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Homework Help: Force between two solenoids

  1. Sep 6, 2014 #1
    1. The problem statement, all variables and given/known data
    Two solenoids are placed in a cartesian xy-coordinate manner with their bottoms pointing in the negative y-direction, and their tops pointing in the positive y-direction. The solenoids are placed a distance x from each other (on the x-axis).

    The solenoids are identical, each having length L, diameter d, N turns, resistance R and negligible self-inductance L.

    The left-most solenoid has current I running through it.

    Find the force, as a function of distance x between the solenoids, that is required to pull the right-most solenoid away from the left, with a constant velocity.

    2. Relevant equations
    I am not sure.

    3. The attempt at a solution
    My understanding is that the left-most solenoid having current running through it will produce a magnetic field, drawing the right-most solenoid towards it. I'm looking for a force that exceeds this magnetic force in magnitude, in order to get the solenoid moving in the +x-direction.

    It makes sense to me that the force needed should be less for increasing distance from the left-most solenoid, since the magnetic field from it is weaker when moving away from it.

    However, I can't seem to find a good expression for the magnetic field, radially outward from the left-most solenoid. All I find on the internet is some crazy formula containing "elliptical integrals of 1st and 2nd order"..

    I'm starting to think I'm looking at the problem the wrong way.

    Help! =)
  2. jcsd
  3. Sep 6, 2014 #2

    Simon Bridge

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    Hint: think induction.
  4. Sep 7, 2014 #3
    Hi Simon and thank you for your reply! I gave your hint a shot, and this is what I came up with:

    Since the coil is of N turns, the induced emf in the coil running current through it should be given by
    ε = -N(dø/dt).​

    ε is also given by Ohm's law, ε = IR, meaning
    IR = -N(dø/dt) ⇔ dB/dt = -IR/(NA)​
    since the cross-sectional area of the coil is constant I took it out of the derivative. To clarify, A is the cross-sectional area I'm talking about,
    A = (d/2)2π → dB/dt = -IR/(N(d/2)2π).​

    From this I think it's pretty fair to say that dB/dt is constantly decreasing, meaning B(t) is decreasing linearly.

    Is this decrease along the symmetry axis of the solenoid though, or is it actually appliable to the radial axis, the axis I think I'm looking to get the B-field for?

    How does this help me in terms of finding the force required to push the right-most solenoid away from the left-most? I don't see the connection force-induction.
  5. Sep 7, 2014 #4

    Simon Bridge

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    Sketch the situation... How would you normally treat that?
    I'd try either treating as two magnets or use work-energy.
    You are right... You need the radial field or some approx. I'm thinking you have a set of notes to deal with that.
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