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: Electric Current as a function of time

  1. Mar 19, 2012 #1
    1. The problem statement, all variables and given/known data
    8. An electric current in a conductor varies with time according to the expression
    I(t) = 100 sin (120*pi*t), where I is in amperes and t is in seconds. What is the total charge passing a given point in the conductor from t = 0 to t = 1/240 s?

    2. Relevant equations

    3. The attempt at a solution
    I have attempted to substitute the values of t into the equation and use the difference, but I do not feel that was the correct way. Next, I attempted to perform a definite integral, but I get 0 for both numbers.
  2. jcsd
  3. Mar 19, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    welcome to pf!

    hi jmuduke! welcome to pf! :smile:
    yes, I = dQ/dt, so Q = ∫ I dt

    it's only over 90°, so you shouldn't get 0 for both limits :confused:

    show us what you did :smile:
  4. Mar 19, 2012 #3
    Thanks for the reply Tim!

    I calculated the integral and got -(5 cos(120*pi*t))/6*pi

    Originally, my calculator was set in radians, so that could have been why I got 0 for both limits. I changed it to degrees and got -0.265 for both limits then, but that result in the definite integral being 0, correct?
  5. Mar 20, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper

    hi jmuduke! :smile:

    (just got up :zzz:)
    yes, cos(0) = 1, so that's correct for the t = 0 limit :smile:

    but for t = 1/240, cos(120πt) = cos(π/2) = cos90° = 0 :wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook