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Homework Help: Magnetic field question?

  1. Jun 26, 2012 #1
    1. The problem statement, all variables and given/known data

    The conductive bar CD with a length of 20 cm and resistance R, moves with speed v=5 m/s. The magnetic field is perpendicular with the plan and the induction is B=2 T.
    The mass of the object in the figure is m=80 grams and g=10 m/ s^2
    a) find the direction and the intensity of the induced current that moves in CD
    b) The resistance of this bar

    2. Relevant equations

    Figure : http://s1171.photobucket.com/albums/r549/Elaia06/?action=view&current=hrtfg.jpg

    3. The attempt at a solution
    I tried using Emf=B*L*v and then I=emf/R but i dont know the R..
    I JUST WANT TO KNOW HOW TO RELATE Tension and weight with induction//
  2. jcsd
  3. Jun 26, 2012 #2
    If the bar is moving at a constant speed this tells you something about the resultant force on the bar?
    There is a force due to the weight of the mass and there is a force due to any induced current flowing in the wire CD.
  4. Jun 26, 2012 #3
    Hello Elaia06,
    You have to find the induced current first which as truesearch points out bases itself on the fact that the rod is moving at a constant velocity.Tension can be found out basing your derivations on the same fact.Direction of induced current comes out as a result of the same(analyse the forces on the rod and see which direction of current corresponds to equilibrium).Then for resistance you have adequate expressions in your attempt .
  5. Jun 27, 2012 #4
    Ah, the resultant force is zero.This means the only force acting on the bar is the magnetic force F=B*i*l.
    Now we have the Tension= mg-ma but acceleration is zero so we write
    Magnetic force = Force on the string
    I know how to solve this now
    The magnetic force will have the direction <---- based on the figure and the induced current the direction from left to right .

    Last edited: Jun 27, 2012
  6. Jun 27, 2012 #5


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    you haven't used v or R :confused:
  7. Jun 28, 2012 #6
    after i find I, then I use I=b*L*v/R and so I find R :)
    Is it correct in total?
  8. Jun 28, 2012 #7


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    yes, that's correct :smile:
  9. Jun 28, 2012 #8
    Thank you very much sweetheart! :D
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