Capacitance

What these notes cover

These notes cover capacitance for Cambridge International A Level Physics (9702): what capacitance means, how capacitors combine in series and parallel, the energy stored in a charged capacitor, and how a capacitor discharges through a resistor. They include the exponential discharge equation and the time constant, which appear regularly in Paper 4 questions.

Key formulas and definitions

• Capacitance: C = Q / V (unit: farad, F = C V⁻¹).
• Capacitors in parallel: C = C₁ + C₂ + …
• Capacitors in series: 1/C = 1/C₁ + 1/C₂ + …
• Energy stored: W = ½QV = ½CV² (the area under a potential difference–charge graph).
• Time constant: τ = RC
• Discharge through a resistor: x = x₀ e^−t/RC, where x is the charge, current or potential difference.