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Alternating currents: WJEC A‑Level Physics questions

Every WJEC A-Level Physics exam question on 4.A Alternating currents, 2016–2025, with its mark scheme and examiner’s report.

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1,173 questions

  1. Q7(a)(i)

    Summer 2025 · Unit 4

    4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(a)(i): peak a.c. voltage;

  2. Q7(a)(ii)

    Summer 2025 · Unit 4

    4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(a)(ii): period of an a.c. supply;

  3. Q7(a)(iii)

    Summer 2025 · Unit 4

    5 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(a)(iii): d.c. voltage. [5] You may add to the diagrams to assist your answer.

  4. Q7(b)(i)

    Summer 2025 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(b)(i): Calculate the reactance of the inductor in the following LR circuit. [2] Frequency of supply = 141 kHz 53 μH 47 Ω

  5. Q7(b)(ii)

    Summer 2025 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(b)(ii): In the following RC circuit, calculate the frequency at which the reactance of the capacitor is equal to the resistance of the resistor. [2] a.c. supply of rms

  6. Q7(b)(iii)

    Summer 2025 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(b)(iii): Calculate the rms pd across the resistor when this occurs. [2]

  7. Q7(b)(iv)

    Summer 2025 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(b)(iv): Hence, or otherwise, determine the resonance frequency and Q factor of the following LCR circuit. [2] 24.0 nF 53 μH 47 Ω

  8. Q7(b)(v)

    Summer 2025 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(b)(v): Explain why the following two circuits have the same total impedance. [2] XL = 94 Ω XL = 200 Ω 47 Ω 47 Ω XC = 24 Ω XC = 130 Ω

  9. Q7(c)

    Summer 2025 · Unit 4

    5 marks4.A Alternating currents
    The whole paper ↗
    Summer 2025 Unit 4, question 7(c): Steve requires the following circuit to have a resonance frequency of 2.70 MHz and a Q factor of 90 Determine the correct values of L and R. [5] 82pF R L

  10. Q7(a)(i)

    Summer 2024 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(a)(i): Explain why the pd across the resistor decreases as the frequency of the supply increases. [2] variable frequency supply Vrms = 2.5 V 53 μH 47 Ω

  11. Q7(a)(ii)

    Summer 2024 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(a)(ii): Calculate the frequency when the pd across the resistor and inductor are equal. [2]

  12. Q7(a)(iii)

    Summer 2024 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(a)(iii): Calculate the rms pd across the resistor when it is equal to the rms pd across the inductor. [2]

  13. Q7(b)(i)

    Summer 2024 · Unit 4

    3 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(b)(i): Explain in terms of Faraday’s law why the emf is proportional to each of B and A. [3]

  14. Q7(b)(ii)

    Summer 2024 · Unit 4

    1 mark4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(b)(ii): Explain at which point in the cycle the peak emf occurs. [1]

  15. Q7(c)(i)

    Summer 2024 · Unit 4

    1 mark4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(c)(i): at resonance; [1]

  16. Q7(c)(ii)

    Summer 2024 · Unit 4

    4 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(c)(ii): at twice the resonance frequency. [4]

  17. Q7(d)

    Summer 2024 · Unit 4

    5 marks4.A Alternating currents
    The whole paper ↗
    Summer 2024 Unit 4, question 7(d): Tomos wants to display a wave trace on an oscilloscope for an a.c. power supply of frequency 67.5 kHz and rms pd 240 mV. Determine whether or not the settings o

  18. Q7(a)(i)

    Summer 2023 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2023 Unit 4, question 7(a)(i): Calculate the peak induced emf in the coil. [2]

  19. Q7(a)(ii)

    Summer 2023 · Unit 4

    2 marks4.A Alternating currents
    The whole paper ↗
    Summer 2023 Unit 4, question 7(a)(ii): Calculate the flux linkage of the coil when t = 5.0 ms. [Hint: put your calculator in radian mode.] [2]

  20. Q7(a)(iii)

    Summer 2023 · Unit 4

    1 mark4.A Alternating currents
    The whole paper ↗
    Summer 2023 Unit 4, question 7(a)(iii): Calculate the induced emf in the coil when t = 5.0 ms. [1]
All 49 questions on Alternating currents, as a list
  1. Summer 2025, Unit 4, Q7(a)(i): peak a.c. voltage;
  2. Summer 2025, Unit 4, Q7(a)(ii): period of an a.c. supply;
  3. Summer 2025, Unit 4, Q7(a)(iii) (5 marks): d.c. voltage. [5] You may add to the diagrams to assist your answer.
  4. Summer 2025, Unit 4, Q7(b)(i) (2 marks): Calculate the reactance of the inductor in the following LR circuit. [2] Frequency of supply = 141 kHz 53 μH 47 Ω
  5. Summer 2025, Unit 4, Q7(b)(ii) (2 marks): In the following RC circuit, calculate the frequency at which the reactance of the capacitor is equal to the resistance of the resistor. [2] a.c. supply of rms
  6. Summer 2025, Unit 4, Q7(b)(iii) (2 marks): Calculate the rms pd across the resistor when this occurs. [2]
  7. Summer 2025, Unit 4, Q7(b)(iv) (2 marks): Hence, or otherwise, determine the resonance frequency and Q factor of the following LCR circuit. [2] 24.0 nF 53 μH 47 Ω
  8. Summer 2025, Unit 4, Q7(b)(v) (2 marks): Explain why the following two circuits have the same total impedance. [2] XL = 94 Ω XL = 200 Ω 47 Ω 47 Ω XC = 24 Ω XC = 130 Ω
  9. Summer 2025, Unit 4, Q7(c) (5 marks): Steve requires the following circuit to have a resonance frequency of 2.70 MHz and a Q factor of 90 Determine the correct values of L and R. [5] 82pF R L
  10. Summer 2024, Unit 4, Q7(a)(i) (2 marks): Explain why the pd across the resistor decreases as the frequency of the supply increases. [2] variable frequency supply Vrms = 2.5 V 53 μH 47 Ω
  11. Summer 2024, Unit 4, Q7(a)(ii) (2 marks): Calculate the frequency when the pd across the resistor and inductor are equal. [2]
  12. Summer 2024, Unit 4, Q7(a)(iii) (2 marks): Calculate the rms pd across the resistor when it is equal to the rms pd across the inductor. [2]
  13. Summer 2024, Unit 4, Q7(b)(i) (3 marks): Explain in terms of Faraday’s law why the emf is proportional to each of B and A. [3]
  14. Summer 2024, Unit 4, Q7(b)(ii) (1 mark): Explain at which point in the cycle the peak emf occurs. [1]
  15. Summer 2024, Unit 4, Q7(c)(i) (1 mark): at resonance; [1]
  16. Summer 2024, Unit 4, Q7(c)(ii) (4 marks): at twice the resonance frequency. [4]
  17. Summer 2024, Unit 4, Q7(d) (5 marks): Tomos wants to display a wave trace on an oscilloscope for an a.c. power supply of frequency 67.5 kHz and rms pd 240 mV. Determine whether or not the settings o
  18. Summer 2023, Unit 4, Q7(a)(i) (2 marks): Calculate the peak induced emf in the coil. [2]
  19. Summer 2023, Unit 4, Q7(a)(ii) (2 marks): Calculate the flux linkage of the coil when t = 5.0 ms. [Hint: put your calculator in radian mode.] [2]
  20. Summer 2023, Unit 4, Q7(a)(iii) (1 mark): Calculate the induced emf in the coil when t = 5.0 ms. [1]
  21. Summer 2023, Unit 4, Q7(a)(iv) (3 marks): Vanessa states that her answers to parts (ii) and (iii) are consistent with the coil having completed a quarter of a cycle. Determine whether or not she is corr
  22. Summer 2023, Unit 4, Q7(b)(i) (2 marks): Calculate the resonance frequency of the following circuit. [2] Vrms = 2.5 V C = 150 pF R = 180 Ω L = 16 mH
  23. Summer 2023, Unit 4, Q7(b)(ii) (2 marks): Explain why the rms current at resonance is approximately 14 mA. [2]
  24. Summer 2023, Unit 4, Q7(b)(iii) (4 marks): Calculate the rms current in the circuit when the frequency is 105 kHz. [4]
  25. Summer 2023, Unit 4, Q7(b)(iv) (2 marks): Explain why your answers to parts (b)(i), (b)(ii) and (b)(iii) suggest that the circuit has a high Q factor. [2]
  26. Summer 2023, Unit 4, Q7(b)(v) (2 marks): Thomas considers the effect, on this circuit, of increasing the capacitance only. He believes that this will decrease the Q factor. Determine whether or not he
  27. Summer 2019, Unit 4, Q6(a)(i) (3 marks): when the circuit is not in resonance; [3] Space for diagram.
  28. Summer 2019, Unit 4, Q6(a)(ii) (2 marks): when the circuit is in resonance. [2]
  29. Summer 2019, Unit 4, Q6(b)(i) (1 mark): the Q factor of the circuit; [1]
  30. Summer 2019, Unit 4, Q6(b)(ii) (1 mark): the rms current; [1]
  31. Summer 2019, Unit 4, Q6(b)(iii) (2 marks): the frequency of the power supply; [2]
  32. Summer 2019, Unit 4, Q6(b)(iv) (2 marks): the inductance of the inductor. [2]
  33. Summer 2019, Unit 4, Q6(b)(v) (2 marks): Sketch a graph of the instantaneous pd across the capacitor on the grid provided. (The instantaneous pd across the resistor is shown.) [2] Instantaneous pd / V
  34. Summer 2019, Unit 4, Q6(b)(vi) (2 marks): Without further calculation, explain why the current decreases when the frequency of the power supply is increased. [2]
  35. Summer 2019, Unit 4, Q6(c) (5 marks): Sam claims that the output pd (VOUT) in the following circuit is greater than 6.0 V when the frequency is greater than 20 kHz, but less than 6.0 V when below 20
  36. Summer 2018, Unit 4, Q6(a)(i) (2 marks): The output of the rotating coil is used to power a small lamp of known resistance. Explain how you would calculate the rms input power to the lamp from the know
  37. Summer 2018, Unit 4, Q6(a)(ii) (5 marks): Draw a typical trace that might be seen on the oscilloscope screen with the oscilloscope settings shown. [5] Space for calculations. VARIABLE VOLTS/DIV .1 50 .2
  38. Summer 2018, Unit 4, Q6(b)(i) (3 marks): Explain why the resonance frequency of the circuit occurs when: [3] 1 ω = L ω C
  39. Summer 2018, Unit 4, Q6(b)(ii) (2 marks): Calculate the resonance frequency of the circuit. [2]
  40. Summer 2018, Unit 4, Q6(b)(iii) (3 marks): Calculate the rms current when the frequency of the supply is 324 kHz. [3]
  41. Summer 2018, Unit 4, Q6(b)(iv) (5 marks): A student claims that the following circuit cannot have a peak pd above 1.5 kV across the capacitor. Investigate whether or not he is correct. [5] (It may be us
  42. Summer 2017, Unit 4, Q6(a)(i) (2 marks): Use Faraday’s law to explain why the emf is proportional to the angular velocity of the coil. [2] The peak emf induced in a rotating coil is given by: E = ωBAN
  43. Summer 2017, Unit 4, Q6(a)(ii) (3 marks): Discuss the agreement of the graph with the equation for the peak emf and suggest a reason for any disagreement (see diagram of coil opposite). [3]
  44. Summer 2017, Unit 4, Q6(b)(i) (2 marks): The minimum impedance of the circuit below occurs when the frequency of the a.c. supply is zero. Explain why this is so. [2] variable frequency a.c. supply Vrms
  45. Summer 2017, Unit 4, Q6(b)(ii) (3 marks): Calculate the rms current when the rms pd across the resistor and inductor are equal. [3]
  46. Summer 2017, Unit 4, Q6(c)(i) (3 marks): Explain how resonance arises in the LCR circuit shown. [3] variable frequency a.c. supply Vrms = 12 V 5.2 mH 82 Ω 220 nF
  47. Summer 2017, Unit 4, Q6(c)(ii) (2 marks): Calculate the power dissipated in the circuit at resonance. [2]
  48. Summer 2017, Unit 4, Q6(c)(iii) (3 marks): Calculate the rms current when the frequency of the supply is 9.40 kHz. [3]
  49. Summer 2017, Unit 4, Q6(c)(iv) (2 marks): Discuss briefly the effectiveness of the given LCR circuit for providing a sharp resonance curve. [2] ω0L R = 1 L (It may be useful to note that ) C . R

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