10 minutes maximum! Can you do it in 5?

1. Capacitors are made using 'dielectric materials'. Which of these statements best describes a dielectric material?

  • A. A conductor that can be polarised.
  • B. A conductor that cannot be polarised.
  • C. An insulator that can be polarised.
  • D. An insulator that cannot be polarised.

2. A simple capacitor of capacitance C is produced using 2 parallel plates of area A and separation d. (Air is used as the dielectric material). If the separation of the plates is halved to d/2 and the area of both plates is doubled, the new capacitance will be:

  • A. C
  • B. 2 C
  • C. 4 C
  • D. 8 C
3. A 10-4 F capacitor is connected to a 10 V cell. What is the charge and energy stored on the capacitor?  
Charge (C) Energy (J)
10-3 5 x 10-4
10-5 5 x 10-4
10-3 5 x 10-3
10-5 5 x 10-3

4. Two large capacitors of value 1F and 2F are wired in series and then in parallel. What is the total capacitance when combined like this?

Series Parallel
3F 3/2 F
2/3 F 3 F
3 F 2/3 F
3/2 F 3 F

5. Identical capacitors are connected in different combinations. Which of these capacitor networks has the largest capacitance?


capacitor networks x4

6. The formula τ = RC can be used to find the time constant for the discharge of a capacitor through a resistor. The capacitor is initially charged to a potential of 1 V.
At a time t = τ, what is the voltage in volts across the capacitor?

  • A. e
  • B. e-1
  • C. 0.5
  • D. 0
7. Which of the following graphs best shows the potential difference across a capacitor discharging through a resistor?  

capacitor discharge

8. Which of the following graphs best shows the potential difference across a capacitor charging through a resistor?


capacitor charging x4

9. The energy stored in a capacitor is E when charged to a voltage V0. After a time τ the potential difference across the capacitor has fallen to 0.69 V0 as it discharges through a resistor. This means that the energy transferred to the resistor in this time will be:

  • A. 0.31 E
  • B. 0.692 E
  • C. (1 - 0.692) E
  • D. 0.312 E

10. A capacitor discharges through a fixed resistor. The time constant τ for the capacitor and resistor is 10 seconds. At what time will the charge across the capacitor have dropped to half the original value?

  • A. 0.5 e-10
  • B. 10 e-0.5
  • C. 10 ln(-0.5)
  • D. -10 ln(0.5)