CIE A Level Physics
CIE A Level Physics Alternating Currents Exam Questions
Free CIE A Level Physics exam questions for Alternating Currents, with a mark scheme for every pack. Covers sinusoidal alternating currents, peak and root-mean-square values, mean power, half-wave and full-wave rectification and smoothing.
Alternating Currents exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Alternating Currents
Alternating currents is Topic 21 of the Cambridge International AS & A Level Physics (9702) syllabus. It describes sinusoidal alternating current and voltage in terms of period, frequency, peak value and root-mean-square (r.m.s.) value, and explains why r.m.s. values are used for power. You then study rectification using diodes, including half-wave rectification and full-wave rectification with a bridge of four diodes, and smoothing with a capacitor. It builds on Electricity, Capacitance and Magnetic fields. Examiners like questions on mean power in a resistor and on how capacitance and load resistance affect ripple.
What you need to know
- Describing a.c. A sinusoidal current can be written x = x0 sin ωt. Know period, frequency, peak value and root-mean-square value.
- R.m.s. values The r.m.s. value of an alternating current is the steady direct current that gives the same mean power in a resistor. For a sinusoid, Irms = I0/√2 and Vrms = V0/√2.
- Mean power For a sinusoidal supply in a resistive load, mean power is half the maximum power: P = I0V0/2 = IrmsVrms.
- Half-wave rectification A single diode allows current in one direction only, so half of each cycle is removed.
- Full-wave rectification A bridge of four diodes makes the current through the load flow in the same direction for both halves of the cycle.
- Smoothing A capacitor in parallel with the load charges near the peaks and discharges through the load between them. A larger capacitance or larger load resistance (larger time constant) gives less ripple.
Common mistakes in Alternating Currents
- Using peak values in power calculations as if they were r.m.s. Mean power uses r.m.s. values.
- Defining r.m.s. as the average value of the current. The mean of a sinusoidal current over a cycle is zero; r.m.s. is the equivalent steady d.c. for power.
- Thinking the smoothing capacitor goes in series with the load. It is connected in parallel.
- Saying smaller capacitance improves smoothing. Larger RC gives slower discharge and less ripple.
- Drawing a bridge rectifier where current passes through the load in opposite directions in each half cycle.
Try three Alternating Currents exam-style questions
1. A sinusoidal supply has a peak voltage of 325 V. Calculate its r.m.s. voltage. (1 mark)
Show the mark scheme answer
Vrms = V0/√2 = 325/√2 = 230 V.
2. This supply is connected across a 50 Ω resistor. Calculate the mean power dissipated. (2 marks)
Show the mark scheme answer
P = Vrms^2/R = 230^2/50; P = 1.06 × 10^3 W.
3. Explain how a capacitor smooths the output of a full-wave rectifier, and how the smoothing can be improved. (4 marks)
Show the mark scheme answer
Capacitor charges as the rectified p.d. rises to its peak; when the supply p.d. falls the capacitor discharges through the load, maintaining the p.d.; the output falls only slightly before the next peak recharges it; smoothing improves with larger capacitance or larger load resistance (larger time constant).
Alternating Currents FAQs
What does Cambridge AS & A Level Physics Alternating currents cover?
It covers sinusoidal a.c., peak and r.m.s. values, mean power in a resistive load, half-wave and full-wave rectification and smoothing with a capacitor.
Which paper is Alternating currents examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What should I be able to draw for this topic?
Be ready to draw a bridge rectifier circuit with the load and smoothing capacitor, and to sketch output graphs for half-wave, full-wave and smoothed outputs.
