Electric current: rate of flow of charge, I = Q/t. Unit ampere = one coulomb per second. Conventional current flows from positive to negative; electron flow is opposite.
DC: current in one direction only (cells, batteries). AC: current that reverses direction continuously (mains, 50 Hz).
Potential difference V = W/q. Volt = joule per coulomb. EMF: energy given by a source to each coulomb of charge (device converting non-electrical energy to electrical). The p.d. across a source in a closed circuit is less than its emf because of internal resistance.
Ohm's law: at constant temperature, current through a conductor is directly proportional to the potential difference across it: V = IR.
Resistance: opposition to flow of current, unit ohm (Ω). Resistivity: resistance of a wire of unit length and unit cross-section, unit Ω m.
Temperature coefficient of resistance: change in resistance per unit resistance per kelvin (unit K⁻¹).
Formulas
I = Q/t, V = W/q, V = IR, R = ρL/A, A = πd²/4, ρt = ρ0 (1 + αΔT). 1 Ah = 3600 C.
Must-know points
Resistance increases with length and temperature (metals), decreases with area. Semiconductor resistance decreases when temperature rises.
Ammeter is connected in series (low resistance); voltmeter in parallel (high resistance).
Non-ohmic devices (diode, bulb filament, thermistor) give a curved I-V graph; ohmic conductors give a straight line through the origin.
Two differences between d.c. and a.c.: d.c. has constant direction, a.c. reverses direction; d.c. comes from cells, a.c. from generators; a.c. can be stepped up or down by transformers, d.c. cannot.
Why a.c. is preferred for domestic use (three reasons): it can be transformed to high voltage for cheap, low-loss transmission; it is cheap and easy to generate; it can be converted to d.c. easily and works motors and heaters.
A current-carrying wire is neutral because the number of protons equals electrons inside it.
Past-paper questions with model answers
State Ohm's law; resistor 10 Ω with 2 A: V = IR = 20 V. 12 V and 3 A: R = 4 Ω. 6 Ω with 12 V: I = 2 A.
Bulb on 6 V battery drawing 2 A: R = V/I = 3 Ω.
Copper wire 1 m long, 2 mm diameter: A = π(2×10⁻³)²/4 = 3.14 × 10⁻⁶ m²; R = ρL/A = 1.7 × 10⁻⁸ × 1 / 3.14 × 10⁻⁶ ≈ 5.4 × 10⁻³ Ω.
Two differences between d.c. and a.c.; describe d.c. and a.c. with one advantage each; why a.c. for home use.