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Homework Help: Metal bar on conducdting rod take 2 (didn't post first time)

  1. Jul 28, 2008 #1
    . The problem statement, all variables and given/known data

    A metal bar is free to slide without friction along two parallel metal rails. The circuit is completed by a battery. (conducting bars are in the x direction, left side of drawing the conducitn bars are connected by a battery, the right side is where the metal bar is) If the ciurcuit is in a unifrom B as shown, which is into the page, in what direction will the bar move. If the rails have zero resistance but the bar has resistance R, will the current be V/R, > V/R, or <V/R (v is from power source).

    2. Relevant equations
    faraday's law of induction
    right hand rule

    3. The attempt at a solution
    well, i know for part a that because the positive part of the battery is up, and that the magnetic field flows into the page, that the current flows in a clockwise direction bc current flows from the pos. terminal of the battery. This thus forces the bar to move to the right, as determined by the right hand rule.

    Where i get stuck is determining the current. I know that the magnetic force pushes the bar to the right, which causes an increase in area for hte magentic field, and addditonally the electric flux. When the electric flux increases, so does the induced emf. when the emf increases, so does the current. thus, i would like to say that the current is >V/R.

    However, Lenz law continues to pop in my mind. lenz law says that an induced emf czuses the curent to flow in a way to oppose the change. thus the current might flow in the opposite direction to oppose such a change and result in the current being <V/r. i need help please, hw is due in 11 hrs. please help
  2. jcsd
  3. Jul 28, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    You are going about this the right way. You need to find the magnitude and direction of the induced current. Then, if the currents point in the same direction you can add them, otherwise you should subtract them.

    Lenz's law states that the magnetic field created by the induced current will point in the direction opposite to the change in the magnetic flux. What direction is this? What direction must the induced current be moving, according to the right hand rule?
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