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Homework Help: Instantaneous current value in AC circuit

  1. Nov 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A sinusoidal supply defined by ## v(t)= 325.27 sin(314.16t+\frac{\pi}{6}) ## is connected
    across a coil of resistance 25Ω and inductance of 0.5 H. The instantaneous value of the current at 2.968ms is
    13.13 mA
    21.6 mA
    2. Relevant equations
    ## i(t) = \frac{v(t)}{Z} ## where Z is impedance
    ## Z= \sqrt{R^2 + ω^2L^2} ##
    ## ω= 314.16 radian/s ##

    3. The attempt at a solution
    ## i(t) = \frac{v(t)}{Z} ##
    Substituting all values getting,
    2.0315 A which is not matching with any of the options.
    What is wrong here?
  2. jcsd
  3. Nov 6, 2015 #2


    User Avatar
    Homework Helper
    Gold Member

    If I'm not wrong, t=2.968ms is the time elapsed after the circuit was switched on. I think you should go for transient response. Your calculations are correct, but they are for the steady state. 2.031A is the peak current. Time constant of your cicruit is 20ms.
  4. Nov 6, 2015 #3


    User Avatar
    Science Advisor

    When looking for the instantaneous current, you need to consider the phase angle of the impedance. Subtract that phase angle from the v(t) phase and you will get an answer that matches one of your options.
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