This physics grades 9-10 lesson on static electricity notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on static electricity and check yourself with the quiz.
Quick answer: Static electricity is electric charge that builds up on insulators when electrons are transferred by rubbing. Like charges repel and unlike charges attract. A charged object has an electric field around it, and the charge that flows in a current follows Q = I × t, with charge measured in coulombs.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, HS-PS2-4
Static electricity notes: the key ideas
What is static electricity?
A comb pulled through dry hair can lift tiny pieces of paper, and a jumper pulled off in the dark can crackle. These effects come from static electricity: electric charge that stays in one place instead of flowing round a circuit.
Electric charge comes in two kinds, positive and negative. It is measured in coulombs (C). Objects are normally neutral because they hold equal amounts of both kinds. An object is charged when it has gained or lost electrons.
Like charges repel; unlike charges attract. The force is stronger when the charges are bigger or closer together.
How does rubbing charge an object?
Atoms contain positive protons that stay locked in the nucleus and negative electrons that can be pulled away. When two insulators are rubbed together, electrons are scraped off one material and stick to the other. Only electrons move, never protons.
| Pair rubbed together | Gains electrons | Result |
|---|---|---|
| Polythene rod and cloth | Rod | Rod negative, cloth positive |
| Acetate rod and cloth | Cloth | Rod positive, cloth negative |
| Balloon and hair | Balloon | Balloon negative, hair positive |
| Glass rod and silk | Silk | Glass positive, silk negative |
Metals are conductors: their electrons move freely, so charge spreads out or leaks away at once. That is why you only see charging by friction clearly on plastics, glass and rubber. A charged object attracts neutral scraps because its field pushes charge to one side of the scrap, so the nearer side has opposite charge.
Saying that protons move from one material to the other, or that a rod becomes positive by “gaining positive charge”. A rod becomes positive by losing electrons.
What is an electric field?
An electric field is a region where a charged object feels a force. The field direction is the direction of the force on a small positive charge, so field lines point away from positive charges and towards negative ones.
Lines drawn close together show a strong field, which is why sparks form at sharp points. A charged particle placed in a field feels a force: positive particles move along the field lines, and electrons move against them.
Why do sparks happen, and what is the link between charge and current?
When a lot of charge builds up, the field in the air between the charged object and the earth or a conductor becomes so strong that air molecules ionise. The air then conducts for a moment, and the charge rushes across as a spark. Lightning is a huge spark. Sparks are dangerous near fuel vapour, so tankers and aircraft are earthed during refuelling. Electrostatics is also useful: inkjet printers steer charged droplets, smoke precipitators remove dust from chimneys, and spray painting charges droplets so that they coat a surface evenly.
Current is the rate of flow of charge, so charge and current are linked by a simple equation.
Q = I × t
Q = charge (C), I = current (A), t = time (s). One coulomb is the charge carried by a current of 1 A in 1 s. One electron carries 1.6 × 10⁻¹⁹ C.
A torch in Nairobi draws a current of 0.80 A for 25 s. Calculate the charge that flows.
Q = I × t = 0.80 × 25 = 20 C
A balloon carries a charge of 2.4 × 10⁻⁹ C. Calculate how many extra electrons it holds.
N = Q ÷ e = 2.4 × 10⁻⁹ ÷ 1.6 × 10⁻¹⁹ = 1.5 × 10¹⁰ electrons
Convert first: milliamps (mA) ÷ 1000 to amps, minutes × 60 to seconds, nanocoulombs × 10⁻⁹ to coulombs. Then the equation always works.
- Rubbing transfers electrons: the gainer is negative, the loser positive.
- Like charges repel; unlike charges attract.
- Electric field lines point from positive to negative.
- Q = I × t (coulombs = amperes × seconds).
- One electron carries 1.6 × 10⁻¹⁹ C.
Charge, fields and sparks: frequently asked questions
Why does rubbing make an object charged?
Rubbing transfers electrons from one insulator to the other. The material that gains electrons becomes negative and the one that loses them becomes positive. No new charge is created.
What is the unit of electric charge?
The coulomb (C). One coulomb is the charge delivered by a current of 1 ampere in 1 second, and one electron carries 1.6 × 10⁻¹⁹ C.
How do you calculate charge from current?
Multiply current by time: Q = I × t. Use amps and seconds, so 250 mA for 2.0 minutes is 0.25 × 120 = 30 C.
In which direction do electric field lines point?
They point away from positive charges and towards negative charges. They show the direction of the force on a small positive charge placed in the field.
Why are fuel tankers earthed?
Fuel flowing through pipes can build up charge. If it sparked to an unearthed object, the spark could ignite petrol vapour, so an earthing wire lets the charge flow safely to the ground.
