Electric Charge — Static Electricity | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Charging by friction, electric fields and uses/dangers of static electricity tested as shorter questions.

Three states of charge

A particle (or an object) can be:
- positively charged: has fewer electrons than protons
- negatively charged: has more electrons than protons
- neutral (uncharged): has the same number of electrons and protons, so the charges exactly cancel
An ordinary atom is neutral because the number of orbiting electrons matches the number of protons in its nucleus; the equal-but-opposite charges add to zero overall

How an object becomes charged

The only charges that can move at room temperature are electrons, since protons are locked inside the nucleus
Two consequences follow:
- Gaining electrons leaves an object with a net negative charge
- Losing electrons leaves an object with a net positive charge
The shortcut to remember this is the maths: subtracting a negative from zero leaves a positive

0 − (−1) = +1

Exam tip

Explaining how friction makes an object positively charged

What comes up: a question states that rubbing transfers charge between two materials and asks you to explain how one of them (usually the cloth) ends up positively charged.

Write (two marks): (1) electrons are transferred from the cloth to the rod (negative charge transfers); (2) the cloth has lost electrons, leaving it with a net positive charge.

Watch out: the mark scheme rejects any suggestion that protons move — stating "the cloth gains protons" or "protons are transferred" scores zero. Positive charge is always the result of losing electrons, never of gaining protons.

Forces between charges

Two charged objects placed near one another exert an electric force on one another without touching, which is a non-contact force
The direction of the force depends on the signs:
- Like charges repel (both positive, or both negative → they push apart)
- Opposite charges attract (one positive, one negative → they pull together)
The size of the force grows as the charges are brought closer together and shrinks as they are moved apart

Charge of object 1	Charge of object 2	Force is…
positive	positive	repulsive
negative

Exam tip

Demonstrating that an object is charged

What comes up: you are asked to describe an experiment that could show a charged object carries charge.

Write (two marks): (1) bring the object close to a suitable uncharged insulator (e.g. small pieces of paper, a fine water stream, hair, or a suspended second rod); (2) attraction of that uncharged object to the charged one demonstrates charge is present.

Watch out: the mark scheme rejects an answer based on repulsion alone (unless the answer specifically uses a gold-leaf electroscope, where deflection in either direction is accepted). Showing attraction is the safe default.

A charged object also exerts a force on an apparently uncharged object; for example, a rubbed comb picks up small pieces of paper
The mechanism is induced charge separation:
- Inside the neutral object, the electrons are free to shift a little, even if the object as a whole has no net charge
- When a positively charged comb is brought near, the loose electrons in the paper drift towards the comb, leaving the far side of the paper slightly positive
- The negative near-side is now closer to the comb than the positive far-side, so the attractive force on the near-side wins out and the paper is pulled in
The same trick explains how charged balloons stick to walls, why dust clings to a television screen, and why a rubbed plastic ruler bends a thin stream of water from a tap

Static Electricity

Exam Frequency Analysis

Three states of charge

How an object becomes charged

Forces between charges

Why a charged object attracts a neutral one