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Homework Help: Electrical Science - LR Time Constant & Voltage Drops

  1. Feb 23, 2014 #1
    Electrical Science -- LR Time Constant & Voltage Drops

    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.

    Relevant Equations:
    a) T=L/R


    a) T=L/R
    T= 0.2/330
    T= 0.000606
    T= 0.606 ms
    *Quite confident on this one.

    b&c) I'm really struggling trying to ascertain what equations I need to calculate these two answers and that is where I need the help.

    d) T=L/R
    R= 0.2/0.00055
    R= 363.63 Ohms
    Resistance of coil = 363.63 - 330 = 33.64 Ohms
    *Quite confident on this one too.

    If someone has the time to confirm my answers for a) & d) look ok, and to help with the equations and path I need to take to calculate b) & c) it would be much appreciated.

    Thanks in advance :)

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 23, 2014 #2


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  4. Feb 28, 2014 #3
    Thanks, all sorted now... I appreciate your response.
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