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.

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

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

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

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