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Interesting thought about simple induction

  1. Feb 18, 2009 #1
    Hello All,

    Was thinking about some of the basic concepts of magnetic induction, in the case of a stationary solenoid coil with a moving magnet crossing the face of the coil. We know that when the magnet approaches the face of the coil and the field increases, that a votlage potential (emf) is induced in the windings, which generates a current, i, in the windings. Also, as the magnet proceeds past the face of the coil, the magnetic field decreases and the opposite voltage potential is induced in the coil and a current is driven in the opposite direction.

    Now assuming that I'm on par so far, I have made the assumption in the above example that the coil was either shorted to itself or across a load of some sort so that a current can actually flow when the emf from the magnet is induced into the windings. But what would happen if the coil was open-circuited while the magnet was approaching the coil, but then shorted to itself right as the magnet passed over the face of the coil? Would a current still be generated at that point in time (assuming that the magnetic field has not yet started decreasing). If a current is generated at this point, was the energy stored capacitively in the windings while the magnet was approaching and the coil was open-circuited? Or would nothing at all happen?

    Jason O

    EDIT: Argh.. Typos!
    Last edited: Feb 18, 2009
  2. jcsd
  3. Feb 18, 2009 #2
    If the magnet is moving, and the coil is shorted, of course current has to flow. When the coil was open circuited, then the energy was indeed stored capacitively per W = 0.5 * C * V^2. Have I answered your question?

  4. Feb 18, 2009 #3
    Partially. I still have two more questions here. If the energy is stored capacitively in the windings of the coil, then how would one quantify this capacitance? Is it just the parallel parasitic capacitance between the windings? Also, once the winding is shorted with this potential in it, would the current discharge from the coil like a capacitor or would one expect to see the standard inductance behavior in this case? The reason I ask is because in this case, the potential is generated in the coil rather than applied to it from an external source, so I'm curious to know if the capacitive energy would still make the coil behave like a coil or like a capacitor?

    Jason O
  5. Feb 18, 2009 #4
    It behaves like a coil with some series resistance and shunt capacitance. At very low frequencies, the capacitance is negligible. The influence of the capacitance becomes more pronounced as freq increases. As far as quantifying the capacitance, it is non-linear and can be estimated from tables published by inductive component suppliers. An online search should turn up something. Is this helpful?

  6. Feb 18, 2009 #5
    Yes it is, thank you :smile:
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