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Homework Help: Determining power across a certain device.

  1. Sep 18, 2012 #1
    1. The problem statement, all variables and given/known data

    The voltage across and current through a device are given by:

    v(t)= 5cos(4∏t)V
    i(t) = 0.1cos(4∏t)A

    determine the instantaneous power p(t) at t=0 and t = 0.25s

    2. Relevant equations


    3. The attempt at a solution

    for t=0 I got p=0.5W

    for t=0.25 I got p = 0.5W

    the textbook says at t=0 it should be 0. Does power always equal 0 at t=0 or did i mess up on my calculation?
  2. jcsd
  3. Sep 18, 2012 #2


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    Staff: Mentor

    Are you sure that both v and i were specified as cosines, and that neither included a phase angle?
  4. Sep 18, 2012 #3
    I copied it exactly the way it's written in the textbook.
  5. Sep 18, 2012 #4


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    Staff: Mentor

    Well, very strange then; the textbook's answer for t=0 appears to be incorrect.
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