Charging by Friction

Section 1 of 7Atoms and charge

A neutral object

• A neutral object Atoms contain positive protons in the nucleus and negative electrons around it,
  • and in a neutral object the two kinds of charge are equal in number so they cancel out.

Only electrons move

• Only electrons move Protons are fixed inside nuclei, so in solids only electrons can be transferred from one object to another.
• An object that gains electrons becomes negatively charged;
  • an object that loses electrons is left with a positive charge.

Insulators hold their charge

• Insulators hold their charge Plastic, polythene, acetate, cloth and rubber are insulators,
  • so charge put on them stays where it is rather than flowing away.
• This is why charging by rubbing works on insulators.

Section 2 of 7How rubbing charges two insulators

1Rub the surfaces together

• Rub the surfaces together Friction between the two insulators pulls some electrons off the surface atoms of one material.

2Electrons transfer

• Electrons transfer The electrons move across to the other material, for example from an acetate rod to a cloth.

3Opposite charges result

• Opposite charges result The cloth gains electrons and becomes negative, whereas the rod, which has lost electrons, becomes positive.

4Equal and opposite

• Equal and opposite The two objects end up with charges of the same size but opposite sign, because the electrons are conserved.

Section 3 of 7Forces between charged objects

Situation	What happens	Reason
Two positively charged rods	Repel	Like charges repel
Two negatively charged rods	Repel	Like charges repel
A positive rod and a negative rod	Attract	Unlike charges attract
A charged object and an uncharged object	Attract	Charges in the neutral object are separated, so the nearer charge is opposite

Section 4 of 7Demonstrations

Picking up paper

• Picking up paper Combing hair transfers electrons to the comb,
  • which becomes negative while the hair becomes positive;
    • this is why strands of hair may stand apart from each other.
• The negative comb pushes electrons away from the nearer surface of a scrap of paper, leaving that surface positive,
  • so the paper is attracted and jumps up.

Balloon on the wall

• Balloon on the wall Rubbing a balloon on a woollen jumper moves electrons on to the balloon, which becomes negative.
• Near the wall the balloon repels electrons in the wall surface, leaving positive charge nearest it, so the balloon sticks.

Bending a stream of water

• Bending a stream of water A charged rod held near a thin stream of water attracts it,
  • because water molecules have charged regions that are pulled towards the rod.

Section 5 of 7Explaining it in an exam

1

State which materials are being rubbed and that both are insulators.

2

Say which material gains electrons and which loses them.

3

State that the one that gained electrons is negative and the one that lost them is positive.

4

Finish by explaining the observed force using the rule that like charges repel and unlike charges attract.

Section 6 of 7Key terms

Term	Meaning
Electrostatic charge	A build-up of positive or negative charge on an insulating object.
Electron	A negatively charged particle in an atom that can be transferred between objects.
Insulator	A material that does not let charge flow through it easily.
Friction	The rubbing force between two surfaces that causes electrons to be transferred.
Neutral	Having equal amounts of positive and negative charge, so no overall charge.

Section 7 of 7Exam technique

1

Saying that positive charges move from one object to the other. In solids only electrons move.

2

Writing that an object 'makes' or 'creates' charge. Charge is transferred, not created.

3

Forgetting that the loss of electrons gives a positive charge, so the rod becomes positive when electrons leave it.

4

Saying a charged object only attracts other charged objects. It also attracts uncharged objects, but repels only objects with the same charge.

5

Trying to charge a metal rod by rubbing while holding it in the hand, as the charge leaks away through your body.