Charging by friction
- When certain insulating materials are rubbed against each other, they become electrically charged.
- Negatively charged electrons are rubbed off one material and onto the other.
- The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.
- Only electrons move: protons stay in the nuclei of the atoms.
Forces between charges
- When two charged objects are brought close together, they exert a force on each other.
- Like charges repel; unlike charges attract.
- This is a non-contact force: the objects don't need to touch.
Electric fields
- A charged object creates an electric field around itself. The field is strongest close to the object.
- A second charged object placed in the field experiences a force, which gets stronger as the distance between the objects decreases.
- The field around an isolated charged sphere is drawn as straight lines going outwards (from positive) or inwards (to negative), like the spokes of a wheel.
Sparks
- If the charge builds up enough, the electric field can become strong enough to make the air conduct.
- Electrons jump across the gap as a spark, for example between a charged object and an earthed conductor.
- Sparks can ignite fuel vapour, so fuel tankers are earthed while refuelling.
Key terms
- Static electricity
- Charge that builds up on an insulated object.
- Electron
- A negatively charged particle that can be transferred by friction.
- Repel
- Push apart; like charges repel.
- Attract
- Pull together; unlike charges attract.
- Electric field
- The region around a charged object where another charged object feels a force.
- Non-contact force
- A force that acts without the objects touching.
- Earthing
- Connecting an object to the ground so charge can flow away safely.
- Spark
- A flow of charge through the air when the electric field is strong enough.