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Homework Help: Alternating Current (multiple CHoice)

  1. Dec 2, 2011 #1
    1. The problem statement, all variables and given/known data
    An alternating voltage is given by [tex]v = 30\sin(314t)[/tex].The time taken by the voltage to reach 30V for the first time is:
    A. 0.02s
    B. 0.03s
    C. 0.1s
    D. 0.015s

    2. Relevant equations
    google for equation for instantaneaous current and voltage

    3. The attempt at a solution
    30 = 30sin(314t)
    1 = sin(314t)
    pi/2 = 314t
    t = 0.005s
    not in the choices

    30 = 30sin(314t - pi)
    pi/2 = (314t - pi)
    t = 0.015 from the choices

    why do i need to subtract 180 degrees inside the sine eqn instead of adding
    ( Formula for n repeating cycles is v(t + nT) )
    Last edited: Dec 2, 2011
  2. jcsd
  3. Dec 2, 2011 #2


    User Avatar

    Staff: Mentor

    It would appear that the question is incorrect. Starting from time t = 0, the first positive peak occurs at approximately 5ms (milliseconds), which is what you first calculated.

    By the way, is that the complete statement of the question in its original form?
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