Static charge and electric fields

Atoms and charge

An atom is electrically neutral — equal numbers of positive protons and negative electrons. Protons are fixed in the nucleus; electrons can move (conduct) or transfer between insulators (charge by friction).

Charging by friction

When two suitable insulators are rubbed together, electrons transfer from one to the other.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.
• Equal and opposite charges — overall conservation of charge.

Classic example: a polythene rod rubbed with a duster. The polythene gains electrons → negative; the duster loses electrons → positive.

Forces between charges

• Like charges repel.
• Opposite charges attract.
• A charged object can also attract a small uncharged object by induction — the charges in the uncharged object rearrange (e.g. a charged comb attracts pieces of paper).

Electric fields

A charged object is surrounded by an electric field — the region in which another charge would feel a force. Field lines:

• Point from + to −.
• Are perpendicular to the surface of a charged sphere.
• Closer field lines = stronger field.
• A radial field exists around a point charge (or sphere); a uniform field exists between parallel plates.

Field strength decreases with distance — closer charges experience a larger force.

Hazards and uses

• Hazard: a person walking on a synthetic carpet builds up charge; touching a metal handle gives a small shock. In refuelling, a spark can ignite vapour — earthing is essential.
• Uses: photocopiers (charge attracts toner), electrostatic paint sprayers (paint droplets repel each other → even coverage; attracted to oppositely-charged car panel).
• Lightning conductors safely earth charge from clouds → reduces strike damage.

OCR exam tip

For a 4-mark question on a paint sprayer, structure your answer: (1) paint droplets all gain same charge, (2) repel each other → fine spray, (3) car body opposite charge, (4) attracts paint → even coat, less waste.