Electrification: process of giving an object an electric charge (friction, contact, induction).
Conservation of charge: charge can neither be created nor destroyed, only transferred. Quantization: charge exists in whole multiples of e = 1.6 × 10⁻¹⁹ C (q = ne).
Conductor: material with free electrons that allows charge to flow (copper). Insulator: tightly bound electrons, resists charge flow (rubber, plastic).
Electric field: region around a charge where another charge feels a force. Electric field intensity E = F/q, unit N C⁻¹, direction is the direction of force on a positive test charge.
Electric field lines: imaginary lines from positive to negative charge; closer lines mean stronger field; never cross.
Electric potential (potential difference): work done per unit charge, V = W/q (volt = J/C).
Charging by friction: electrons transfer from one material to the other (rubber rod rubbed with fur becomes negative, fur positive). Charging by contact: gives the same sign. Charging by induction: no contact; the induced charge is opposite to the charged object, then the conductor is earthed.
Effect on a test charge: a positive test charge is pushed away from a positive source and pulled towards a negative source. Characteristics (any two): field lines start on positive and end on negative; they never cross; density shows strength; field is uniform between parallel plates; the field has direction and magnitude.
Electroscope detects charge and its sign; leaves diverge because of repulsion between like charges.
Discharging: by conduction (earthing), convection (charged air), corona or spark. A charged insulator is discharged by holding it over a flame because the flame ionises the air, and the ions neutralise the charge. Heating gives the same effect. Moisture in humid air makes the surface conducting so the charge leaks away.
Corona discharge: glow around sharp points of a conductor at high voltage. Lichtenberg figures: fern-like patterns left on an insulator by a high-voltage discharge.
Electrostatic precipitator: charges dust particles and attracts them to plates to clean smoke. Photocopier uses charged drum and toner.
Lightning is a huge discharge between cloud and ground. A lightning rod (pointed metal rod, earthed) gives the charge an easy path to ground. Safe place in a storm: inside a metal car, not under a tree.
Past-paper questions with model answers
What is electric field? How does it affect a test charge? Write two characteristics (every set): use the points above.
Electric potential and work: charge 10 C moved from 120 V to 220 V: W = qΔV = 10 × 100 = 1000 J.
Role of moisture in discharging an insulator; why heating helps an insulator lose its charge; how a charged insulator is discharged on a flame (3 marks each).
Electric field intensity with formula, SI unit and direction (Section C, 2+2+1).
Capacitors 3 F and 4 F in series: Ceq = (3 × 4)/(3 + 4) = 1.71 F.