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Homework Help: The current in the circuit ?

  1. Dec 21, 2009 #1
    The current in the circuit ...??

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

    An AC voltage of the form V = Vmax sin 2*pi*f*t , with frequency 22 Hz and maximum voltage 763 V, is applied across a 17 W light bulb. When the voltage is first applied.

    a) what is the current through the circuit?

    Three Choices :

    1. the maximum current
    2. 0
    3. the rms current

    What is the correct answer and why ... ??

    b) When t = T/4 , where T is the period of the applied voltage, what is the current in the circuit?

    2. Relevant equations

    P = I * V
    I max = [tex]\sqrt{}2[/tex] * I rms
    T = 1/f

    3. The attempt at a solution

    In part a) i think that when the voltage is first applied means that time is ZERO so it could be ZERO the options # 2

    In part b) I substituted t = 0.25T or 0.25/f in this formual

    V = Vmax sin 2*pi*f*t

    I found V then I used P = I * V and solved for I but i got wrong answer.
  2. jcsd
  3. Dec 21, 2009 #2
    Re: The current in the circuit ...??

    17 W is the average power over a full cycle. It's equal to [itex] I_{rms} V_{rms} [/itex]. You used this value as if it was the instantaneous power at t=0.25T
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