Flashcards · 9702 · A Level Topic 21

Alternating currents (A Level) flashcards

10 cards on A Level Topic 21, subtopics 21.1 to 21.2. Pick a subtopic or study them all.

Study mode needs JavaScript. Every card is listed below, so you can still cover the answers and test yourself.

All 10 cards

For a quick read-through or printing. Cover the right-hand side to test yourself.

Equation for a sinusoidal alternating current: I = I₀ sin ωt, where I₀ is the peak current and ω = 2πf = 2π ÷ T.
r.m.s. value of an alternating current: The value of the direct current that would dissipate the same mean power in the same resistor.
r.m.s. and peak values for a sinusoidal supply: I_rms = I₀ ÷ √2 and V_rms = V₀ ÷ √2.
Why is the mean current not used to describe an alternating current?: It spends equal time in each direction, so the mean over a cycle is zero. Squaring makes every value positive, so the mean of the squares reflects the power.
Mean power vs peak power in a resistive load (sinusoidal a.c.): Mean power is half the peak power: ⟨P⟩ = I_rms V_rms = ½I₀V₀.
Mains is 230 V r.m.s. Find the peak voltage.: V₀ = 230 × √2 ≈ 325 V.

Half-wave rectification: A single diode conducts on one half-cycle only. The output is a series of positive humps with flat gaps at zero between them.
Full-wave rectification: Four diodes in a bridge. Both half-cycles give a positive hump (the negative half is inverted), so there are twice as many pulses and no gaps.
How is a rectified output smoothed?: A capacitor in parallel with the load. It charges at each peak and discharges through the load between peaks, so the output falls only slightly before the next peak (ripple).
How do capacitance and load resistance affect the ripple?: A larger capacitance or a larger load resistance gives a longer time constant RC, so the capacitor discharges more slowly and the ripple is smaller. The ripple is never zero.