2.1 — Static electricity
Insulators can be charged by friction: electrons are transferred from one material to the other. The material gaining electrons becomes negatively charged; the one losing electrons becomes positively charged.
Like charges repel; unlike charges attract. A charged object creates an electric field around it — the field lines point away from positive and toward negative.
Conductors allow charge to flow; insulators do not. Metals are conductors because of free (delocalised) electrons.
Dangers of static: sparks near flammable gases, electrostatic discharge in electronics. Uses: inkjet printers, electrostatic precipitators, paint spraying.
2.2 — Current and charge
Electric current is the rate of flow of charge. Conventional current flows from + to −; electrons flow from − to +.
Potential difference (voltage) is the energy transferred per unit charge. An ammeter is connected in series; a voltmeter in parallel.
2.3 — Resistance
Ohm's Law: V = IR. Resistance is measured in ohms (Ω). An ohmic conductor has constant resistance (straight I–V graph).
Resistance of a wire increases with: longer length, smaller cross-sectional area, higher temperature, and certain materials. A thermistor has lower resistance at higher temperature. An LDR has lower resistance at higher light intensity.
I–V graphs: ohmic resistor → straight line through origin; filament lamp → curve (resistance increases with temperature); diode → conducts in one direction only (forward bias).
2.4 — Series and parallel circuits
In parallel, total resistance is always less than the smallest individual resistor.
2.6 — Mains electricity and safety
UK mains supply: 230 V, 50 Hz, AC. AC alternates direction; DC flows one way only.
Three-pin plug wires: live (brown) — 230 V; neutral (blue) — ~0 V; earth (green/yellow) — safety, connected to casing.
A fuse is a thin wire in series with the live wire — it melts if current is too high, breaking the circuit. Circuit breakers and RCDs perform the same function but can be reset. Choose the fuse rating just above normal operating current.
Double-insulated appliances have no metal casing connected to mains — they need no earth wire (class II). Metal-cased appliances must be earthed (class I).