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Homework Help: Current in a capacitor question

  1. May 8, 2017 #1
    This is a question from an a level text book.


    The R starts at 8k ohms (thought its not relevant)
    Switch S is closed - C fully charges up.

    Question:Switch S is opened. Explain why the current decreases and REVERSES direction in the ammeter.

    This is what I think. The decrease in current is easy to explain but the REVERSE in direction is not.
    • When the switch is closed current flows -> (anticlockwise) through the ammeter
    • When the switch is opened current STILL flows anticlockwise as it leaves the capacitor.

    But the questions says there is a reverse of I so something is wrong in my answer.

    Can anyone please point out what I have got wrong!


  2. jcsd
  3. May 8, 2017 #2
    The question or the diagram is wrong.

    The current won't change direction.

    Textbooks aren't always correct.
  4. May 8, 2017 #3


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    Gold Member

    You are correct. Why it says the current reverses I cannot imagine.
  5. May 8, 2017 #4


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    Staff: Mentor

    Is there anything you're not telling us about R? A wirewound resistor perhaps?
  6. May 8, 2017 #5
    Thank you for your replies. Here's the whole question - I don't think I missed anything out... the first 3 parts are simple calculations. Underneath is what the answer in says. I tend to agree with the answers provided but I be really grateful for your considered opinions on the whole question!


    back of book "The capacitor starts to discharge with the current flowing in the opposite direction to the charging current" ---- i think that's a poor answer anyway because it is a statement, rather than an explanation of the reversal...
  7. May 8, 2017 #6


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    Staff: Mentor

    Part (d) doesn't make sense for the given circuit unless by "reverse" they are referring to the current magnitude getting smaller with time (falling back from its initial maximum value). Perhaps the question was originally formulated with a slightly different circuit in mind.

    It would have been more interesting had the ammeter been in series with the capacitor.
  8. May 8, 2017 #7
    Thanks... I dont think they mean that... it would asking the same thing twice, and in a very confusing way. if the ammeter was in series with the capacitor then it would make more sense as you suggest.
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