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Does flux move with the magnet?

  1. Nov 20, 2008 #1
    To my surprise, through experimentation, I have found that the "Flux tubes"/lines of force, do not move when the magnet is turned on its' axis but the field lines do move with the magnet when one flips it from N to S. This would lead me to conclude that the N/S area are a property of the poles and not the magnet it self. Am I interpreting my visual confirmation correctly?

    I've constructed a 3D magnifying glass, much like the iron fillings on a sheet of paper but in
    3D (I'm not prepared to discribe the tool since I would like to put it on the market first) This tool shows clearly the findings above. Additionally I have found that lines of force can be shaped, meaning that they are able to be tightly spaced and as a result, finding stronger gauss readings, larger spacing gives lower gauss readings. Can this be done or are my readings awry?

    And If I may pose one more question, or finding. In-order to shield magnetic influence, the mesh that is used will consist of weaves that intersect each other at a right angle, effectively re-directing the flux allong the plane of the mesh material or does it disapate the field and its' influence?
    Last edited: Nov 20, 2008
  2. jcsd
  3. Nov 20, 2008 #2


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    A magnetic field is not a "material object", and there is no link between a specific field line at one instant and at another. If the material configuration changes from t1 to t2, then there will be a field associated with the configuration at t1 and there will be a field at configuration t2. At every point in space, the field will have changed from its value and direction at t1 to its value and direction at t2, but you cannot say that there has been a MOTION of the field, like a rigid body or something. In certain cases, it might look like a rigid motion, attached to an object. In other cases, not at all.

    A simplistic analogy: suppose that you have 10 lightbulbs on a row. Suppose you switch on one bulb at a time. Does the "brightness" MOVE from point to point ? You can have that impression. What actually happens is that at certain bulbs, the brightness goes down, and at others, it goes up. But there's no "continuous motion" of brightness.
  4. Nov 20, 2008 #3
    Thanks for your input Vanesch, I don't understand magnetic field lines as a physical object any more. I'm not a physicist, so your reference to t1 and t2, if you mean t as being time representing a position (t1) that has moved in time to (t2), makes sense. I understand that these field lines are for all intensive purposes stagnant, there is no movement unless one moves the object. Flux, I understand to be much like electricity, more of an action-reaction process, yes? It's interesting that you would say that there is no motion.. if field lines cut across at a perpendicular angle to the wire/solenoid a force is realized in the wire but if the field lines run parallel no such force is noticed. So form what you have mentioned above.. how is it that this discrepancy exists?

    Let me be clear here.. when I move the bar magnet by flipping it, end to end, the lines tend to move, meaning that the micron powder chain moves with the field lines. When I spin the bar magnet on its' axis the powder chains seem to be anchored and only sway in the 'seemingly' moving field. Which lead to the conclusion above.

    Thanks again.
  5. Nov 21, 2008 #4
    So, the question might be as this: If I rotate a magnetic along it's polar axis, does the magnetic field rotate along with the rotation of the magnet?
    The answer should be a yes or no.

    Any takers?
  6. Nov 21, 2008 #5


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    I thought I answered that: the answer is no. The reason is that if you have your magnet in a certain position at moment t1, and you rotate it to another position (around the polar axis) at t2, then the field calculated at t1 at every point in space will be the same as the field in space calculated at t2. As such, for every point in space, the value and the direction of the magnetic field will not have changed between t1 and t2. You cannot say that the field vector "that was at point P1 at t1 is now at point P2 at t2". You can only say that the field vector at point P1 was such and so at t1, and is such and so at t2, and look at the difference. Field vectors don't have "paint on them" so that you can recognize where they came from and where they go and that "this vector moved from here to there".

    That's why you cannot say that the field "rotated".

    In some cases, it will *look like as if* the field rotated or moved. That is the case if you have a bar magnet at a certain position and direction at moment t1, and move it to a different position and direction at moment t2. If you now have the whole magnetic field distribution undergo the same rotation and translation as the bar magnet from t1 to t2, you will find the correct values of the field in every point. So it "looks as if" the field was a material object that "moved with" the magnet. But it only "looks as if".
  7. Nov 22, 2008 #6
    Thanks vanesch; I have seen this debated all around the Internet and I was getting contraditory impressions. You cleared that up.
  8. Nov 22, 2008 #7
    The foregoing questions and comments were interesting and it made me think of gravitational and electrical fields as well.

    I wonder if you wouldn't comment on gravitational fields as well? Does the earth spin inside a non-moving field or does the field move (rotate) along with the earth? It seems the value of the gravitational field varies with varying densities of the earths surface...? and if I track these changes with time it seems the field would appear to rotate...?
  9. Nov 22, 2008 #8
    Good question.
  10. Nov 23, 2008 #9


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    In Newtonian gravity, you will have a very similar behaviour as with a magnetic field. What you have, at each instant, is a certain gravitational field vector at each point, and if the configuration of the masses changes, then this vector changes at each point in space. If the "generating object" (the earth) is a-symmetrical and rotates, then the *value and direction* of the vector at point P will be equal to the value and direction (rotated...) of the vector at point P', which was the point that maps upon P under the rotation transformation corresponding to the rotation of the earth. But what we have here is again, a change of the local field vector (or no change, if it ends up being the same magnitude and direction), and our "is the same as the vector that we had at P', rotated" is just a trick to calculate it. It DOESN'T MEAN that the VECTOR AT P' WAS DISPLACED PHYSICALLY TO P. It's just that the value and direction of the vector at P now, is the same as the value and direction (rotated) we had previously at P'.

    As such, in some cases, it *looks like as if* the field was rotated, and if you calculate the field this way, you will find the right values, but that doesn't say anything about "the field being physically MOVED from P to P' ".

    If, by symmetry, the field at P now is the same as the field at P before, it doesn't matter if this result is ALSO reached by taking the field at P' and "bringing it to P". The only thing we can say is that the field at P DIDN'T CHANGE.

    So much for Newtonian gravity.

    In Einsteinian gravity, however, there IS an effect of a rotating mass. It is called gravitomagnetism. In normal circumstances, it should be a very very tiny effect, and there is an experiment under way to test it: Gravity probe B http://en.wikipedia.org/wiki/Gravity_Probe_B
  11. Nov 24, 2008 #10
    Excellent, Thank you... If I understand correctly then the earth (or "generating object") is rotating within the field and local irregularities create local "high" or "low" points in the "topology" of the field?
  12. Nov 24, 2008 #11
    That appears to be the main problem here, of which I cannot find a consensus answer.
    Consider an asymmetrical magnet rotating about it's polar axis. Do the flux lines not change during rotation? I would like to see an actual experiment showing this, one way or the other.

    With regards to gravity, my impression is that there is no "polar axis" but I could be wrong.
  13. Nov 25, 2008 #12
    I understand the same as you... no "polar axis" with gravity.

    If I understood the foregoing comments (vanesch) then irregularities or imperfections in objects creating a field (magnetic, electric or gravitation fields) create local "irregularities" in the value of the field at those discrete points. Without further detail, I guess, I would classify an asymmetric as a local "irregularity"?

    A weak attempt at an analogy would be a perfect sphere surrounded by a flexible membrane. The sphere rotates within the membrane and the membrane then assumes the shape of the sphere.

    If, however, the perfect sphere has bump on it's "equator" the sphere still rotates within the membrane however the flexible membrane deflects "outward" as the bump comes around.
    That's how I understand the foregoing comments...

    It would seem someone (at least one of the "greats") would have already have designed an experiment to demonstrate the concept...
  14. Nov 26, 2008 #13
    Perhaps, but gravity is a very weak force and cannot be "shielded"
    Those aspects make it difficult(though not impossible) to conduct experiments.
  15. Dec 17, 2008 #14
    Thanks everyone, The reason I posed this question was based on my findings. So far we have discused the relationship between the source axis and the flux field that appears to be changing, much like Venesch describes, this confirms my findings but does not explain the relationship if the source is flipped from end to end. As I mentioned before the flux lines move with the magnetic poles or appear to move with both ends. This finding had me infur that magnetic flux lines, representing lines of force, is a property of the poles and not the magnet itself. Since the appearance of movement is very very much different if the source is rotated compared to when it's flipped. But then again I'm not learned in this area.

    Anyways give it a shot guys, you'll see what I mean. Regardless of the shape or source of the field lines that are visable, tends to confirm that the flux field is a property of the poles and not the magnet. If this is true than this would explain to a me how it is when I approch a wire at a right angle to the wire I get a current and that I don't get any current when I swipe the magnets parrallel to the wire.

    Thanks for all you input...
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