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Coils & Questions

  1. Jan 27, 2008 #1
    Ok, you know those flashlights that you shake to charge the battery? I'm no physics major, but I get the basic idea. The magnet is passed through the coil, creating an electrical current. So here's my question:
    Imagine I changed the structure of the coil a bit. If I take a series of magnets (arranged in a straight line), wrap a coil around them, and then pass a heavy steel object rapidly over the coil, will it have the same effect? Grant it, the magnets aren't moving inside the coil, but the steel object outside the coil will interact with each magnet inside the coil as it passes over them, creating a traveling flux in the magnetic field inside the coil. So just like in the flashlight, there's an constantly changing magnetic field inside a coil of wires connected to a battery. Will it charge the battery?

  2. jcsd
  3. Jan 27, 2008 #2
    I imagine the changing flux will be significantly less than for the case of moving magnets, and thus the current generated will be significantly less. I predict you couldn't generate enough current to charge the battery enough to be practical.

    Any particular reason you ask? Did you have some kind of device in mind?
  4. Jan 27, 2008 #3
    Oh you know, glory, riches and solving the energy crisis. But I guess you've brought me back down to Earth. Let's look at this one more way. Forgive my ignorance if this is poses the exact same problem.

    This time I have a large coil in which small magnets are attached to every inch of the inner surface. Now I pass a steel object rapidly through the coil... anything?

    As you can see I'm trying to create a coil that generates electricity without the benefit of a moving magnet. I can only use moving metal objects. Any insight?
  5. Jan 27, 2008 #4
    I don't see any advantage to this. Magnets are metallic, so why do you prefer to have additional metal in there?
  6. Jan 27, 2008 #5
    It's not an advantage, it's a limitation :)
    Forgive me for being purposely vague
  7. Jan 27, 2008 #6


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    You'll find that whatever the mechanism there's a force between the moving object and the coils by which the mechanical energy is being converted (via negative work) to the electrical energy. Or as Larry Niven's Lunies would say: "TANSTAAFL"
  8. Jan 27, 2008 #7


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    Let me add, that any metal you have moving around the magnets will have eddy currents generated within them since they are conductors as well as potentially ferromagnetic. This will add greatly to the loss of energy in whateveritisyourtryingtodo. Think of the piece of metal as a coil which is shorted out.
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