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Motional emf

  1. Mar 17, 2016 #1
    1. The problem statement, all variables and given/known data
    Two long parallel horizontal rails, a distance d apart and each having a resistance λ per unit length, are joined at one end by a resistance R. A perfectly conducting rod MN of mass m is free to slide along the rails without friction (see the figure). There is a uniform magnetic field of induction B normal to the plane of the paper directed into the paper. A variable force F is applied to the rod MN such that, as the rod moves, a constant current flows through R.
    Find the velocity of the rod and the applied force F as a function of the distance x of the rod from R.

    2. Relevant equations

    3. The attempt at a solution
    The emf induced will be Bvd, where v is the velocity of the rod at that instant.
    The current is i = emf/resistance = Bvd/(R+2λx).
    I know F = FB = idB = B2d2v/(R+λx)
    However, I don't know how to get rid of v in the F expression, and I don't know how to express v in terms of x. So, my aim is to figure out v, because then I'll get F as well.
    I figured since the current is constant, di/dx = 0
    ##\frac{d}{dx} \frac{Bvd}{R+2λx}## = 0

    ##( \frac{dv}{dx}) (R+2λx) = (v)(2λ)##

    ## \int \frac{1}{v} \, dv = \int \frac{2λ}{R+2λx} \, dx ##

    ## v = c(R+2λx) ## where c is an arbitrary constant.

    I don't know how to find c. Please help.
  2. jcsd
  3. Mar 17, 2016 #2


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    You have mass 'm' of the rod. You can replace F by m(dv/dt) and further using chain rule, F=mv(dv/dx).
  4. Mar 17, 2016 #3
    This might be a really dumb question but isn't Fnet = ma? And here Fnet = 0 as the variable force F is equal and opposite to the Lorentz force at any instant? I got the answer with this approach though. I just have this stupid question.
  5. Mar 17, 2016 #4


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    If the net force were 0, the rod would move with a constant velocity and the current would be decreasing since the resistance of the loop is increasing with distance.
  6. Mar 17, 2016 #5


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    Instead of what I said in #2, I believe your approach in the OP is correct.
    Here's how I think the problem should be..The rod is initially moving with some velocity u at some distance xo from the starting point, which drives a current Bud/(R+2λxo). A variable force is applied to maintain this current. I believe u and xo need to be specified. If xo is assumed to be 0, still u is unknown. If xo and u are known, constant of integration in your solution attempt can be eliminated.
  7. Mar 17, 2016 #6
    Wouldn't this be incorrect then? Because net force would be F - FB, I guess. I wrote F (variable external force) = FB (Lorentz force) assuming that they would be equal and opposite. However, I do agree with your conclusion that the current would then be decreasing and we'd have a constant velocity. I'm just not sure how to resolve F = FB dilemma.
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