Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Question concerning AC circuits

  1. Jan 25, 2007 #1
    Hi everyone,
    I ran into the following little problem:

    1. The problem statement, all variables and given/known data
    Given v=40 cos(100*pi*t + pi/3), the question is:
    what is the minimum number of millisec that the function must be shifted to the left if the expression for v is 40 cos(100*pi*t)

    2. Relevant equations

    3. The attempt at a solution
    ok, I started with:
    40 cos(100*pi*(t+t0) + pi/3) = 40 cos(100*pi*t)
    hence 100*pi*(t+t0) + pi/3 = 100*pi*t
    the 100*pi*t cancels out on each side and we can solve for t0 but t0 < 0

    BUT the book starts with:
    40 cos(100*pi*(t+t0) + pi/3) = 40 cos(100*pi*t + 2pi)
    and they solved for t0... and t0 is now > 0

    My question why they added the 2pi! Although the more I think about it the more it makes sense to me but I'd like to be sure... Is it because we look for the min time but still positive???
    If that's the case, what bothers me is: how are we supposed to know ahead of time to add this '2pi'?

    Thanks for your answer(s) o:)
  2. jcsd
  3. Jan 25, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    I think the answer is that they said "shift the function to the left" implying a positive time shift was being looked for.
  4. Jan 25, 2007 #3
    Oh my god... :surprised you are right, I can't believe I didn't think of it this way... That totally makes sense !!!:biggrin:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook