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Inductance in DC Networks

  1. Jul 11, 2012 #1
    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.
    (a) the time constant of the circuit
    (b) the voltage drop across the inductor after two time constants
    (c) the voltage drop across the resistor after three time constants
    (d) the resistance of a 0.2 H coil used to replace the inductor if the
    circuit’s time constant falls to 0.55 ms.

    This is the full question, I have calculated A, B, and C, however I am struggling with D, any help would be great and clarification of my current answer would be even better

    (a) 0.606ms
    (b) 1.62V
    (c) 11.4V
    (d) :'(
  2. jcsd
  3. Jul 11, 2012 #2


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    Homework Helper

    The coil is equivalent with an ideal inductor and a resistor connected in series with it. The time constant is related to the inductance and the total resistance in the loop. You know the total resistance from the new time constant...

  4. Jul 11, 2012 #3
    so, resistance = inductance / time constant
    R = 36 ohms????
  5. Jul 11, 2012 #4


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    Staff: Mentor

    Close. Try calculating it again.
  6. Jul 11, 2012 #5
    Hmmm, I still get the same answer. Am I along the right track with the equation used?
  7. Jul 11, 2012 #6


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    Staff: Mentor

    Show your calculations.
  8. Jul 11, 2012 #7
    I actually don't know where i got that answer from, my calculations are now something way different.

    I am completely off track here, im all confused.
  9. Jul 11, 2012 #8
    is the answer 33 ohms!!!
  10. Jul 11, 2012 #9


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    Staff Emeritus
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    Education Advisor

    You're probably not going to get much help if it's only going to be a guessing game as to what you've actually done. Show your calculations.
  11. Jul 12, 2012 #10


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    You don't normally get many marks for the answer, most of the marks are for showing how you got there.
  12. Jul 12, 2012 #11


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    Staff: Mentor

    That looks right.
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