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Two ferromagnetic balls, rolling down a ruler

  1. Jan 16, 2007 #1
    Here's the situation:

    We have a ruler, inclined slightly with a strong ceramic magnet at the end. Now, we put two identical ferromagnetic (steel, for example) balls on the top of the ramp, touching each other. We let the balls roll down the ramp, and, perhaps against intuition, the balls stop before touching the magnet.

    I'm trying to figure out why this is, but I am not sure what's causing the forces in the end. Are the induced magnetic fields polarizing the balls in a certain way? I have a grasp of what's going on, but the conceptual details are escaping me.

    Thanks in advance.
  2. jcsd
  3. Jan 16, 2007 #2
    If it is induced EMF, the ball wouldn't stop completely, since if it does stop completely, there will no longer be any EMF, and gravity will pull the ball down.

    So, it shouldn't be just induced EMF... other than that, I'm clueless as well.
    Last edited: Jan 16, 2007
  4. Jan 17, 2007 #3
    I thought so as well, but then I thought that maybe the induced magnetic field is sufficiently strong to magnetize the balls, which would create permanent dipoles.

    I'm still not sure, though.
  5. Jan 18, 2007 #4
    I am assuming that the balls are rolling down the ruler at the same time and are in a single file line.

    In the above situtation what happens is as the balls roll down the slightly inclined ramp the approach the magnet. The closer the balls get to the magnet the stronger the polarization of the steel balls. As the balls polarize they then attract one another. Once the two balls are attracted strongly enough they balls essentially become one object looking very much like a peanut, which as you might guess does not roll down hill end over end.
  6. Jan 19, 2007 #5


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    I'm not sure if it's relevent to here but if you have a round and uniform magnetic bowl then if you drop a ferromagnetic ball into it the ball will hover in the air. the reason for this is that the magnetic field of the bowl create eddy currents on the surface of the ball which in turn create magnetic fields which push the ball out of the bowl.
  7. Jan 19, 2007 #6


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    I believe quinn have provided a good explanation, if the interpretation of the set-up is correct.
    How do Eddy currents provide anything more than a damping force? You are claiming they provide a restoring force. You will then need to address the point raised by tim above: when the ball is hovering above the cup (i.e., stationary), there should be no induced eddy current, and hence no upward force on the ball.
  8. Jan 19, 2007 #7
    Your assumptions of the system are correct.

    Here's what I thought -- is this correct?

    Induced currents in the balls will polarize one of the balls. This induced magnetic field in the one ball will induce a current and an opposite magnetic field in the other ball. The balls will magnetize with opposite dipoles -- thus they will attract each other.
  9. Jan 20, 2007 #8


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    Your're right - but if the balls are superconductive then the eddy currents caused initially will continue for ever.

    quinn: are you assuming that there's enough friction so that the balls can't slide without rolling?
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