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Simple question about capacitor

  1. Aug 7, 2012 #1
    My friends, it is probably an easy one, but I dont know it and I have an exam tomorrow :X

    1. The problem statement, all variables and given/known data
    How much time does it take for a capacitor to reach %80 of its max. potential difference?

    2. Relevant equations

    Thats the part I m needing.

    3. The attempt at a solution


    Thank you for your help!
     
  2. jcsd
  3. Aug 7, 2012 #2

    lewando

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    It depends on the charging circuit. If an ideal capacitor is attached to an ideal voltage source through a switch, when the switch is closed, the capacitor will charge instantaneously. Non-ideal devices will have some resistance which will affect the charging time.
     
  4. Aug 7, 2012 #3
    Hmm... I have translated the question from my test completely, and there isnt any other information given. So, does it mean that the question is incomplete?
     
  5. Aug 7, 2012 #4

    cepheid

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    As mentioned before, in real life, even if there are no components in the circuit but the capacitor, the circuit will always have some non-zero resistance. So, if you assume that the capacitor is charging through a resistor, you can then answer the question.

    How long does it take for the capacitor to reach 80% of full charge in an *RC* circuit?
     
  6. Aug 7, 2012 #5

    tiny-tim

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    Hi Cetullah! :smile:
    That's like asking "How long is a piece of string?" :confused:

    You have to know something about the capacitor (and the circuit it's in) to answer that!

    Are you sure a "time constant" isn't given?​
     
  7. Aug 7, 2012 #6
    Hi tiny-tim :smile:

    Yeah, thats a weird question, but that's what exactly writes on the paper.

    Our teacher is a weird man anyways.

    But I guess, if sufficient data were given, we would use the equation "Vc=V*(1-e^(-t/RC)), right?
     
  8. Aug 7, 2012 #7

    cepheid

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    That equation seems about right. The question I asked you in my previous post still stands. There is not reason why you cannot express an answer in terms of the time constant.
     
  9. Aug 7, 2012 #8

    tiny-tim

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    i agree :smile:
     
  10. Aug 7, 2012 #9
    Oh cepheid, so you advice me to substitute the "t" with something like "kRC"?

    Edit: Did so, and found one of the answers on paper "1.6"
     
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