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Force on a Permanent Magnet Falling through a Coil

  1. Jun 18, 2009 #1
    What is the drag force on a magnet falling through a coil? I have found several sources online describing the drag force on a magnet falling through a conductive pipe. How might this vary if a coil is used in place of a pipe? Does the coil need to be connected to a circuit to create this force?
     
  2. jcsd
  3. Jun 22, 2009 #2
    The coil needs to be shorted for it to have an effect otherwise I don't think that the results differ that much.
     
  4. Jun 22, 2009 #3
    I think this is pretty much correct. The magnet would like to induce eddy currents, and a large piece of metal allows currents to form in loops. This is why transformer, and motor cores are often laminated to reduce eddy currents. However, an open coil structure does not allow very big circular loops to form, and sort of acts like a laminated piece of metal, only better. Once, the coil is shorted, you have a very nice low-resistance loop! Perhaps, in this case, you would refer to the currents as generator currents, rather than eddy currents, since you now have a simple linear generator.
     
  5. Jun 24, 2009 #4
    Thanks guys. From doing a little bit of lab testing and pondering, I gather that dropping a magnet down a conductive pipe produces lots of drag due to the Eddy currents forming inside the pipe. The same magnet falling down a coil experiences little drag and the drag force is probably negligible.

    You guys mention shortening the coil. Would that lower the force? Ultimately the goal of my project is power production. I am trying to model how this thing might behave and I was curious about a magnetically-induced drag force. But since the force seems tiny and more power is the primary objective, would a longer coil be better?
     
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