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Homopolar motor

  1. Oct 18, 2007 #1
    how does this work? (i made one and it does work pretty well, mine was spinning for about 20 minutes) the wire has to be copper so it conducts the electricity, but is not directly magnetic until the applied current produces the magnetic field. Also the magnet has to be very strong, i used an N50 grade neodynium

    I cant see where the rotational force is coming from as there are no changes in the direction of the magnetic field or electric current, so i would have thought that the wire would just stay in equilibrium or get attracted to the magnet, as with other standard North - South pole interactions.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Oct 18, 2007 #2
    This is a fascinating and clever variation of the motor invented by Michael Faraday.


    The one in the video is great because it does away with the need for a fluid (mercury or salt water) contact for the wire. Very clever.

    The metal casing of the battery is made into a single north or south pole by being placed on one pole the magnet. The current in the piece of wire flows from top (neg) to bottom (pos) on each side and simple doubles the single wire used by Faraday. The torque results from the Lorentz force: a charge moving in a uniform magnetic field (in this case the one created around the battery casing) experiences a force at right angles to both the field and the direction of the current flow. There doesn't have to be any change in the direction of the magnetic field or the current. The "change" that keeps it moving is simply the flow of charge through the wire.

    The motor could be made more powerful by the addition of more top to bottom wires, by using thicker wires, by using a stronger magnet, or a battery able to deliver more current, or all of the above.

    It's a very interesting video. Thanks for posting that.
    Last edited by a moderator: Sep 25, 2014
  4. Oct 21, 2007 #3
    Thats interesting. I'm presuming the energy efficientcy of it is quite small as it is effectively shorting out both ends of the battery?

    I wonder what would happen if you made one with a car battery, with thick copper wires and a huge 13,200 gauss permanent neodynium magnet (like this one), that would create a very powerful motor. actually I expect the battery would explode! the small ones heat up quite significantly when you do it with them.
  5. Oct 21, 2007 #4


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  6. Oct 21, 2007 #5
    You're saying the battery itself got hot in the one you made?
  7. Oct 22, 2007 #6
    yes, very much so. After about 5 minutes of spinning it was very warm to touch. I also kept seeing tiny sparks between the wire and the magnet. Its hard to make it quickly, as you have to arrange it so the rotational force is constantly pushing the wire towards the magnet, or else the wire will just disconnect as it spins. Also you have to make small indent in the battery so the wire sits there confortably and does not spin off and i found that wetting the wire a bit helped the current flow and sped it up considerably.
    Last edited: Oct 22, 2007
  8. Oct 22, 2007 #7
    That's very interesting. When the wire spins fast enough to disconnect the device ceases to be a motor and becomes a generator. Momentum carries it around cutting through the magnets lines of force and apparently a rather high voltage is developed: enough to jump the gap in the form of a spark. The actual voltage depends on the density of the magnetic field lines and the speed of the rotor in cutting them, as per Faraday.

    However, there is probably a form of switching surge as well: when the current flows in the wires as normal the magnetic field around the wires is going to be distorting the magnetic field of the magnet. When contact is broken and the magnetic field around the wire collapses the magnet's field can "relax" back to normal, and as it does so, its lines of force cut through the wire, generating current at a fairly high voltage. (This is also straighforward Faraday: a changing magnetic field will induce current in a conductor.)

    In practice, as soon as the lines of force start generating current in the wire its magnetic field comes back, distorts the magnet's field again so its lines are no longer cutting the wire and current is no longer generated, whereupon the magnet's field re-relaxes, generating current, and so on, over and over, such that you get a high frequency oscillation. As Rutherford figured out, these sparks, as in the discharge of a capacitor, are actually alternating current phenomena. (He discovered that discharging a leyden jar through a magnetised needle would weaken its magnetism demonstrating that the discharge was a decaying AC current.)

    The heating of the battery may simply be due to the lack of resistance in the circuit: it is discharging a lot of current very quickly. Or it may be due to the fact the battery is being subjected to the high voltage of the spark. Or both. An easy way to test the former possibility is to just connect the terminals of a battery with a short length of the same gage wire and see if it starts to heat up. I am sure the wire will get hot, but don't know about the battery.

    Your idea of wetting the wire prevents contact from being broken and the motor works much better since the current isn't always being interrupted. You might also try a thicker gage of wire that won't bend so easily. The guy in the video seems to have put some kind of aluminum cap over his magnet with a groove around the sides for his contacts to ride in. I'm sure that helps constrain the position of the wire eliminating the need to put a dimple in the negative terminal as you did.
  9. Oct 22, 2007 #8

    Ivan Seeking

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    I don't think that we know that this happens. Have you calculated dphi/dt?

    The principle of superposition is what applies here. The fields don't distort each other.

    Again, have you calculated this, or are you just guessing?

    No distorting; superposition.


    Either way, heat is generated by the flow of current.

    Bad idea. Yes, short circuiting a battery will make it hot and can be dangerous!

  10. Oct 22, 2007 #9


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    As Ivan mentioned - Yes! HPG's have very low resistance - the circuit is mostly metal - wires and metal disk. That's the whole point of an HPG - high current at low voltage. DO NOT drive it with a battery, especially a car battery where the hydrogen could ignite.
  11. Oct 22, 2007 #10
    What is it, specifically, you think we don't know is happening?
  12. Oct 23, 2007 #11
    Yes. Of course I had to try it and eventually realized these sparks aren't electric arcs at all: it's the wire glowing red hot where the contact is made through very small irregularities created when the enamel is scraped off.

    I did this with a AA battery, figuring that if the motor in the video didn't explode, I'd be safe doing it for short periods. I tapped the wire to close the circuit intermittantly, feeling the battery for heat, and to my surprise saw the "tiny sparks" RA mentioned. Needless to say, I was baffled as to where such voltage could be coming from and kept doing it till the battery and wire were noticably heated up. Then it occured to me that the same current that was heating the battery and wire was flowing through very small points where the wire touched the battery, and these points were heating up to the point of glowing.
  13. Oct 23, 2007 #12

    Ivan Seeking

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    I'm not entirely sure what all is happening.

    Part of what bothers me is that there is virtually no inductance and the current is relatively low. Also, once current is established, to an extent, an arc can be drawn out without the need for an increase in the voltage.

    What you were saying might be correct or partly correct, but I don't think we can say for sure without properly analyzing the problem.
  14. Oct 24, 2007 #13
    If you have a link to something that explains this (about the arc), it could be helpful. I went to the library yesterday and found the book from which I learned out self-inductance and switching surge. Something I'd forgotten is that the actual voltage of the spark depends on how quickly you open the circuit; how much of an air gap you create how quickly. The larger the gap you make in a given amount of time the higher the voltage rises to jump it. There doesn't seem to be any way to open it so fast no spark occurs. However, all that is for the case of a coil, and the book doesn't explain how the voltage adjusts itself to the situation. To the extent you can draw an arc out once a current is established it may involved the same phenomenon of the voltage adjusting itself to the gap in question.

    I am now sure it is only partly correct. Once contact is broken and the rotor is turning by momentum only in the magnetic field it automatically becomes a generator, but it is not possible that it could generate a "fairly high" voltage and complete the circuit through a spark. There was a long thread on homopolar generators a few years back in which a guy pointed out that, due to the short length of the conductor being subjected to the magnetic field it is not practically possible to get anything but low voltages from Faraday generators. (The current, though, can be amazingly high for the voltage. As Astronuc mentioned this kind of generator is all about current.) RA stated he used a very strong magnet, so I was thinking this is where a high voltage could be accounted for: plenty of field lines to cut per unit time. In fact, though, that only counts for something in the case of a conventional coil.
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