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Homework Help: 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


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    Gold Member

    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


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    Staff: Mentor

    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


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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


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    Staff: Mentor

    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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