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Homopolar motor explanation

  1. Jan 29, 2016 #1
    How these are working:





    Is current flowing? In the first case I don't think so. In the other... the wire is insulated like in a coil I think and then no current also. But why need the battery and why the battery is discharging?
    Can current flow through a magnet without demagnetize it? What does the magnet do here at all?
     
  2. jcsd
  3. Jan 29, 2016 #2

    Dale

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    Yes, current is flowing in both cases! The second one will not work with insulated wire. Depending on the design a homopolar motor can draw enough current to make the wire hot.
     
  4. Jan 29, 2016 #3
  5. Jan 29, 2016 #4

    CWatters

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    +1 to that.

    In both cases the battery supplies the power needed to cause motion. Neither will work without the battery. Can't be done with magnets alone because they aren't an energy source.
     
  6. Jan 29, 2016 #5

    sophiecentaur

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    Both demos are very cool.
     
  7. Jan 30, 2016 #6
    But electrons are flowing through the magnets. Is it demagnetize them? I mean e- particles change, one leave and one arrive which has to allign to the right direction to keep up the magnetic field.

    Most turns in the tunnel (on the second demo) are short circuited because contact each others without insulation. Why is it not problem?

    If you have time I would like a detailed explanation how does it work.
     
    Last edited: Jan 30, 2016
  8. Jan 30, 2016 #7

    sophiecentaur

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    Can you think of a reason why the net number electrons drifting through the magnet due to the Current would behave any differently from the same electrons that are in constant thermal motion? This only involves the valence electrons that are dissociated from any particular atom. If there were sufficient current to heat the magnet, the permanent magnetism could be destroyed, I guess.
     
  9. Jan 30, 2016 #8

    Dale

    Staff: Mentor

    No. The current through the magnet does not change the alignment of the magnetic domains unless it is really large.

    Here is a detailed explanation
    https://www.google.com/url?sa=t&sou...4X6xcPmEpVS7XVKEQ&sig2=j8PIIOm6TRIGy0oTm9d2uQ
     
    Last edited: Jan 30, 2016
  10. Jan 30, 2016 #9
    Electrons don't have to change their spin because of a little thermal motion. Big is demagnetize the metal of course. Current flow force spin to allign but electrons are already alligned to an other direction and will resist. The degree of R is up to the direction of the current flow.
     
  11. Jan 30, 2016 #10
    I read the resistance of a magnet is very high. I was thinking... this resistance is up to the direction of the current flow. Those electrons in a magnet stick on their position much more than on a non-magnetized metal. They have to change their spin against the other electrons and full domains. If the direction of current keep the left hand rule the resistance is low, the opposite direction give the highest R and so one...
    Am I right?

    Thanks, this pdf is really useful!
     
  12. Jan 30, 2016 #11

    Dale

    Staff: Mentor

    This is not correct, but even if it were it would not make a magnet demagnetize. The magnetization of a ferromagnetic material is due to the bulk alignment of the magnetic domain crystals. The spin of conduction electrons doesn't matter much.
     
  13. Jan 30, 2016 #12

    sophiecentaur

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    And they are in pretty much the same amount of motion even when there is no current. A charge flow of mm per second can hardly make any difference, except to add a small change in the external magnetic field.
     
  14. Jan 30, 2016 #13
    two things I want to say here , first of all observe! that most if not all demos involving a homopolar motor in its various layouts and forms involve the use of neodymium magnets , mostly round in shape.almoust all neodymium magnets have a conductive metallic coating which acts as a conductor.
    Because of this the neodymium magnet itself can be used as a motor even without any other coils or parts , ofcourse the battery is still needed as the energy source or any other energy source for that matter.
    an example here :


    It;s basically a homopolar disc with a magnet attached to it ,the disc being the conductive coating.


    the second thing is more of a question i want to add from myself, in the first demo and youtube is full with such demos , a battery stands in the middle with a small neodymium and a twisted wire rotating around it.Now I don't want to go in length here about the basis of my question as for this post but what would happen if the battery with the magnet was rotating itself? if spinned in the direction of wire movement would the wire now spin with the rpm of rpm battery + rpm of the wire itself?
    it is said that for a homopolar motor the actual physical spinning of the magnet doesn't matter as long as there is a uniform field which there is for a cylindrical magnet.
     
  15. Jan 31, 2016 #14
    You are right! I forgot the coating!
    But where is the Lorentz force in this demo? The electron flow in the same direction with the flux of those magnets. These has to be perpendicular to each others.
     
  16. Jan 31, 2016 #15
    But they are look closely , imagine this setup as two faraday discs with a magnet inbetween , the discs are the coating on the magnet.
     
  17. Jan 31, 2016 #16
    I found the answer for my own question!

    http://www.school-for-champions.com/science/magnetism_lorentz.htm

    "A magnetic field is created by the motion of an electrically charged particle—such as a proton or electron. If that electrical charge is moving through an external magnetic field, there will be a magnetic attraction or repulsion force, depending on how the two magnetic fields interact."
     
  18. Feb 2, 2016 #17
    And I found a better video about the second demo, the train. This one explain how to build it and how does it work:

     
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