1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Given voltage and current equations, find instantaneous Power

  1. Sep 5, 2009 #1
    1. The problem statement, all variables and given/known data
    The voltage across and current through a certain device are given by:
    v(t) = 5cos(4*pi*t)
    i(t) = .1cos(4*pi*t)

    (a) the instantaneous power p(t) at t= 0 and t = .25 seconds
    (b) the average power pav, defined as the average value of p(t) over a full time period of the cosine function (0 to .5 seconds)

    2. Relevant equations
    Power = Voltage * Current

    3. The attempt at a solution

    (a) I plugged 0 and .25 in for t into both of the equations, and multiplied the answers together. For both t = 0 and t = .25, I got .5 Watts for my answer. In the back of the book, it says for t = 0 it should be 0 watts, and for t=.25 s .5 watts. I got the answer correctly for t=.25s, but what am I missing for the beginning part? I have been going over this problem for a while now and I cannot figure it out for the life of me.

    (b) 1/(b-a) *integral (i(t)*v(t)) evaluated from a to b, a being 0 and b being .25 seconds. I'm pretty sure this is the right direction for this part.

    Thanks in advance for your time / help!
  2. jcsd
  3. Sep 6, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    (a) I agree with your answer, the power is 0.5 W at t = 0 s. You might double-check that the expressions for v and i you used are correct, but it sounds like you've looked things over pretty carefully already.

    (b) Yes, that's exactly the way to do it. Integrate p over 1 period.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook