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Homework Help: Electromagnetism, finding velocity and acceleration

  1. Jun 2, 2013 #1
    1. The diagram attached shows a 0.010 kg metal rod resting on two long horizontal frictionless rails which remain 0.40 m apart. The circuit has a resistance of 3.0 W and is located in a uniform 0.20 T magnetic field. Find the initial acceleration and maximum velocity for the rod.

    a) Initial acceleration: 120 m/s2; maximum velocity: 300 m/s
    b) Initial acceleration: 40 m/s2; maximum velocity: 190 m/s
    c) Initial acceleration: 120 m/s2; maximum velocity: 190 m/s
    d) Initial acceleration: 40 m/s2; maximum velocity: 300 m/s

    2. Relevant equations


    3. The attempt at a solution

    v=[itex]\frac{ε}{Bl}[/itex] = [itex]\frac{15}{0.20×0.40}[/itex] = 187.5 m/s

    Now I'm not exactly sure how to find acceleration.

    I know the initial v was 0 m/s and the final v was 187.5 m/s. I don't have the length of the rods or anything.

    Attached Files:

  2. jcsd
  3. Jun 3, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    Your emf = BLv equation assumes the only source of emf is the velocity of the bar in the B field. Buth there is also a second source of emf, to wit, the 15V battery.

    Ponder the following statement: "as long as there's current flowing thru the bar, the bar experiences force and therefore acceleration".
  4. Jun 3, 2013 #3
    Okay, I got it!

    I just use F=ma and solve for the acceleration, since both F and m are given.
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