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: Metal bar on conducting rails

  1. Jun 30, 2007 #1
    1. The problem statement, all variables and given/known data

    This is a Problem 7.7 fom Griffiths Introduction to Electrodynamics (3ed)

    A metal bar of mass m slides frictionlessly on two parallel conducting rails a
    distance l apart. A resistor R is connected across the rails and a uniform magnetic field B, pointing into page, fills the entire region.

    If the bar moves to the right at speed v, what is the current in the resistor ?

    2. Relevant equations


    3. The attempt at a solution

    my reasonig is:

    magnetic flux is:

    A0 is initial surface, and A1 is surface which bar makes moving to the right with spead v.

    A1=x*l=v*t*l , but v is also function of t, so: A1=v(t)*t*l

    I know that equation for A1 is wrong, becouse when I try to get electromotive froce
    I get this:


    in solution manual it's:


    Can someone explain why my reasoning is wrong, it seams logical to me.
  2. jcsd
  3. Jun 30, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Setting x = v*t assumes that v is constant.

    In any case, what matters is the rate at which flux changes, which depends on the speed at the moment in question:
    d(A1) = l*v*dt
    d(A1)/dt = l*v, even if v is changing.
    Last edited: Jun 30, 2007
  4. Jun 30, 2007 #3
    Ok, than you.
    That will help me to answer other question from that problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook