons between the two materials. Electrons are rubbed off the cloth and on to the rod, so the rod gains extra electrons, which makes it negative. The cloth has lost those same electrons, so it is left with an equal positive charge.

Name the particle, give its direction, and finish the pair. 'Charge moved' or 'positive charges moved' doesn't earn those points.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Rub two insulators together and electrons hop from one to the other. That one short journey explains the charges, the pushes and pulls, and the sparks.

What you need to know

Rubbing certain insulators together charges them by moving electrons from one material to the other.
Gain electrons: negative. Lose electrons: left with an equal positive charge.
Same type of charge repels; different types attract, and the force acts without contact.
If enough charge builds up, it can jump a gap as a spark.

The big picture

Rubbing certain insulating materials together moves negatively charged electrons off one material and on to the other. The material that gains electrons becomes negatively charged; the one that loses them is left with an equal positive charge. If charge keeps building up, it can jump a gap as a spark. Charged objects exert forces on each other without touching: the same type of charge repels and different types attract, so these are non-contact forces.

Key points

1Electrons are negatively charged. When two insulators are rubbed together, electrons are rubbed off one material and on to the other.

2In charging by rubbing, positive charges don't move. An object becomes positive by losing electrons.

3The two charges are equal in size and opposite in type, because every electron one object loses, the other gains.

4Two charged objects brought close together exert a force on each other: the same type of charge repels, different types attract.

5Attraction and repulsion between charged objects are non-contact forces. A charged object moving when nothing touches it is the evidence.

6Charge that keeps building up on an insulator can jump across a gap to a nearby object as a spark.

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Worked example

Problem

Rod X is rubbed with a cloth, and rod X loses electrons. That cloth is then held near rod Y, which gained electrons when it was rubbed. Do the cloth and rod Y attract or repel?

⚠ Watch out

Saying that positive charges (or protons) moved on to the object that became positive. Nothing positive moves in charging by rubbing. That object is positive because it lost electrons.

🧠

Memory hook

Only the electrons travel. The gainer goes negative; the loser's left positive. Same type? Shove apart. Different types? Pull together.

✓

Check yourself

A rod rubbed with a cloth ends up positive. Which way did the electrons move, what charge does the cloth have, and do the two attract or repel?

Flashcards

(12)

What moves when two insulators are rubbed together?

Electrons: negatively charged particles rubbed off one material and on to the other.

What kind of material becomes charged by rubbing?

Certain insulating materials.

An object gains electrons. What charge does it get?

A negative charge.

An object loses electrons. What charge is it left with?

A positive charge, equal in size to the negative charge on the object that gained them.

Why can't rubbing make an object positive by adding positive charge to it?

The positive charges (protons) stay in the atoms and don't move. Only electrons transfer.

Why are the two charges always equal in size after rubbing?

Every electron one object loses, the other gains. Charge is moved, not created.

Two objects with the same type of charge are brought close. What happens?

They repel.

Two objects with different types of charge are brought close. What happens?

They attract.

What kind of force is the attraction or repulsion between charged objects?

A non-contact force: it acts without the objects touching.

Describe evidence that charged objects exert forces on each other without touching.

A charged rod hung so it can turn freely swings towards or away from a charged object held near it, even though they never touch.

What can happen when a lot of charge builds up on an insulator?

The charge can jump across a gap to a nearby object as a spark.

Why is it called static electricity?

The charge stays where it was put on the insulator instead of flowing away.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Static electricity and charging by friction (physics only)

Follow the electrons

So what happened to the cloth?

Push apart or pull together?

Now you explain it

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