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How can there be a current in this magnet/coil configuration

  1. Aug 2, 2015 #1
    Here is a rotating magnet inside some coil tubes: Screenshot_2015_08_02_18_04_54.png

    I recently learned about Faraday's law. Here since the bulb lights, there must be a changing magnetic flux inducing a current throught the bulb.

    Where is the magnetic flux changing? To me, the magnetic field does not change throught the whole circular area enclosed by the big loops.

    However, if the loops are empty tubes, then yes, there is a changing magnetic flux throught the area enclosed by each small cross section of each tube. BUT shouldn't the currents profuced then be running along the small cross sections circumferences of each tube, rather than along the length of the tubes? But if the bulb lights, the induced current must go along the length of the tubes!

    So my questions are basically : where is the magnetic flux is changing and in what direction goes the induced current in the above picture?
  2. jcsd
  3. Aug 2, 2015 #2


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    The induced voltage/emk(1 must be directed along the tube, and the flux must change through an area, surrounded by the coil.

    I see some magnet spinning in the background, but cannot see the angle of the plane it's spinning in, neither the angle of the plane, that the tube is coiled in.

    (1: Faraday says that an emk is induced, not a current. The current comes from: I = V / R.
  4. Aug 2, 2015 #3
    Thank you for replying!

    The image is from a video. It's hard to tell but I think the magnet is spinning in the center of the coiled tube. And in the same plane.

    But how can magnetic flux throught the area surrounded by the coil change? The quantity of magnetic field lines going in or out of the area is always the same here, no matter if their direction are rotated.
  5. Aug 2, 2015 #4


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    Now assuming this isn't one of the many bogus videos on the net ......

    consider for a moment the magnet as stationary. It appears to be a bar magnet.
    What do you know about the magnetic field around a bar magnet ?
    Now consider the motion of that field as the magnet spins.

    I must also point out ... it appears as those coils of copper are not insulated. This means they are essentially shorted out giving just one single
    large loop of copper

  6. Aug 2, 2015 #5
    Well, the field goes from one pole to another, forming closed loops of field lines. And the strength of the field decreases with distance.

    So, the field lines at the north pole go out of the surface area. And they go back in, at the south pole. And to me it's the same when the magnet spins. The same exact lines go out and back in, they have just all rotated. Actually I would say the flux is 0 all the time.

    Obviously I am missing something, since the bulb lights. But what? How the flux can vary here?

    EDIT : I found a link to the video. It srarts at 12:28. It's in french though.

    Last edited: Aug 2, 2015
  7. Aug 2, 2015 #6


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    If you watch the video closely when he starts, the bulb oscillates between light and dark. A single loop might have the same effect: the loop is not completely closed at the bottom due to the connections to the light bulb.
  8. Aug 2, 2015 #7
    Ah, interesting! Indeed there is a bottom loop going to the bulb, and in this loop the flux changes.

    Would that mean that the other loops are totally useless?
  9. Aug 2, 2015 #8


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    draw a loop of wire, draw a bar magnet in the middle of the loop, draw the field lines around the magnet so that they are also intersecting the loop
    Now imagine the magnet rotating, what do you see as an interaction of the field lines and the loop ?
    can you see that the loop is being subjected to a varying magnetic field ... that is
    Take a single point on the loop and see how the moving field lines interact with that point

  10. Aug 3, 2015 #9
    I can see one field line, while rotating, moving on the area enclosed. I see all the lines moving, but none is moving more through the area, or less through it. No angle between any line and the area changes. And of course the area is fixed as well. I can't see something varying here. Except the positions of the intersecting points of the field lines on the area, but that doesn't affect the flux.

    Some lines BTW are completely in the plane of the area. While others move "out" of the area and then back to it, from north pole of the magnet to its south pole.

    Some picture to help me visualizing:

    http://www.welsch.com/gallery/3d/Magnetfeld_eines_Stabmagneten_006.jpg [Broken]
    Last edited by a moderator: May 7, 2017
  11. Aug 3, 2015 #10


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    I don't think you are not visualising it correctly .... have a look at this drawing I did

    magnet in loop.JPG

    blue is the wire loop
    red is the rotation direction of the magnet about that centre red point, just as in the video you posted
    under such conditions, I can visualise the loop being constantly subjected to a varying magnetic field

    I have never built something like this, don't know how successful / efficient it would be ?
    maybe you could build one and tell us how you get on :smile:

    its changing constantly. rotate the magnet in my drawing step by step say at 45 deg steps and look at the changing angle between the field lines and the coil

  12. Aug 3, 2015 #11
    Thank you for the drawing!

    The magnetic field is rotating, but why is the flux affected by this rotation? According to Faraday's law, only the rate of change of the flux matters. There are 3 possibilities then: the field lines get closer or farther from the area, the angle between the lines and the area change, or the area changes.

    Here, the lines move, but their angle does not seem to change, as well as their distance to the area. And the area is fixed. What varies then?

    Indeed I think if I can't find out what I am missing I'll try to setup a similar experiment. :)

    EDIT: However as @mfb suggested, there is an area for which the flux changes: the area of the loop parallel to the table, with the tubes going to the bulb. The question is now: is this flux change the only one to induce the emf/current, or is there also a magnetic flux change through the other loops that I am missing.
    Last edited: Aug 3, 2015
  13. Aug 3, 2015 #12


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    Yes, I believe this is a type of paradox that falls under what is known as the Faraday Paradox. It happens because Faraday's law is only an approximation. From here: https://en.wikipedia.org/wiki/Faraday_paradox

    "These paradoxes are generally resolved by the fact that an EMF may be created by a changing flux in a circuit as explained in Faraday's law or by the movement of a conductor in a magnetic field."
  14. Aug 3, 2015 #13


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    @davenn: the flux is not changing in that setup. The magnetic field at different places changes, yes, but there is no flux through the area because all field lines are parallel to it (=in the drawing plane, within the circular part of the loop). No flux also means no change in flux.

    It could be related to the Faraday paradox, but there the geometry is different (there is a field orthogonal to a current which we don't have here).
  15. Aug 3, 2015 #14
    I am amazed that Faraday's law is actually not always valid! Or at least that there are some subtleties, like the fact the magnetic may not rotate with the magnet in Faraday's paradox experiment.

    However I don't get why the wiki article says at some point : "If the lines of flux are imagined to originate in the magnet, then they would be stationary in the frame of the magnet, and rotating the disc relative to the magnet, whether by rotating the magnet or the disc, should produce an EMF, but rotating both of them together should not." To me even if only the magnet or only the disc is rotating, the flux through the disc is still constant, since any rotation is made around the symmetry axis the magnet and the disc.

    Well, that's maybe another subject/question. I have the answers I was looking for, and even more. Much thanks all for helping!
  16. Aug 3, 2015 #15


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    Yes I was considering that ... but I was also considering the field lines off the ends of the magnet, which are not parallel to the loop

    As I said in my previous post, I don't know if it would work or not

    I am still considering that the originally posted video is garbage and that there is another unseen power source lighting the globe
    As stated a way back, I didn't like the idea of shorted turns

  17. Aug 3, 2015 #16
    I might be wrong but I think even for the field lines off the end, the flux is still 0. Because any line going out of the magnet, even an off pole line, goes back in. So if you take one line, the effect of going out then going in just cancels. Again, might be wrong, it's just what seems to happen for me.

    The video is certainly not made to be scientifically correct. Its a french tv show which tries to explain technologies and science to common people, without any mathematics or so, and very big simplifications. So it does contains errors and even maybe misconceptions indeed. I spotted some in this same video. BTW it is still a pretty good show overall, when you just want to get some idea of what's going on behind something.

    I would say in this particular case that nobody is behind to light the bulb because the bulb seems, at the beginning, to alternate between dark and light at the same rate of the rotating magnet. I suspect then that the current is indeed induced by the changing flux through the bottom loop, but the guys making up the show might not have realized that, or even really know Faraday's law.
  18. Aug 3, 2015 #17


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    and if not scientifically correct, then must be a fake to deceive people
    Afterall, all the comments above say that a current shouldn't be produced
  19. Aug 4, 2015 #18


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    Furthermore: Looking at the picture in #1 and speaking of "center of the coil", "area of the coil", "angle of plane" doesn't make sense, because the coil itself is a mess: What is the area of such a coil? The angle of the plane of a single winding could easily be 10°, and so on.

    Say we want to calculate the B-field at some point, using Biot-Savart with a current = 1A through the coil, then we would have to express a mathematical path along the windings to integrate along: It's impossible, based on the picture. We could only adapt the location of a center, adapt some mean diameter as for a circle representing all the windings and adapt a plane wherein this circle is placed.
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