Static electricity
Charge, electric fields, rubbing, induction, hazards and the electrostatic precipitator for G3 Pure and O-Level Physics.
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Before you begin
Rub a balloon and it clings to a wall; pull off a jumper and it crackles. This topic explains where that charge comes from, how it pushes and pulls through an electric field, how a metal can be charged without being touched, why charge that builds up can be dangerous, and how it cleans the smoke from a power station.
Be comfortable with: forces, and atoms as positive nuclei surrounded by negative electrons.
Learning goals
- State that charges are positive or negative and are measured in coulombs
- State that like charges repel and unlike charges attract
- Describe an electric field as a region where charge experiences force
- Recall that the field lines of an isolated point charge give the direction of the force on a positive test charge
- Draw the directed electric field of an isolated point charge
- Interpret the electric-field pattern between two isolated point charges
- Draw the electric-field pattern between two isolated point charges
- Explain charging by rubbing as electron transfer
- Describe experiments that charge a conductor by induction
- Describe potential electrostatic-charging hazards
- Describe an electrostatic precipitator and transfer the principle
Syllabus statements covered
- state that there are positive and negative charges and that charge is measured in coulombs
- state that unlike charges attract and like charges repel
- describe an electric field as a region in which an electric charge experiences a force
- draw the electric field of an isolated point charge and recall that the direction of the field lines gives the direction of the force acting on a positive test charge
- draw the electric field pattern between two isolated point charges
- show an understanding that electrostatic charging by rubbing involves a transfer of electrons
- describe experiments to show electrostatic charging by induction
- describe examples where electrostatic charging may be a potential hazard
- describe the use of electrostatic charging in an electrostatic precipitator and apply the use of electrostatic charging to new situations.
Lessons
Work through them in order.
Charge and induction foundations
- Charge and charging by rubbingPositive and negative charge, and why rubbing two insulators leaves them with equal and opposite charges.
- Conductors, insulators and earthingWhy a metal held in your hand will not stay charged, and how earthing lets charge flow to or from the ground.
- Charging a conductor by inductionCharge a metal sphere without touching it, by earthing it or by separating two spheres, and why the order of the steps matters.
Electric fields
Hazards and applications
- Electrostatic hazardsHow charge that builds up becomes a spark that can light fuel or dust, and how earthing and lightning conductors make it safe.
- Electrostatic precipitators and applicationsHow an electrostatic precipitator cleans smoke, and how the same charge-then-attract idea works in new situations.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | What to remember |
|---|---|
| Electric charge | positive or negative; measured in coulombs (C) |
| Forces between charges | like charges repel; unlike charges attract |
| Charging by rubbing | electrons move from one material to the other; gain electrons → negative, lose → positive |
| Electric field | a region in which an electric charge experiences a force |
| Field direction | the direction of the force on a positive charge: away from +, towards − |
| Charging by induction | rod near → earth on → earth off → rod away; the conductor ends with the opposite sign |
| Hazards | charge builds up → spark → ignites fuel vapour or dust; earthing lets the charge away |
| Precipitator | charge the dust → attract it to earthed plates → shake it into a hopper |
Core knowledge to remember
- There are positive and negative charges, and charge is measured in coulombs.
- Unlike charges attract and like charges repel.
- An electric field is a region in which an electric charge experiences a force.
- The direction of a field line is the direction of the force on a positive test charge.
- Field lines go from positive to negative charges, never cross, and are closer together where the field is stronger.
- Charging by rubbing transfers electrons. Protons do not move.
- In charging by induction, the earth connection must be removed before the charged rod.
- Electrostatic charging is a hazard wherever a spark could ignite fuel vapour or dust; earthing prevents the build-up.
- An electrostatic precipitator charges dust particles and attracts them to collecting plates.
Common mistakes
- Saying protons or positive charge move. In every process in this topic, only electrons move.
- Saying a neutral object has no charge. It has equal amounts of positive and negative charge.
- Drawing field arrows from − to +, or drawing field lines that cross.
- Removing the charged rod before the earth connection during induction, which leaves the conductor neutral.
- Saying static charge itself explodes. The charge makes a spark, and the spark ignites something flammable.
- Describing a precipitator without saying how the dust is charged, or how it is removed from the plates.