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: LR circuit, book mistake?

  1. Apr 2, 2012 #1
    problem: switch of LR circuit is closed at time = 0; what is ratio of inductor's self-induced emf ( E(L) ) to battery's emf ( E(bat) ) at t = 2τ?

    my solution:
    switch is closed so current begins to build up according to equation:
    i = (E(bat)/R) (1-e^(-t/τ) )
    multiplying R on both sides and plugging in for t = 2τ:
    E(L) = E(bat) (1-e^(-2) )

    E(L) / E(bat) = 1-e^(-2) ≈ 0.865

    But my book says the answer is 0.135, which is just the value of e^(-2), which you would obtain if you used the equation for current decay

    i = (E(bat)/R) (e^(-t/τ))

    instead of the equation for current rise. But the current in this case is clearly rising. So is my book wrong or am I wrong?
  2. jcsd
  3. Apr 2, 2012 #2


    User Avatar

    Staff: Mentor

    R*I is not the voltage across the inductor, it's the voltage across the resistor...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook