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: Free fall of straight wire in a homogeneous magnetic field

  1. Apr 4, 2012 #1
    hello every body. I have a high school problem
    a straight horizontal wire is falling freely in a homogeneous horizontal magnetic field, perpendicular to the wire and i want to find the inductive voltage.
    I said E= Blv=Blgt
    But I can also say E=ΔΦ/Δt=BΔΑ/Δt=Βl1/2gt [t][2]/Δt=1/2Blgt
    why doew this difference of 1/2 arise?
  2. jcsd
  3. Apr 4, 2012 #2

    Your problem arises because of something like this:

    v = gt, but... v = 1/2gt too?

    That's because on the first equation, you were finding INSTANTANEOUS velocity, so therefore INSTANTANEOUS EMF.
    On the second equation, you somehow got average velocity... I'm still trying to figure it out.

    A-ha! Okay, I think I got it figured out.
    When you substituted 1/2gt for X, and divided by t to get 1/2gt, you actually found the average velocity, since it's NOT instantaneous velocity. The velocity constantly changes because the wire is accelerating.

    Both of your equations are right, except the first one is instantaneous EMF after a certain T, and the second equation is average EMF with respect to T.
    Last edited: Apr 4, 2012
  4. Apr 5, 2012 #3
    thanks a lot
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook