Lesson 7.3.2

7.3.2 Charge and discharge of resistor-capacitor circuits Quiz: Pearson Edexcel Physics, Unit 7

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 7.3.2, Charge and discharge of resistor-capacitor circuits: 20 multiple choice questions for the Pearson Edexcel Physics (9PH0), Unit 7: Electric and Magnetic Fields, written with Revision Ninja.

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The 20 questions

  1. What are the units of the time constant RC of a resistor-capacitor circuit?

    • Ohms
    • Volts per second
    • Seconds
    • Farads

  2. After one time constant RC has elapsed during discharge, what fraction of the initial charge remains on the capacitor?

    • About 50 per cent, which is one half
    • About 10 per cent, which is 1/e squared
    • About 63 per cent, which is 1 minus 1/e
    • About 37 per cent, which is 1/e

  3. For the discharge Q = Q0 e^(-t/RC), what is the gradient of a graph of ln Q against t?

    • -RC
    • -1/Q0
    • RC / Q0
    • -1/RC

  4. During discharge of a capacitor through a resistor, how does the current vary with time?

    • It increases linearly, with I proportional to t
    • It decreases exponentially, with I = I0 e^(-t/RC)
    • It decreases linearly to zero in exactly one time constant
    • It remains constant at the initial value until the capacitor is empty

  5. In a discharge through a resistor, how long does the charge take to fall to 1/e of its initial value?

    • Half a time constant, RC / 2
    • Two time constants, 2RC
    • The product of R and C squared, (RC)^2
    • One time constant, RC

  6. A 2.0 megohm resistor is connected in series with a 4.0 microfarad capacitor. What is the time constant?

    • 8.0 s
    • 0.50 s
    • 2.0 s
    • 8.0 x 10^-12 s

  7. A 100 microfarad capacitor charged to 12 V discharges through a 10 kilohm resistor. What is the potential difference across it after 1.0 s?

    • 6.0 V
    • 0.44 V
    • 4.4 V
    • 8.0 V

  8. Using the same circuit, how long does the potential difference take to fall to half its initial value?

    • 1.4 s
    • 0.50 s
    • 1.0 s
    • 0.69 s

  9. A capacitor with initial charge 2.0 millicoulomb has a time constant of 0.50 s. What is the charge on it after 0.50 s of discharge?

    • 1.0 mC
    • 0.27 mC
    • 0.14 mC
    • 0.74 mC

  10. A 12 V supply charges and then discharges through a 10 kilohm resistor and a 100 microfarad capacitor. What is the current at t = 1.0 s during discharge, given the initial current is 1.2 mA?

    • 4.4 mA
    • 0.44 mA
    • 0.12 mA
    • 1.2 mA

  11. A capacitor charged to 9.0 V discharges through a resistor with time constant 2.0 s. What is the potential difference after 4.0 s?

    • 3.3 V
    • 4.5 V
    • 0.45 V
    • 1.2 V

  12. A capacitor is charged through a 1.0 kilohm resistor from a 10 V supply. What is the potential difference across the capacitor after one time constant of 1.0 s?

    • 10 V
    • 8.6 V
    • 3.7 V
    • 6.3 V

  13. Which change increases the time constant of a resistor-capacitor discharge circuit?

    • Decreasing the resistance while keeping the capacitance fixed
    • Increasing either the resistance or the capacitance
    • Connecting a second identical capacitor in parallel with the first
    • Decreasing the capacitance while keeping the resistance fixed

  14. A graph of ln V against t for a discharging capacitor is a straight line with gradient -0.20 s^-1. What is the time constant?

    • 5.0 s
    • 0.80 s
    • 0.20 s
    • 2.0 s

  15. A capacitor starts discharging from 6.0 V. What is its potential difference after three time constants?

    • 2.0 V
    • 0.03 V
    • 0.30 V
    • 0.60 V

  16. A 2.0 kilohm resistor is in series with a 1000 microfarad capacitor being charged from a 10 V supply. How long does the capacitor take to reach 90 per cent of the supply voltage?

    • 0.90 s
    • 6.9 s
    • 4.6 s
    • 2.0 s

  17. A capacitor with time constant 0.80 s discharges through a resistor. How long does its stored energy take to fall to one quarter of its initial value?

    • 0.55 s
    • 0.40 s
    • 0.20 s
    • 1.6 s

  18. On a graph of ln V against t, a discharging capacitor's line passes through ln V = 2.30 at t = 0 and ln V = 0.00 at t = 10 s. What is the time constant RC?

    • 10 s
    • 4.3 s
    • 2.3 s
    • 0.23 s

  19. Why is the discharge current greatest at the start of a discharge?

    • The capacitance increases during the early part of the discharge
    • The current is constant throughout, but the measured value is largest at the start
    • The resistance of the resistor is lowest at the start of the discharge
    • The p.d. across the capacitor is greatest at the start, and current is proportional to this p.d.

  20. A student says that a capacitor discharges completely in exactly one time constant. Which response is correct?

    • The student is right, but only if the resistance is greater than the capacitance
    • The student is wrong: after one time constant about 37 per cent of the charge remains
    • The student is right, because the charge falls to zero after one time constant exactly
    • The student is wrong because capacitors do not discharge at all through a resistor

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