Lesson 3.7.4.4

3.7.4.4 Capacitor charge and discharge Quiz: AQA Physics, Unit 7

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.7.4.4, Capacitor charge and discharge: 20 multiple choice questions for the AQA Physics (7408), Unit 7: Fields and their consequences (A-level only), written with Revision Ninja.

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The 20 questions

  1. What does the time constant RC of a capacitor and resistor circuit represent?

    • The time for the pd to reach the supply pd exactly
    • The time for the capacitor to store half of its maximum energy
    • The time for the charge or pd to fall to 1/e (about 37%) of its initial value during discharge
    • The time taken for the current in the resistor to fall to zero

  2. Which expression gives the half-life for the charge on a discharging capacitor?

    • 0.69 RC
    • 2.0 RC
    • 0.36 RC
    • 1.44 RC

  3. Which equation describes the charge on a capacitor discharging through a resistor?

    • Q = Q0 exp(t/RC)
    • Q = Q0 exp(-RC/t)
    • Q = Q0 (1 - exp(-t/RC))
    • Q = Q0 exp(-t/RC)

  4. Which equation describes the charge on a capacitor charging through a resistor from a constant supply?

    • Q = Q0 exp(t/RC)
    • Q = Q0 (1 + exp(-t/RC))
    • Q = Q0 exp(-t/RC)
    • Q = Q0 (1 - exp(-t/RC))

  5. Which shape describes the pd across a discharging capacitor plotted against time?

    • A curve that rises to the supply pd and then levels off
    • A sine wave of constant amplitude
    • An exponential decay curve that approaches zero asymptotically
    • A straight line that reaches zero at a finite time

  6. During charging through a resistor from a constant supply, how does the current change with time?

    • It rises exponentially from zero towards the supply current
    • It falls exponentially from its maximum at the start towards zero
    • It falls linearly to zero at the time RC
    • It stays constant at the supply value until the capacitor is full

  7. A resistor of 10 kiloohm is connected in series with a 100 microfarad capacitor. What is the time constant?

    • 0.10 s
    • 0.010 s
    • 10 s
    • 1.0 s

  8. A 2.2 megaohm resistor is in series with a 1.0 microfarad capacitor. What is the time to halve the charge during discharge?

    • 0.32 s
    • 2.2 s
    • 3.2 s
    • 1.5 s

  9. A capacitor charges from zero through a resistor. After one time constant, what fraction of its final charge is stored?

    • about 63%
    • about 86%
    • about 50%
    • about 37%

  10. A capacitor charged to 10 V discharges through a resistor. What is its pd after one time constant?

    • 5.0 V
    • about 6.3 V
    • about 0.37 V
    • about 3.7 V

  11. A 10 V supply charges a 2.0 microfarad capacitor through a 1.0 megaohm resistor. What is the pd after 4.0 s?

    • 3.7 V
    • 9.8 V
    • 8.6 V
    • 6.3 V

  12. A capacitor has initial charge 0.50 mC and discharges through a resistor. What is the charge after two time constants?

    • 6.8 x 10^-5 C
    • 1.8 x 10^-4 C
    • 2.5 x 10^-4 C
    • 5.0 x 10^-5 C

  13. A capacitor discharges from 12 V through a resistor with RC = 5.0 s. How long for the pd to fall to 3.0 V?

    • 2.5 s
    • 15 s
    • 1.7 s
    • 6.9 s

  14. A 5.0 kiloohm resistor is in series with a 20 microfarad capacitor. How long does the charge take to halve during discharge?

    • 0.10 s
    • 0.035 s
    • 0.069 s
    • 0.14 s

  15. On a graph of ln(V) against time for a discharging capacitor, what does the gradient represent?

    • -RC
    • 1/(RC)
    • -1/(RC)
    • RC

  16. The gradient of a ln(V) against t graph for a discharging capacitor is -0.50 per second. What is the time constant RC?

    • 0.25 s
    • 0.50 s
    • 2.0 s
    • -2.0 s

  17. How many time constants does a capacitor take to reach 90% of its final charge when charging?

    • about 4.6
    • about 2.3
    • about 0.9
    • about 1.0

  18. A 100 microfarad capacitor charged to 9.0 V discharges through a 50 kiloohm resistor. What is its charge after 5.0 s?

    • 1.7 x 10^-4 C
    • 6.5 x 10^-4 C
    • 9.0 x 10^-4 C
    • 3.3 x 10^-4 C

  19. The pd across a discharging capacitor falls from 6.0 V to 2.0 V in 10 s. What is the time constant?

    • 3.3 s
    • 5.0 s
    • 9.1 s
    • 20 s

  20. During discharge, what is the gradient of a Q-t graph at any instant equal to?

    • minus the current in the circuit at that instant
    • the capacitance multiplied by the resistance
    • the pd across the capacitor at that instant
    • the energy stored in the capacitor at that instant

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