Static Electricity
Electricity
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
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
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 | negative | repulsive |
| positive | negative | attractive |
| negative | positive | attractive |
Why a charged object attracts a neutral one
- 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
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.