Static electricity covers the two types of electric charge, how charges interact, and how objects become charged. Every charging process is a movement of electrons, never of protons. The topic covers electric fields and field lines, charging by rubbing and by induction, and the hazards and applications of electrostatics.
It is the foundation for Topic 14 (Current Electricity), because current is simply charge in motion. It appears every year in Paper 1, and often in Paper 2 Section A. Most marks are lost to imprecise wording rather than to missing knowledge.
What these notes cover
- •Electric charge: the two types
- •The law of electrostatics
- •Conductors and insulators
- •Electric fields and field lines
- •Field patterns of point charges
- •Charging by rubbing
- •Charging by induction: two spheres and earthing
- •Hazards: refuelling, lightning and dust explosions
- •The electrostatic precipitator
- •Spray painting and photocopier contexts
- •Misconceptions to unlearn
- •Exam technique for full-mark answers
Key definitions
- Law of electrostatics
- Like charges repel; unlike charges attract. The force acts along the line joining the two charges and gets stronger as the charges get closer.
- Electric field
- A region in which an electric charge experiences an (electric) force.
- Direction of an electric field
- The direction of the force on a small positive test charge placed at that point.
- Electrical conductor
- A material containing many free (mobile) electrons that can move throughout it, e.g. copper, silver, steel, the human body and damp air.
- Electrical insulator
- A material whose electrons are tightly bound to their atoms and cannot move freely, e.g. polythene, Perspex, rubber, dry silk and glass.
Key formulas
Unit of charge
Q is measured in coulombs (C); 1 μC = 10⁻⁶ C
Q is the symbol for charge. One coulomb is a very large charge. A rubbed plastic rod typically carries only around 10⁻⁸ C.
Charge on one electron
e = 1.6 × 10⁻¹⁹ C
Charge is quantised: it comes in whole-number multiples of e. This is enrichment beyond the syllabus.
Charge transferred by n electrons
Q = ne
n is the whole number of electrons gained or lost. This is a school-paper extension beyond the national syllabus.
Number of electrons transferred
n = Q ÷ e
Q is the size of the charge in coulombs and e = 1.60 × 10⁻¹⁹ C.
Conservation of charge
total charge before rubbing = total charge after rubbing = 0
The two rubbed bodies acquire equal and opposite charges. For example, a rod at −8.0 nC means the cloth is at +8.0 nC.
Worked example
A polythene rod is rubbed with a woollen cloth. The rod acquires a charge of −4.8 × 10⁻⁹ C. (a) State the charge on the wool. (b) How many electrons were transferred, and in which direction?
- 1By conservation of charge, the wool loses exactly the electrons the rod gains, so the wool carries +4.8 × 10⁻⁹ C, equal in magnitude and opposite in sign.
- 2Using the size of the charge, n = Q ÷ e = (4.8 × 10⁻⁹) ÷ (1.6 × 10⁻¹⁹) = 3.0 × 10¹⁰ electrons.
- 3The rod became negative because it gained electrons, so 3.0 × 10¹⁰ electrons were transferred from the wool to the polythene rod.
Watch out: Only electrons move. The wool becomes positive by losing electrons, not by gaining positive charge. Writing “positive charges moved” loses the mark.
Common mistakes
Which particle moves?
Wrong: During charging, protons move onto or off the object.
Right: Only electrons are ever transferred, because protons are bound inside the nucleus. An object becomes positive by losing electrons and negative by gaining electrons.
Does attraction prove the object is charged?
Wrong: If a charged rod attracts an object, the object must carry the opposite charge.
Right: A charged rod also attracts neutral objects by inducing charge separation in them, so attraction is ambiguous. Only repulsion is a sure test of charge.
Order of the induction steps
Wrong: Remove the rod first, then separate the spheres or disconnect the earth.
Right: While the rod is still near, separate the spheres or remove the earth connection, and remove the rod last. Otherwise the electrons flow back and the object ends up neutral.
Questions students ask
What is the definition of an electric field in O-Level Physics?
An electric field is a region in which an electric charge experiences an (electric) force. All three ideas must appear: a region, an electric charge and a force. The definition is worth 1–2 marks whenever it is asked.
How do you charge a metal sphere by induction?
Bring a charged rod near the sphere without touching it, so that its free electrons are repelled to, or attracted towards, one side. While the rod is still near, earth the sphere, then remove the earth connection first and the rod last. The sphere ends up with a charge opposite in sign to the rod, and the rod keeps all of its own charge.
Why can electric field lines never cross?
The direction of a field line at a point is the direction of the force on a small positive test charge placed there. If two lines crossed, a test charge at that point would experience a force in two directions at once, which is impossible.
Why is earthing needed when refuelling aircraft?
Friction between the fast-flowing fuel and the pipe transfers electrons, and charge builds up because the tyres and hose linings are insulators. The charge builds until the potential difference is large enough for a spark, which can ignite the fuel vapour. A bonding wire is connected to earth before pumping starts, so charge flows away continuously and no spark can occur.
In the full PDF
- •Field-line diagrams for isolated positive and negative charges, unlike charges, and like charges with the neutral point
- •Step-by-step diagrams of the two-sphere and earthing induction methods
- •Further worked examples on deducing unknown charges, induction sequences, and forces at points A, B and C
- •Hazards of lightning and dust explosions in silos, the electrostatic precipitator diagram, and spray-painting and photocopier questions
- •The remaining misconceptions and full-mark answer templates for exam questions
- •Section summaries covering the whole topic