SP11 · Static electricity · Topic 11 — Static electricity | GCSE Edexcel Separate Sciences

Revise the key ideas

Rubbing two suitable insulating materials can transfer electrons between them. The material gaining electrons becomes negatively charged; the material losing electrons has an equal positive charge if no charge is lost elsewhere.
Rubbing suitable insulators transfers electrons, not protons. An object becomes positive when it has lost electrons (has an electron deficit); no new positive particles are created.
Charge is conserved: equal and opposite changes do not create net charge from nothing. Insulators retain localised charge because charges cannot move freely through the whole material; conductors allow mobile charge to redistribute.
Like charges repel and unlike charges attract. The force is non-contact and acts through an electric field; attraction alone does not prove that two objects carry opposite net charges.
A charged object can attract a neutral object by induction. Charges in the neutral object move or shift slightly (polarisation), making the near side effectively oppositely charged. Attraction to the near side is stronger than repulsion from the far side.
A rubbed balloon can stick to a wall, and a charged comb can pick up small pieces of paper. They cause charges in the neutral material to shift slightly (polarise). The wall or paper does not need a permanent overall charge.

A large potential difference can make air conduct and allow a sudden charge transfer, producing a spark. Everyday shocks after walking on a carpet arise from a charge imbalance discharged when a conducting path forms.
Lightning is a large electrical discharge associated with charge separation in clouds and between clouds or ground. It transfers charge through ionised air; it is not frictional heating of rain alone.
Earthing connects an object to Earth through a conductor. Electrons can move to Earth from a negatively charged object or from Earth to a positively charged object, reducing the imbalance.
Electron direction depends on the initial sign of the object's charge.
During fuel transfer, charge build-up and sparks can ignite flammable vapour. Bonding conducting parts and earthing suitable equipment reduce dangerous potential differences and charge accumulation.
An electrostatic sprayer gives droplets the same charge so they repel one another and spread out. Attraction towards an appropriately charged or earthed target can improve coverage and reduce waste; use appropriate safety controls.
Electrostatic methods can be useful, but evaluate exposure, ignition risks, target coverage and unintended drift in context. Charge effects do not remove all problems with an insecticide.

An electric field is a region where an electric charge experiences a force. Field direction is defined as the direction of force on a small positive test charge; a negative charge experiences force in the opposite direction.
Electric field lines point outwards in all directions from a positive point charge and inwards towards a negative point charge. They show the field direction; they are not physical threads or compulsory paths for particles.
The direction is the force direction for a positive test charge; these are separate isolated-charge diagrams.
Between large parallel oppositely charged plates, the central field is approximately uniform, with straight parallel equally spaced lines from the positive plate to the negative plate. Near the edges the field curves.
Edge effects are omitted from this central-region model.
Closer field-line spacing represents a stronger field; the number and density of drawn lines are a representation, not a count of electrons. Lines do not cross because the field has one direction at a given point.
The field model explains non-contact forces and induction: charges affect nearby charges through the field even when objects do not touch. Force direction depends on both field direction and charge sign.
In simple qualitative diagrams, use arrowheads and +/− labels so colour alone is not needed. Distinguish conventional field direction from electron motion.