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Faraday's Law and electric force

  1. Dec 31, 2013 #1
    I'm working on personal research problem while taking leave from school to work a Co-op. The basic idea of what i'm trying to create is a device that will produce electrical charge without having to be charged or have some outside dependence of electric force. the only way I could think to do this -and work with the design limitations i have- is to use a magnet, some copper wire, and a plastic tube. I'm trying to produce a small device that puts out about 40 mV rms. I know that -basically- according to Faraday's Law if you have a changing magnetic flux passing across a wire you will get a current to flow through the wire and a potential will form at the terminals. The way i'm implementing this design is to wrap the wire around the plastic tube -thickness about 2.75mm- and then put a spherical magnet -3mm- on the inside of the tube (I don't have a gauss meter to tell me how strong the magnet is). The magnet will move inside the tube and across the coil of wire and induce a voltage. What i need to know is: with the thickness of this tube; how many turns would I need to create the desired 40 mV; i know the answer lies with Faraday's law but i'm having trouble deriving equations to fit my application.

    Can someone possibly give some direction?

    Thank you,
  2. jcsd
  3. Jan 1, 2014 #2

    Simon Bridge

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    Welcome to PF;
    You have just described a perpetual motion machine - this is not possible.
    Please review:
    ... discussion of perpetual motion is not permitted in these forums.


    The device you have described in your application requires some outside source to move it about - so the magnet moves through the coil. i.e. it needs to be charged.

    Perhaps you are happy with making a normal electric generator?

    The fastest approach, since wire is cheap, will be to experiment - put a meter on the terminals and shake it for different numbers of windings and see what the actual relationship is.
    Last edited: Jan 1, 2014
  4. Jan 1, 2014 #3
    You can use a normal compass to find out how many gauss your magnet is. Procedure here. The whole thing is pretty fun and recommended just for learning magnet stuff, but you only need B for your magnet to solve your practical problem.

    Once you have that, plug into here and solve for N.

    As you can see though, it needs a changing B. So you will have to kick it around or shake it. Note that when macroscopic objects collide like that you are experiencing the electromagnetic force. Any solution will require energy in the form of work.
  5. Jan 1, 2014 #4


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    Hey Tyler! Just out of curiosity, why not use a solar panel for this device (assuming a light source is available)?
  6. Jan 1, 2014 #5
    The application I'm needing to use this for is underwater and I'm not one hundred percent sure, but, I don't think a solar panel would be practical :)
  7. Jan 1, 2014 #6

    I can see where you would think that; perhaps I was not clear in my explanation: I have mechanical force moving the magnet inside the coil, I was simply referring to the fact that something like a capacitor or a battery wold be impractical for what I'm needing to do.

    Thanks to all of you for your reply's! :)
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