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I Magnet + Disc + Headphones

  1. May 26, 2017 #1
    I did a little experiment the other day. It involved connecting headphones to a copper disc and a battery. Then I placed a magnet under the disc. When current flows with the magnet present I get this strange sound similar to squeezing ice. What am I hearing? I dont believe it to be the electrical contacts.

    Think of a homopolar motor with the disc held stationary and headphones connected in series.
  2. jcsd
  3. May 26, 2017 #2


    Staff: Mentor

    That's a fun question. Moving the magnet will cause eddy currents in the disc which might be detected by the pickup. Other than that, I have no idea.
  4. May 26, 2017 #3
    Do you think its possible that its the sound of electrons squeezing past impurities of the conductor? Or maybe the sound of electrons interacting with a magnetic field.
  5. May 26, 2017 #4


    Staff: Mentor

    No. Headphones don't react to sounds, they respond to electric signals in the wires. Headphones convert those electric signals to sound.

    A microphone converts sound to electric signals, but you don't have one.
  6. May 26, 2017 #5


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    Science Advisor

    I'd have to imagine that the transduction would work both ways, in both devices, to at least some extent.
  7. May 26, 2017 #6


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    Staff: Mentor

    Can you post a picture or sketch of how you are setting this up? Thanks.
  8. May 26, 2017 #7
    There is a similar experiment done by connecting a transformer winding to the input of a hi fi amplifier and bringing a magnet close to the core of the transformer. You can hear clicks as the magnetic boundaries pass over each other.
  9. May 26, 2017 #8
    Ill post a picture after work.

    Yes, but the magnet is stationary. It is sitting under the copper disk.

    Im suggesting the electrons are carrying some type of noise into the signal.
  10. May 26, 2017 #9


    Staff: Mentor

    That's wrong thinking. If the electrons made sound, you could hear it with a doctor's stethoscope, not electronic headphones.

    Don't forget that a magnet rubbing a disc is metal rubbing on metal, that makes a scrape noise even if there is no magnet.
  11. May 26, 2017 #10
    I dont think Im explaining it well. When I say noise I dont mean something that can be heard. I mean interference or static that is converted into sound by the headphones. Again, Im not sure why this is happening. But, I do want to articulate my theory so that if I am wrong, I can understand why.
  12. May 26, 2017 #11


    Staff: Mentor

    That's better. Then I return to my answer from post #2.

    What you called "interference or static", I called "eddy currents". For an explanation, read this article. https://en.wikipedia.org/wiki/Eddy_current
  13. May 26, 2017 #12


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    Gold Member

    This sound like breathing or clicking is when the domains of the magnet are being aligned by the applied field. It is only heard when the field is changing.
  14. May 26, 2017 #13


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    2017 Award

    again so we could all understand what you are doing .... a photo or two would help lots, else we are all just guessing
  15. May 26, 2017 #14
    This is how I set it up. This time, I didnt get the effect without holding the wire. So I will charge the batteries and try again tomorrow.

    The very first time I did it I used more than 1 disc. They were disc cut out of aluminium foil and stacked where the current flows out of the center of 1 disc and into the center of another. The set up in the picture is what I used a few days ago.
    Last edited by a moderator: Jun 19, 2017
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