4.2.1Electricity and Magnetism

Electric Charge — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 10 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

4.2.1 Electric Charge — IGCSE PHYSICS notes: Describe the two kinds of charge, the forces between them, charging by friction, conductors and insulators, and electric fields.Types — there are two kinds of electric charge, positive and negative.Inside an atom — electrons are negati…

Only electrons move
Non-contact force
Polythene rod and cloth
Acetate rod and cloth
Charge stays
Conductor test

Part 1 of 9Two kinds of charge

Two kinds of charge

Types

Types there are two kinds of electric charge, positive and negative.

Inside an atom

Inside an atom electrons are negative, protons are positive and neutrons are neutral.
An atom has equal numbers of protons and electrons, so its charges cancel and it is neutral overall.

Unit

Unit electric charge is measured in coulombs (C) (Extended).

Only electrons move

Only electrons move protons stay fixed in the nucleus, so charging always involves electrons being gained or lost.

Part 2 of 9Forces between charges

Rule

Rule opposite charges attract and like charges repel.

Non-contact force

Non-contact force the force acts at a distance, like the force between magnetic poles, but it is a different phenomenon.

Object 1	Object 2	Result
positive	positive	repel
positive	negative	attract
negative	positive	attract
negative	negative	repel

Part 3 of 9Charging by friction

1Process

Process rubbing two different insulators together transfers electrons from one to the other.
The object that gains electrons becomes negative.
The object that loses electrons becomes positive.
The two objects end with equal and opposite charges, so they attract each other.

2Polythene rod and cloth

Polythene rod and cloth electrons move from the cloth to the rod, so the rod is negative and the cloth is positive.

3Acetate rod and cloth

Acetate rod and cloth electrons move from the rod to the cloth, so the rod is positive and the cloth is negative.

4Charge stays

Charge stays insulators cannot let the charge flow away, so it stays where it was made.

Part 4 of 9Conductors and insulators

Conductor

Conductor a material that lets charge (electrons) flow through it easily, such as a metal.
In a metal the outer electrons are delocalised (free), so they can move through the lattice of positive ions.

Insulator

Insulator a material that does not let charge flow easily, such as plastic, rubber, glass and dry wood.

Earthing

Earthing connecting a charged conductor to the ground lets electrons flow to or from earth and discharges it.

Part 5 of 9Testing materials and detecting charge

Conductor test

Conductor test touch a charged gold-leaf electroscope with the material.
A conductor makes the leaf fall quickly because charge flows away.
An insulator leaves the leaf raised or lets it fall only slowly.

Detecting charge

Detecting charge a charged object attracts small pieces of uncharged paper.

Finding the sign

Finding the sign a charged rod is tested by seeing whether it is repelled by a rod of known charge.
Repulsion is the reliable test because an uncharged object is also attracted.

Part 6 of 9Testing a rod of unknown charge in order

Charge a reference rod of known sign (for example polythene, negative) and hang it from a stirrup.

Bring the test rod near one end of the suspended rod.

Observe whether the suspended rod moves away or towards the test rod.

Conclude that repulsion means the test rod has the same sign as the reference rod.

Part 7 of 9Electric fields (Extended)

Electric field

Electric field a region of space in which an electric charge experiences a force.

Direction

Direction the direction of the field at a point is the direction of the force on a positive charge placed there.

Field lines

Field lines they go outward from a positive point charge and inward to a negative point charge.

Parallel plates

Parallel plates between two oppositely charged plates the field is uniform, with parallel, equally spaced lines from + to −.

Part 8 of 9Uses and dangers of static charge

Uses

Uses photocopiers, printers and paint spraying rely on charged particles being attracted to oppositely charged surfaces.

Dangers

Dangers lightning and sparks can ignite fuel, so fuel pipes and tankers are earthed.

Part 9 of 9More from the specification

Key terms

Term	Meaning
Electric charge	Property of matter that causes electric forces; positive or negative, measured in coulombs (C).
Charging by friction	Rubbing two insulators so that electrons transfer and the objects gain opposite charges.
Conductor	Material that allows charge to flow through it easily.
Insulator	Material that does not allow charge to flow through it easily.
Earthing	Connecting a conductor to the ground so that charge flows away.
Electric field	Region of space in which a charge experiences a force.

Avoid these mistakes

Saying positive charge moves in a solid — only electrons move, so a positive object has lost electrons.

Saying attraction proves two objects are oppositely charged — an uncharged object is attracted too, so only repulsion is a sure test.

Mixing up electric and magnetic forces — they are different phenomena.

Drawing electric field lines pointing the wrong way — they run from positive to negative.

Last updated: 15 October 2026