Capacitance and energy storedEdexcel A-Level Physics: Subtopic test

1. 1
   A 2200 μF capacitor is connected across a 9.0 V supply and allowed to charge fully.

   (a)

   Which statement defines the capacitance of a capacitor?

   [1 mark]

   • AThe energy stored per unit charge
   • BThe charge stored per unit potential difference across it
   • CThe potential difference across it per unit charge stored
   • DThe charge stored per unit time

   (b)

   The capacitor is charged by increasing the charge stored from zero. What does the area under a graph of potential difference against charge stored represent?

   [1 mark]

   • AThe capacitance
   • BThe charge stored
   • CThe energy stored
   • DThe reciprocal of the capacitance

   (c)

   Calculate the charge stored and the energy stored when the capacitor is fully charged.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The flash unit of a camera uses a capacitor of capacitance 120 μF, which is charged to a potential difference of 330 V before the flash is fired.

   (a)

   Which expression gives the energy stored in the capacitor?

   [1 mark]

   • A½CV
   • BQ²/C
   • CV²/2C
   • D½CV²

   (b)

   The capacitor is charged to 165 V instead of 330 V. What is the energy stored compared with the original value?

   [1 mark]

   • AOne quarter
   • BOne half
   • CDouble
   • DThe same

   (c)

   Calculate the energy stored in the capacitor at 330 V.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A memory-backup circuit uses a 0.47 F capacitor charged to 5.0 V. When the supply fails, the capacitor powers a data logger that needs a potential difference of at least 3.0 V across it to work. While it works the logger transfers energy at a constant 0.80 W.

   (a)

   Calculate the energy stored in the capacitor and the charge stored on it when it is fully charged.

   [3 marks]

   (b)

   Calculate how long the logger can work after the supply fails.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student charges an initially uncharged 470 μF capacitor through a resistor from a 12 V battery of negligible internal resistance, until the capacitor is fully charged.

   (a)

   The capacitor has capacitance C. Show that the energy stored at a final potential difference V is ½QV, and hence that the energy stored is also ½CV² and Q²/2C.

   [6 marks]

   (b)

   The student says that the energy stored is QV, because the battery moves a charge Q through a potential difference V. Evaluate this statement.

   [6 marks]

   Total for question 4: 12 marks

End of questions