 µ IB | Capacitance

10 quick questions

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)
A
10-3 5 x 10-4
B
10-5 5 x 10-4
C
10-3 5 x 10-3
D
10-5 5 x 10-3

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

Series Parallel
A
3F 3/2 F
B
2/3 F 3 F
C
3 F 2/3 F
D
3/2 F 3 F

5. Identical capacitors are connected in different combinations. Which of these capacitor networks has the largest value? 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? 8. Which of the following graphs best shows the potential difference across a capacitor charging through a resistor? 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)