Alternating Currents · revision map · A-Level 9702, Topic 21
TOPIC 21 · ALTERNATING CURRENTS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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ALTERNATING CURRENTS21
The Sinusoidal a.c.
Current (or voltage) that reverses each cycle, traced as a sine wave.
x = x0 sin ωt
x is I or V;
x0 = peak value
x0 = peak value
- It swings from +x0 to −x0; the mean over a cycle is zero.
- Period T, frequency f = 1/T, angular frequency ω.
e.g. mains supply: f = 50 Hz
R.M.S. Values & Mean Power
The r.m.s. value is the steady d.c. that delivers the same mean power.
- Power ∝ I², so the mean of I² is what counts.
- For a sinusoid only, r.m.s. = peak ÷ √2.
⟨P⟩ = ½I0V0 = Irms²R = Vrms²/R
Rectification
A diode forces the current one way only, blocking the reverse direction.
- Half-wave: one diode removes the negative halves.
- Full-wave: a four-diode bridge flips the negative halves upward.
- Output is bumpy but always one sign: rectified, not yet steady.
Smoothing
A capacitor in parallel with the load flattens the ripple to near d.c.
- It charges at each peak, then discharges through R between peaks.
- Larger C or larger R means a bigger RC, so less ripple.
- Result: a steady d.c. that can power electronics.
BUILDS ON · Oscillations · Capacitance · Magnetic fields
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A.C.
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LEADS TO · Quantum physics · Medical physics