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What is the time constant in an LC circuit.

  1. Nov 28, 2016 #1
    1. The problem statement, all variables and given/known data
    I wrote an exam today and a question worth quite a bit of points asked Determine the value of a capacitor in a series circuit that will give a time constant of 4.0 ms and is isn series with an inductance of 3mH

    2. Relevant equations


    3. The attempt at a solution
    I do not believe there is a time constant since the circuit is in natural response so therefore this voids the problem
     
    Last edited: Nov 28, 2016
  2. jcsd
  3. Nov 28, 2016 #2

    robphy

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    Could it be asking for the period of oscillation?
     
  4. Nov 28, 2016 #3
    Sorry let me update the description exactly, The question was: Determine the value of a capacitor in a series circuit that will give a time constant of 4.0 ms and is in series with an inductance of 3mH. I thought about the period of oscillation but it particularly said time constant. They are very strict and won't let you ask anything.
    My thoughts were it was a trick question
     
  5. Nov 28, 2016 #4

    NascentOxygen

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    I think it would be true to say that any expression comprising circuit constants and which has units of time (e.g., seconds) can be termed a time constant.

    The reciprocal of seconds is written s–1 and can be pronounced "Hertz".



     
  6. Dec 4, 2016 #5

    rude man

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    The question is misleading and confusing. A time constant is associated with an exponential rise or decay, which is not the case here. A period is not a time constant in electrical parlance.
     
  7. Dec 4, 2016 #6

    LvW

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    rude man - yes, in general I do support your view .
    However, in this case, I am inclined to say that the ENVELOPE of the decaying sinusoidal step response (exponential envelope function) could be the answer (most probably)
     
  8. Dec 4, 2016 #7

    NascentOxygen

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    Doesn't that sound too easy, given that the circuit presented is lossless? Especially when ...

    In exams, be guided by the number of marks awarded.

    No examiner gives marks for a candidate stamping his feet and complaining that a question is not well-phrased. The examiner has a particular answer in mind, and marks are awarded according to how well candidates manage to divine his answer from the clues he casts about.
     
  9. Dec 7, 2016 #8
    So the exam was returned and the answer was just the period of this circuit. However since the question can perhaps create confusion everyone was given full credit on the problem.
     
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