Capacitance and energy storedEdexcel International A Level Physics: Subtopic test

1. 1
   A capacitor of capacitance 470 μF is connected across a 12 V d.c. supply until it is fully charged.

   (a)

   What is the charge stored on the capacitor when it is fully charged?

   [1 mark]

   • A3.9 × 10⁻⁵ C
   • B2.6 × 10⁴ C
   • C5.6 × 10⁻³ C
   • D5.6 × 10³ C

   (b)

   What is the energy stored in the capacitor?

   [1 mark]

   • A3.4 × 10⁻² J
   • B6.8 × 10⁻² J
   • C2.8 × 10⁻³ J
   • D5.6 × 10⁻³ J

   (c)

   The capacitor is now charged using a 6.0 V supply instead of 12 V. Calculate the energy stored and explain why it is not half of the energy stored at 12 V.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The flash unit of a camera contains a capacitor of capacitance 1500 μF. It is charged to a p.d. of 300 V and then discharged through the flash lamp in a pulse lasting 1.0 ms.

   (a)

   What is the charge stored on the capacitor when it is charged to 300 V?

   [1 mark]

   • A5.0 × 10⁻⁶ C
   • B2.0 × 10⁵ C
   • C4.5 × 10⁵ C
   • D0.45 C

   (b)

   What is the energy stored in the capacitor when it is charged to 300 V?

   [1 mark]

   • A135 J
   • B67.5 J
   • C0.225 J
   • D6.8 × 10⁷ J

   (c)

   Calculate the average power delivered to the flash lamp during the pulse.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A capacitor stores a charge of 2.4 mC when the potential difference across it is 8.0 V. The p.d. across a capacitor is directly proportional to the charge stored on it.

   (a)

   Use a graph of potential difference against charge stored to derive an expression for the energy stored in the capacitor in terms of Q and V.

   [3 marks]

   (b)

   Calculate the capacitance and the energy stored. Show that W = Q²/2C gives the same energy, and find the energy stored if the p.d. is doubled to 16 V.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A portable defibrillator contains a capacitor of capacitance 100 μF. The capacitor is charged until it stores 200 J of energy. It is then discharged through the patient's chest in a pulse lasting 5.0 ms, delivering 80% of the stored energy.

   (a)

   Calculate the p.d. to which the capacitor is charged, the charge it stores, and the average power delivered to the patient during the pulse.

   [6 marks]

   (b)

   An engineer proposes using a 50 μF capacitor in place of the 100 μF capacitor, still storing 200 J. Calculate the p.d. and charge needed for this capacitor, and evaluate the proposal.

   [6 marks]

   Total for question 4: 12 marks

End of questions