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Homework Help: 2 hard physics question?

  1. Sep 13, 2006 #1
    Hi, I have been set a sheet of questions for homework and there are two which I cannot do. Any help you could give would be greatly appretiated.

    1)
    I have tried doing this but keep getting into cycles of inputting equations into it and just going around in circles.

    2) This is a two part question and I have done the first part.

    I = V/R

    I = 6/(5.6*10^-3)

    I = 1.071*10^-3A

    I dont have a clue how to do this!
     
    Last edited: Sep 13, 2006
  2. jcsd
  3. Sep 13, 2006 #2

    Hootenanny

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    Welcome to the Forums,
    Okay, start with the basic equations for capacitance and resistance;

    [tex]R = \frac{V}{I}[/tex]

    [tex]C = \frac{Q}{V}[/tex]

    Now using the fact that current is the rate of flow of charge ([itex]I = \frac{Q}{t}[/itex]) can you eliminate the current (I) from the first equation?
    Do you know the definition of the time constant?
     
  4. Sep 13, 2006 #3
    "Do you know the definition of the time constant?"

    No.
     
  5. Sep 13, 2006 #4

    Hootenanny

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    Okay, do you know any equations involving the time constant (besides t = RC)?
     
  6. Sep 13, 2006 #5
    "Okay, do you know any equations involving the time constant (besides t = RC)?"

    No sorry - t=RC is the only one given in the question.
     
  7. Sep 13, 2006 #6

    Hootenanny

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    Okay, well in this case the equation you need to know is;

    [tex]I = I_{0}e^{\frac{-t}{RC}}[/tex]

    Where I is the current at time t and I0is the intial current value. Has your tutor not given you this information?
     
  8. Sep 13, 2006 #7
    No. I have never been shown this equation beofre either. Thanks for all your help, but I think it would be best if I went and asked tomorrow, because I think he may have made a mistake.
     
  9. Sep 13, 2006 #8

    Hootenanny

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    No problem. Have you any thoughts on the first problem?
     
    Last edited: Sep 13, 2006
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