Electric ChargeCambridge IGCSE Physics: Revision notes

Section 1

Positive and negative charge

There are two types of electric charge: positive and negative.

Two positive charges repel each other
Two negative charges repel each other
A positive charge and a negative charge attract each other

Charge is measured in coulombs (C).

Key termspositive chargenegative chargecoulomb

Section 2

Charging by friction

Rubbing two insulating materials together can transfer charge between them — this is called electrostatic charging by friction.

Crucially, charging by friction always involves the transfer of electrons (negative charge) from one material to the other:

The material that gains electrons becomes negatively charged
The material that loses electrons becomes positively charged (it is left with more protons than electrons)

Simple experiments — such as rubbing a balloon on a jumper — can produce and detect this charge, for example by observing the charged object attracting small pieces of paper.

Key termscharging by friction

Common mistake

It is always electrons that move during charging by friction — never protons. Positive charge appears because electrons are lost, not because positive charge itself has moved.

Section 3

Electric fields

An electric field is a region in which an electric charge experiences a force. The direction of an electric field at a point is defined as the direction of the force that would act on a positive charge placed at that point.

Common field patterns:

Around an isolated point charge: field lines radiate outwards (from a positive charge) or inwards (towards a negative charge)
Around a charged conducting sphere: field lines radiate outward evenly, as if from a point at the centre
Between two oppositely charged parallel plates: field lines run straight and evenly spaced from the positive to the negative plate (away from the very edges)

Key termselectric field

Exam tip

Field direction is always defined using a positive test charge — the same convention as for gravitational and magnetic fields using appropriate test objects.

Section 4

Conductors, insulators and the electron model

Whether a material can be charged easily, and how it responds to charge, depends on whether it is a conductor or an insulator:

Conductors (e.g. metals) contain free electrons that can move through the material, so charge does not stay in one place — this is why metals are hard to charge by friction and quickly lose any charge
Insulators (e.g. plastic, rubber) do not have free electrons able to move through the material, so charge stays where it is placed — this is why insulators are the materials used in charging-by-friction experiments

An experiment can distinguish a conductor from an insulator: connect a charged object to each material in turn and observe whether the charge disperses (conductor) or remains localised (insulator).

Key termsconductorinsulator

Must Know

There are two types of charge, positive and negative; like charges repel, unlike charges attract
Charge is measured in coulombs
Charging by friction always transfers electrons; gaining electrons gives negative charge, losing electrons gives positive charge
An electric field is a region where a charge experiences a force; its direction is the direction of the force on a positive charge
Field patterns exist around a point charge, a charged sphere, and between oppositely charged parallel plates
Conductors let charge move freely (free electrons); insulators keep charge fixed in place

That's the notes covered.

Carry on to the next subtopic.