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Coil resistance question

  1. Sep 16, 2014 #1
    Good afternoon all,

    I recently submitted a assignment and my lecturer has questions a figure i have put as an answer.

    Question was; An inductor of negligible resistance and an inductance of 0.2 H is connected in series with a 330 Ω resistor to a 12V d.c. supply.

    Determine: the resistance of a 0.2 H coil used to replace the inductor if the circuit’s time constant falls to 0.55 ms.

    I used R=L/t = 0.2/0.00055 = 363.64ohms
    circuit original resistance is 330Ω so an additional 63.64Ω would be apostioned to the new coil??? He has asked where the 63.64ohms has come from??

    Am i right in what i've siad or have i totally misunderstood?

    Any help would be great, i still passed the assignment but just want to be sure.

    Thanks
     
  2. jcsd
  3. Sep 16, 2014 #2

    gneill

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    Staff: Mentor

    Your method new total resistance look fine. But pay attention to the old resistance: was it 300 or 330 Ohms? :wink:
     
    Last edited: Sep 16, 2014
  4. Sep 16, 2014 #3
    Sorry my bad, it should read 33.64ohms
     
  5. Sep 16, 2014 #4

    gneill

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    Staff: Mentor

    Ta da! :smile:
     
  6. Sep 16, 2014 #5
    Hi sorry the 33.64ohms was the answer I had put above was just a typo. Why is he questioning this figures origin?
     
  7. Sep 16, 2014 #6

    gneill

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    Staff: Mentor

    If your answer was 33.64 Ω then I don't know why he would question it, unless you didn't show your work.

    The alternative is that he misunderstood the question and assumed that the 330 Ω resistor was replaced as well. Then the new coil would have to have the full resistance value of about 364 Ω.
     
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