Alternating Current

Physics, HSSC-Punjab Second Year. This chapter is split into 52 topics and has 1,263 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. RLC parallel circuit: properties and applications 22 questions
2. Series resonance: current, voltage magnification and power 26 questions
3. Phase, phase lead and lag, and phasor representation 23 questions
4. RC and RL series circuits 24 questions
5. Characteristics of alternating current 23 questions
6. Exercise questions and conceptual reasoning on alternating current 21 questions
7. Tuning a resonant circuit: frequency, L and C 28 questions
8. Mean power, the heating effect and AC in a resistor 22 questions
9. RLC series circuits and resonance 24 questions
10. Series RLC circuits (numerical) 26 questions
11. Mutual inductance (numerical) 22 questions
12. Phase of AC and phasor representation 22 questions
13. Self-induction and energy stored in an inductor 25 questions
14. Household appliances on AC mains: current, resistance and energy 28 questions
15. Impedance and phase in RC and RL circuits 30 questions
16. Phase angle, impedance and power factor of series circuits 30 questions
17. Resonance in LCR circuits (numerical) 28 questions
18. Reactance problems for inductors and capacitors 30 questions
19. Rectification: diodes, half-wave and full-wave bridge 26 questions
20. Parallel RLC circuits (numerical) 22 questions
21. Rectified output: average value, rms and ripple 24 questions
22. AC through a capacitor and impedance 23 questions
23. Mutual induction 22 questions
24. Choke coil: construction, uses and power factor 23 questions
25. How reactance and current scale with frequency, L and C 26 questions
26. Self-inductance and stored magnetic energy (numerical) 28 questions
27. Peak, mean, rms and peak-to-peak values: concepts and relations 23 questions
28. Inductor in AC: back emf, reactance and zero average power 22 questions
29. Voltage addition in series LCR circuits 24 questions
30. AC through a resistor, inductor and choke 26 questions
31. Reading the equation of an alternating voltage 22 questions
32. Self-induction, the henry and energy stored in an inductor 22 questions
33. Rectifier output and smoothing (numerical) 22 questions
34. AC versus DC and the vocabulary of the AC waveform 24 questions
35. Instantaneous values and timing of an AC wave 24 questions
36. Impedance, RC and RL series circuits and the power factor 20 questions
37. Self-inductance, mutual inductance, induced emf and stored energy 30 questions
38. Smoothing the rectified output and ripple 23 questions
39. Period, frequency, angular frequency and timing of an AC wave 24 questions
40. Peak, rms, average and peak-to-peak conversions 24 questions
41. Half-wave rectification and the diode 21 questions
42. Average, peak and instantaneous values of AC 20 questions
43. Choke coils, power factor and inverters 24 questions
44. RLC parallel circuits 22 questions
45. Full-wave bridge rectifier and smoothing 25 questions
46. Capacitor in AC: lead, reactance and blocking DC 23 questions
47. Mean power in resistive AC circuits 28 questions
48. RLC series circuit, resonance and its properties 21 questions
49. Root-mean-square values and mean power 23 questions
50. Inverter efficiency: converting DC power to AC power 26 questions
51. Inductive and capacitive reactance 30 questions
52. Mutual induction: definition, coupling and the unit 22 questions

Sample questions: Alternating Current

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which behaviour is correct for a capacitor and an inductor?

• A. both block DC and pass AC freely
• B. both pass DC and AC equally well
• C. a capacitor passes DC and an inductor blocks DC
• D. a capacitor blocks DC and passes AC; an inductor passes DC and impedes AC(correct answer)

Answer: D.This is the summary given in the chapter, so the answer is a capacitor blocks DC and passes AC; an inductor passes DC and impedes AC.

Q2. The reason the capacitor in an RC series circuit does not dissipate mean power is that:

• A. the current and the capacitor voltage are 90° out of phase(correct answer)
• B. the current in the circuit is zero
• C. the capacitor has no charge on its plates
• D. the capacitor has a very large resistance

Answer: A.The average of V(C)I over a cycle is zero, so the answer is the current and the capacitor voltage are 90° out of phase.

Q3. Which of the following is the advantage of a full-wave rectifier over a half-wave rectifier?

• A. it produces AC from the input
• B. only half of the input wave is used
• C. both half cycles are used, giving a smoother and larger output(correct answer)
• D. it uses fewer diodes than half wave

Answer: C.Both halves give current to the load, so the answer is both half cycles are used, giving a smoother and larger output.

Q4. A coil has a self-inductance of one henry if:

• A. a current of 1 A produces a flux density of 1 T
• B. an emf of 1 V is induced when the current is 1 A
• C. an emf of 1 V is induced when the current changes at 1 A per second(correct answer)
• D. a charge of 1 C passes in 1 s through it

Answer: C.This is the definition of the henry, so the answer is an emf of 1 V is induced when the current changes at 1 A per second.

Q5. How does self-induction show itself when a current is switched on in a circuit?

• A. the induced emf opposes the rise so the current grows gradually(correct answer)
• B. the current falls to zero as the switch closes
• C. the current rises instantly to its final value
• D. the circuit becomes a capacitor at once

Answer: A.The back emf delays the growth of current, so the answer is the induced emf opposes the rise so the current grows gradually.

Q6. Mutual inductance is utilised in pacemakers and metal detectors because:

• A. the changing magnetic coupling between coils produces measurable emfs(correct answer)
• B. they use no coils in their circuits
• C. they need friction to work
• D. they depend on DC currents only

Answer: A.Both devices depend on inductive coupling, so the answer is the changing magnetic coupling between coils produces measurable emfs.

Q7. Our heart is driven by electric pulses, so high frequency AC current can:

• A. make the heart stronger for a long time
• B. stop the pulse permanently but harmlessly
• C. have no effect on the heart at all
• D. affect the frequency of the heart and lead to a heart attack(correct answer)

Answer: D.The chapter makes this safety statement, so the answer is affect the frequency of the heart and lead to a heart attack.

Q8. The reason a choke is preferred over a resistor in an AC circuit is that:

• A. a choke limits current without wasting energy as heat(correct answer)
• B. a choke has no reactance at all
• C. a choke increases the current
• D. a choke produces more heat than a resistor

Answer: A.The power factor of a choke is almost zero, so the answer is a choke limits current without wasting energy as heat.

Q9. Which statement correctly lists four applications of resonance in LRC circuits?

• A. rectification, transformation, welding and cooling
• B. heating, cooling, lighting and cooking
• C. radio tuning, band-pass filters, induction heating and current magnification(correct answer)
• D. battery charging, switching, rectifying and storing

Answer: C.These are applications from the chapter, so the answer is radio tuning, band-pass filters, induction heating and current magnification.

Q10. The rate of heat production in a resistor depends on the current as:

• A. I, only for direct current
• B. I squared, whether the current is direct or alternating(correct answer)
• C. I squared, only for direct current
• D. zero for alternating current

Answer: B.The sign of the current does not matter, so the answer is I squared, whether the current is direct or alternating.