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LearnCambridge97022025–2027Paper 4 · A Level Structured QuestionsAlternating currents

Paper 4 · Module

Alternating currents

2 subtopics≈ 65 min in total

Start the module: Characteristics of alternating currents Open graphing calculator

Characteristics of alternating currents
An alternating current reverses direction periodically: $I=I_0\sin\omega t$ , with peak value $I_0$ , period $T$ and frequency $f=\dfrac1T$ . The power in a resistor, $I^2R$ , varies between $0$ and $I_0^2R$ , so the mean power is $\tfrac12I_0^2R$ . The r.m.s. value is the steady d.c. value giving the same mean power: $I_{\text{rms}}=\dfrac{I_0}{\sqrt2}$ and $V_{\text{rms}}=\dfrac{V_0}{\sqrt2}$ . Pakistan's mains is $230$ V r.m.s., peaking at about $325$ V.
≈ 30 min
Rectification and smoothing
Rectification converts a.c. to d.c. using diodes, which conduct in one direction only. A single diode gives half-wave rectification: only alternate half-cycles pass. A bridge of four diodes gives full-wave rectification: both halves pass, in the same direction through the load. A capacitor in parallel with the load smooths the output: it charges near each peak and discharges slowly through the load in between. Larger $RC$ (compared with the time between peaks) means less ripple.
≈ 35 min