Section 1 of 4Charging by Friction
Static charge builds up on insulators when they are rubbed against each other.
Friction
- Friction When two insulating materials are rubbed together, electrons are transferred from one material to the other.
Positive charge
- Positive charge The material that loses electrons becomes positively charged, because it now has more protons than electrons.
Negative charge
- Negative charge The material that gains electrons becomes negatively charged, so the two materials carry equal and opposite charges.
Conservation
- Conservation The total charge stays the same, so the charge gained by one material always equals the charge lost by the other.
Section 2 of 4Discharge and Sparks
Static charge stays on an insulator until it has a path to flow away.
1
Charge builds up on an insulator because electrons cannot flow freely through it to the earth.
2
When the potential difference becomes large enough, electrons jump across the gap as a spark, which is a rapid discharge.
3
Lightning is a large-scale electrostatic discharge between clouds, or between a cloud and the ground.
4
Earthing a charged object provides a conducting path for charge to flow away safely, so the build-up is removed.
Section 3 of 4Forces and Uses
Electrostatic forces are useful in some industrial processes but dangerous in others.
Forces and Uses
Attraction and repulsion
- Attraction and repulsion Like charges repel each other,
- while unlike charges attract,
- which explains why a charged object can pick up small neutral pieces of paper.
Photocopiers
- Photocopiers Charged toner particles are attracted to a charged drum, so they can be transferred onto paper.
Spray painting
- Spray painting Paint droplets are given a charge, so they are attracted to an earthed object and coat it evenly.
Hazards
- Hazards Sparks from static electricity can ignite flammable gases or dust, so fuelling tankers must be earthed.
Section 4 of 4Exam Checklist & Tips
2.4.2 – Key Points for the Exam
- Static electricity A build-up of electric charge on the surface of an insulator.
- Electron A negatively charged particle that can be transferred between materials by friction.
- Earthing Connecting a charged object to the ground so that charge can flow away.
- Electrostatic discharge A rapid transfer of charge that happens when the potential difference becomes large enough.
- Insulator A material that does not allow charge to flow through it easily.
- Electric charge
- A property of particles that causes electric forces;
- electrons carry negative charge.
- A property of particles that causes electric forces;
Common Mistakes
- Saying protons are transferred during friction;
- only electrons move, because protons are held firmly in the nucleus.
- Describing a positively charged object as having gained protons; it has lost electrons.
- Thinking that friction creates charge; friction only transfers existing charges from one material to another.
- Forgetting that static charge can only be removed quickly when there is a conducting path to earth.