Current Electricity

What these notes cover

These notes cover current electricity for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): electric charge and current, electromotive force and potential difference, resistance, and electrical energy and power. They also explain the current–voltage graphs of a fixed resistor, a filament lamp and a diode, and how current and potential difference behave in series and parallel circuits.

Key formulas and definitions

• Current is the rate of flow of charge: Q = It (charge in coulombs, C).
• e.m.f. is the electrical work done by a source in moving unit charge around a complete circuit; p.d. is the work done by unit charge passing through a component. Both are measured in volts (V).
• Resistance: R = V / I (in ohms, Ω).
• For a metal wire, resistance is directly proportional to length and inversely proportional to cross-sectional area.
• Electrical power: P = IV; electrical energy: E = IVt
• Resistors in series: R = R₁ + R₂ (the same current flows through each).
• Resistors in parallel: 1/R = 1/R₁ + 1/R₂ (the same p.d. across each; the combined resistance is less than either resistor).