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Capacitor Charge & Discharge (AQA A Level Physics): Exam Questions

Exam code: 7408

3 hours30 questions

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Capacitor Charge & Discharge

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1
1 mark

A capacitor in a circuit is initially uncharged. The switch is closed at time t = 0.

Which of the following graphs shows how the potential difference V across the capacitor, the charge Q on the capacitor, and the current I in the circuit change with time t?

7-7-mcq-q1-medium-aqa-a-level-physics

    Choose your answer

    2
    1 mark

    A capacitor of capacitance C discharges through a resistor of resistance R. 

    Which one of the following statements is true?

    • the time constant will decrease if C increases

    • the time constant will increase if R increases

    • the time constant will increase if R decreases

    • the time constant will increase if C decreases

    Choose your answer

    3
    1 mark

    A capacitor in a circuit is initially fully charged. The switch is opened at time t = 0.

    Which of the following graphs shows how the potential difference V across the capacitor, the charge Q on the capacitor, and the current I in the circuit change with time t?

    7-7-mcq-q3-medium-aqa-a-level-physics

      Choose your answer

      4
      1 mark

      What is the capacitance of a capacitor in a circuit with a time constant of 90 s when it is discharged through a 300 kΩ resistor?

      • 3.3 kF

      • 0.3 mF

      • 3.3 F

      • 0.3 F

      Choose your answer

      5
      1 mark

      What is the final current in a circuit after a capacitor of 450 mF is discharged through a 200 Ω resistor in 1 minute? The initial current is 0.7 A.

      • 0.14 A

      • 0.69 A

      • 0.36 A

      • 0.57 A

      Choose your answer

      1
      1 mark

      A 650 µF capacitor is charge to a pd of 25 V. It is then discharged through a 120 kΩ resistor. 

      What is the time taken for the pd to fall to 20 V?

      • 5 s

      • 45 s

      • 30 s

      • 17 s

      Choose your answer

      2
      1 mark

      The graph shows how the charge on a capacitor varies with time as it is discharged through a resistor.

      7-7-mcq-q2-medium-aqa-a-level-physics

      What is the time constant for the circuit?

      • 5.50 s

      • 12.5 s

      • 20.0 s

      • 4.70 s

      Choose your answer

      3
      1 mark

      A capacitor of capacitance 20 μF is fully charged and the potential difference across its plates is 4.0 V. It is then connected into the circuit as shown.

      tokXDbM8_7-7-mcq-q3-medium-aqa-a-level-physics

      The switch S is closed at time t = 0 s. Which one of the following statements is incorrect?

      • The time constant of the circuit is 12 s

      • After a time equal to the time constant, the charge on the capacitor is 29 µC

      • After a time equal to twice the time constant, the charge remaining on the capacitor is  Q0e2 where Q0  is the charge at time t = 0

      • The initial charge on the capacitor is 80 µC

      Choose your answer

      4
      1 mark

       A 1600 µF capacitor C, initially uncharged, is then charged by a battery with an emf of 40 V.

      7-7-mcq-q4-medium-aqa-a-level-physics

      What potential difference does the resistor reach after 3 minutes?

      • 34.8 V

      • 0.094 V

      • 35 V

      • 5.2 V

      Choose your answer

      5
      1 mark

      When a capacitor discharges through a resistor, the current is 50 % lower in 12 s.

      What is the time constant of the capacitor–resistor circuit?

      • 17 s

      • 6.0 s 

      • 3.0 s

      • 10 s

      Choose your answer

      6
      1 mark

      A capacitor of capacitance C is charged and then discharged through a resistor of resistance R. The graph shows how the current I varies with time t during the discharge.

      Current-time graph for capacitor discharge with t on the horizontal axis, and I on the vertical axis. A single exponential decay curve (solid line) starts at I nought on the vertical axis and decays towards zero.

      The experiment is repeated using a resistor of resistance 2R. The capacitor is charged to the same potential difference (p.d.) before being discharged.

      Which graph correctly shows the new discharge curve (solid) compared with the original (dashed)?

      Four graphs labelled A–D, each showing the same axes (I vs t) with the original curve as a dashed line and a new solid curve. All graphs use the same axis scales. A: The solid curve starts at half the original starting current. It decays more slowly than the dashed curve — at any given time after the start, the solid curve has not fallen as far proportionally. The solid curve takes noticeably longer to reach near-zero current. B: The solid curve starts at half the original starting current. It decays more steeply than the dashed curve — the solid curve reaches near-zero current sooner than the original. C: The solid curve starts at the same starting current as the original. It decays more slowly than the dashed curve — the solid curve sits above the dashed curve at all later times and takes longer to reach near-zero current. D: The solid curve starts at the same starting current as the original. It decays more steeply than the dashed curve — the solid curve sits below the dashed curve at all later times and reaches near-zero current sooner.

        Choose your answer

        1
        1 mark

        A voltage sensor and a datalogger are used to record the discharge of a 120 mF capacitor in series with a 834 Ω resistor from an initial pd of 7.5 V. The datalogger is capable of recording 1000 readings in 20 s.

        7-7-mcq-q1-hard-aqa-a-level-physics

        Which line, A to D, in the table gives the pd and the number of readings made after a time equal to the time constant of the discharge circuit? 

         

         potential difference / V

        number of readings

        A

        B

        C

        D

        2.8

        6.1

        2.8

        6.1

        5000

        5000

        200

        200

          Choose your answer

          2
          1 mark

          A capacitor of capacitance 500 µF is attached to a potential difference of 15 V and a resistor R. It charges to a potential difference of 12 V in 0.7 s. 

          What is the resistance of R?

          • 700 Ω

          • 630 Ω

          • 520 Ω

          • 870 Ω

          Choose your answer

          3
          1 mark

          A capacitor is charged through a 1000 kΩ resistor as shown in the graph below

          7-7-mcq-q3-hard-aqa-a-level-physics

          What is the potential difference across the capacitor after a time equal to one time constant?

          • 73 V

          • 46 V

          • 35 V

          • 12 V

          Choose your answer

          4
          1 mark

          Switch S in the circuit is held in position 1, so that the capacitor C becomes fully charged to a pd V and stores energy E.

          7-7-mcq-q4-hard-aqa-a-level-physics

          The switch is then moved quickly to position 2, allowing C to discharge through the fixed resistor R. It takes 53 ms for the pd across C to fall to V2. 

          What period of time must elapse, after the switch has moved to position 2, before the energy stored by C has fallen to E64?

          • 159 ms

          • 106 ms

          • 190 ms

          • 128 ms

          Choose your answer

          5
          1 mark

          An uncharged 7.4 nF capacitor is connected to a power supply and becomes fully charged. The potential difference across the capacitor is equal to 1.5 V after 0.3 s and it has a time constant of 0.8 s. 

          How many electrons are transferred to the negative plate of the capacitor at the end of the charging process?

          • 3.3 × 1011

          • 1.1 × 1011

          • 2.2 × 1011

          • 6.6 × 1011

          Choose your answer