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Capacitance and energy storedEdexcel International A Level Physics: Flashcards

What these 12 flashcards ask

  • Define capacitance.
  • What is the unit of capacitance?
  • How is the energy stored in a capacitor found from a graph?
  • Why is the area under the V–Q graph a triangle?
  • State the three equations for energy stored in a capacitor.
  • Derive W = ½CV² from W = ½QV.
  • Derive W = Q²/2C from W = ½QV.
  • What happens to the stored energy if the p.d. across a capacitor doubles?
  • Convert 470 μF to farads.
  • How much work is done in adding a small charge ΔQ at p.d. V?
  • Why is the stored energy ½QV and not QV?
  • For the same stored energy, how does the p.d. change if the capacitance is halved?

Exam questions on Capacitance and energy stored

  1. A capacitor of capacitance 470 μF is connected across a 12 V d.c. supply until it is fully charged.
    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
  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.
    Calculate the average power delivered to the flash lamp during the pulse.2 marks
  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.
    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
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).