Static Electricity: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 15 · Read time: ~8 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Charging by friction
When two insulators are rubbed together, electrons are transferred from one to the other. The object that gains electrons becomes negatively charged, and the object that loses electrons becomes positively charged, with an equal amount of charge.
For example, rubbing a polythene rod with a cloth transfers electrons from the cloth to the rod, so the rod becomes negative and the cloth becomes positive.
Conductors (metals) allow electrons to move freely through them, so a charge placed on a conductor spreads out or flows away to earth. Insulators (plastic, glass, rubber) hold charge where it's placed.
Trap: Only electrons move. Never write that 'positive charges were transferred' or 'protons moved to the cloth'. An object becomes positive by losing electrons, not by gaining protons. This one word costs a lot of marks.
Charging by induction
A conductor can be charged without touching it with the charged object. To charge an isolated metal sphere positively using a negative rod:
1. Bring the negative rod near (not touching) the sphere. Electrons in the sphere are repelled to the far side. 2. Earth the far side by touching it with a finger, so the repelled electrons flow away to earth. 3. Remove the earth connection first, then remove the rod. The sphere is left with an overall positive charge, which spreads evenly over its surface.
Induction also explains why a charged rod attracts small, neutral pieces of paper: the near side of the paper gets the opposite charge, and the attraction to that near side is stronger than the repulsion from the far side.
Trick: The charge you end up with by induction is always the opposite of the charge on the rod. Use that as a quick check on your final answer.
Electric fields, uses and hazards
An electric field is a region where a charge experiences an electric force. Field lines point in the direction of the force on a small positive charge: away from positive charges and towards negative charges.
Uses: electrostatic precipitators remove smoke particles from factory chimneys, electrostatic spray painting gives an even coat with less waste, and photocopiers and laser printers use charged drums to attract toner.
Hazards: lightning, and sparks when refuelling aircraft or vehicles, which could ignite fuel vapour. The fix is earthing, connecting the object to the ground with a conductor so excess charge flows away safely.
Trap: When drawing field lines, the arrows must point from positive to negative and the lines must never cross. Arrows pointing the wrong way lose the mark even if the pattern is right.
Worked Example
A polythene rod is rubbed with a dry woollen cloth and becomes negatively charged with a charge of 3.2 × 10⁻⁹ C.
(a) Explain how the rod becomes negatively charged. [2]
Friction between the rod and the cloth causes electrons to be transferred from the cloth to the rod.
The rod gains extra electrons, so it has more negative charge than positive charge.
(b) State the charge on the cloth after rubbing. [1]
+3.2 × 10⁻⁹ C (equal in size and opposite in sign to the rod's charge).
(c) Calculate the number of electrons transferred. The charge on one electron is 1.6 × 10⁻¹⁹ C. [2]
n = Q ÷ e
= 3.2 × 10⁻⁹ ÷ 1.6 × 10⁻¹⁹
= 2.0 × 10¹⁰ electrons
Why this question is a good test of the topic: part (a) is where most students drop a mark by saying charges or protons moved. The calculation in part (c) is easy once you know the rod's charge and the electron's charge, the marks are really in the explanation.
Frequently Asked Questions
How does an object become charged by friction?
Rubbing two insulators together transfers electrons from one to the other. The one that gains electrons becomes negative, the one that loses electrons becomes positive.
Do protons move when an object is charged?
No. Only electrons move. An object becomes positively charged by losing electrons, not by gaining protons.
How do you charge a metal sphere by induction?
Bring a charged rod near the sphere, earth the sphere while the rod is still near, remove the earth connection, then remove the rod. The sphere ends up with the opposite charge to the rod.
Which way do electric field lines point?
In the direction of the force on a small positive charge: away from positive charges and towards negative charges.
Why are aircraft earthed during refuelling?
Fuel flowing through the pipe can build up static charge. Earthing lets the charge flow safely to the ground so a spark can't ignite the fuel vapour.
Still unsure about charging by induction?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author, with a team of tutors.