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Electrical current and direction of magnetic field

  1. Sep 19, 2006 #1
    Magnetic field has direction.
    It has one side and the other side. (we called them south and north...)
    If we imagine that magnetic field is actualy a string of directed particles of "mass of space" that go one behind the other to form magnetic field lines, then: WHAT THE DIRECTION OF FLOW OF ELECTRICITY HAS TO DO WITH THE DIRECTION OF MAGNETIC FIELD?

    Actually - my question here is: why electrical current passing thorough (e.g. a wire) makes the magnetic field that it creates "swirl" and always in the same direction???

    (current runs forward(!!) - I mean: what inside conductor's material (atoms) makes field go around that forward direction of current, and why is it ALWAYS the same direction of "swirl"? what mechanism, what logic is implied there??)
    Last edited: Sep 20, 2006
  2. jcsd
  3. Sep 19, 2006 #2
    Choosing which direction the magnetic field is, is merely a matter of choice. Once chosen, we then define motion of charged particles either along or against that field vector depending on whether the charge is positive or negative.

    In any case, if the current ran "backward," then the particle in the magnetic field would feel a force in the "other" direction.
  4. Sep 20, 2006 #3
    What I was asking on the other hand is: what direction of current has to do with how magnetic field forms?



    Explanation: that up there is a wire; '>' shows direction of flow of electrons thorough wire; 'O' is magnetic field encircleing the wire.

    Next example:

    Here: 'O' is the same as above; and '>' shows how compass orients in that magnetic field.


    So: flow of electricity is perpendicular to crossection of magnetic field it created ('O') - how does that "perpendicular motion" of electrons affect atoms of a wire to create magnetic filed "going around" (and always the same way according to the direction of electron flow thorough the wire)?

    (Where I epmhasize "perpendicular motion" and "going around" (those two aren't logical together))

    It's like as if a wire is a tube and inside of tube there is spral (coiled) "sub-tube", so when you pass water thorough that "sub-tube" from one direction it spirals one way and vice versa...
    Wire does not have a "sub-tube" (from this analogy with hydraulics) - IT'S SOLID!!!!!!!
    SO: what inside of a wire makes that "surface-wise perpendicularily-aligned going-around" of the magnetic field??? (which has its laws!! - magnetic field "goes" (orientates) that specific way when electicity flows from left to right (and vice versa))?


    Electrons flow straight, magnetic field is perpendicular; how come?
  5. Sep 21, 2006 #4
    I am not sure that anyone has proven that electrons flow straight, just that their overall direction of motion ends up being along the length of the conductor from negative to positive. Regardless, I think if I wanted to try and unravel the perpendicular direction of the magnetic field in a current carrying conductor I'd sit and do some pondering about the Lorentz force as well, since they surely must cast some light on each other.
  6. Sep 21, 2006 #5
    "I am not sure that anyone has proven that electrons flow straight"

    How do they propose electrons move then?

    Does it has to do with orientation of atoms/molecules of conductors, and how come they are always orinentated in such a way that the magnetic field is always orientated the same - by that rule?

    (If I wanted to pounder things I wouldn't be asking a question here (that's what this forum is about - to give instant answers to those who don't know things by those who know things (I mean - I'm just interested, free mind... I on my own without any enforced need want to know stuff (how things work in nature (things so common - viewable every day; but can't really explain their origins (strange things - like: what inside matter makes the actual refraction of light, and this magnetic field paradox...))))))

    I looked at wikipedia:
    and (consequently)

    I don't understand it. It doesn't answer my question.
    I seek an instant answer.
  7. Sep 21, 2006 #6
    A crooked sort of path, but one which has a general average direction. This site shows the concept I've seen proposed most often:


    I'm not sure what you're referring to here.

    I haven't ever encountered a physical model that explains why the electron movement in a conductor causes a magnetic field at right angles. I think this is probably because no one has proposed a physical model that could be definitively tested.

    That is highly unsatisfying, but when it comes down to it, you don't need such a model to work with the fact that the field behaves this way. Simply knowing that it does gives you useful information for designing circuits and devices.

    Physics is packed with situations like this. Because things like electrons are not directly observable much of what they do can't be reliably explained in terms of straightforward mechanics. It took a long time before people figured out very, very clever, but very indirect, ways of gathering evidence that such a thing as an electron actually existed.

    What I meant when I said you'd have to ponder the Lorentz force as well, was aimed at you or anyone who wanted to buckle down and try to work out a reasonable physical explanation for the same force at right angles we see in both cases. It wasn't clear to me that you didn't realize that no one has probably ever satisfactorily worked out the answer to your question.
  8. Sep 21, 2006 #7


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    I am going to make a simplifying assumption to your question. As best as I can understand it, you're asking for why the geometry of the magnetic field due to a long, straight current-carrying wire is the way it is. My short answer to this is the symmetry of the situation.

    Here's the long answer. First, let's consider a long, infinite LINE CHARGE. For clarity, let's say it is along the z-axis. If you walk out of the room and I translate the line charge by a certain amount along the z-axis, when you walk back in, you won't notice any differnce. If you again walk out of the room and I rotate the wire around the z-axis, and you come back in, you also won't notice any difference. Do the same thing but this time, I transpose z to -z (i.e. make a mirror reflection), you also won't notice any difference to the line charge. To you, each one of these symmetry operations doesn't change anything with respect to how you see the charge is arranged.

    Thus, the RESULTING FIELD must also have the same symmetry. It must have the same translation, rotational, and mirror-image (or parity) as the source. The only possible geometry of the E field for this charge distribution is then the radial field. You'll notice that this field geometry obeys the same symmetry as the source.

    Now let's tackle the line current. Again, for simplicity, let's have the current flowing in the +z direction. Repeat all of the symmetry operations I have mentioned above. You'll notice that it is invariant to all of the symmetry operations EXCEPT for one. If I flip the wire from z to -z, the direction of the current is now different! You will walk back into the room and would notice that the current is now flowing in the -z direction instead of +z. So the field must also reflect this. The curled field reflect this because flipping the direction of current changes the rotational SENSE of the field. This field geometry still obeys the invariance in rotation and translation along the z-axis.

    That's why geometry of the magnetic field is the way it is.

  9. Sep 22, 2006 #8
    I understood him to be asking about the mysterious fact that a magnet placed near this wire will be deflected at right angles at all, instead of being pushed along in the same direction as the current flow like a car hit by a train, or a cork dropped into a flowing river.
  10. Sep 22, 2006 #9


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    But that "magnet" is only behaving due to the field geometry. It is the same way if you put either a test charge, or a electric dipole, in an electric field. It is only reacting to the geometry of the field.

  11. Sep 22, 2006 #10


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    But suppose the field were parallel to the wire, in the same direction as the current, with a magnitude that depends only on the distance from the wire. Wouldn't that have the same symmetry?

    (hmmm... I just remembered that the magnetic vector potential does run parallel to the wire, in the Coulomb gauge!)
    Last edited: Sep 22, 2006
  12. Sep 22, 2006 #11


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    I actually did ask that question to my prof. one time when this was explained to me, because I brought up the infinite, long solenoid case to him. His explanation was that we already need to know maxwell equation and that the magnetic field B or H (not A, which is the magnetic vector potential that you mentioned), cannot be parallel to the direction of motion of the electrons. So you have to rule that one out a priori.

    Still, I would love to hear if someone has an answer to this based simply on symmtry principle.

  13. Sep 22, 2006 #12

    "I'm not sure what you're referring to here."
    (Does it has to do with orientation of atoms/molecules of conductors, and how come they are always orinentated in such a way that the magnetic field is always orientated the same - by that rule?)

    Never mind; I guess it has to do with elctrons themselves, not conductor. (I guess even electrons passing thorough vacuum of Space create magnetic fields... right?)
  14. Sep 22, 2006 #13
    You explained above that the only possible geometry was the radial field. It's not clear to me, though, how that radial field exerts torque on the magnet such that it always takes a position perpendicular to the direction of current flow. For instance, I don't see that the radial field has a north pole on one side to attract the south of the magnet.

    Also: Is the radial field physically rotating around the wire? I keep having visions of the rotating cylindrical brush in a vacuum cleaner. Is the radial field traveling along the wire at the drift velocity of the electrons giving rise to the field?
  15. Sep 22, 2006 #14


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    The "radial field" was for the example of a line charge.

    I was illustrating why the electric field geometry looks the way it does for that charge configuration. This is not the magnetic field.

  16. Sep 22, 2006 #15
    Understood. The magnetic field arises in the second circumstance, the line current. The only difference between the two, you pointed out, is that the line current doesn't demonstrate symmetry when flipped end to end because the current has changed direction.

    So, you're saying that the radial field geometry is not present in the line current? If not, what is it's geometry and how does this lead to the deflection of the magnet perpendicular to the direction of the flow of current?
  17. Sep 22, 2006 #16


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    The geometry is the circular "curl" field with a rotation sense. The sence of rotation is arbitrarily set by convention, but once it is set, then you have a particular rotational direction of the field. Flipping it via a mirror reflection doesn't change the geometry of the field, but changes the sense of rotation.

    Other than the field parallel to the wire that has been discussed and ruled out via Maxwell equation, this is the only symmetry that I know of that also follows the symmetry of the "source", which is the straight, long line current.


  18. Sep 25, 2006 #17
    Yeah, you just pointed-out to the probable sorce of the phenomenon (that difference arising from the fact that particles now move*), not the explanation itself.
    * But, it is not the only difference (except geometricaly (which I think was his point)), there is another difference which is maybe even more important: electric field is caused by difference in amounts of positive/negative particles; while in magnetc field conductor is electro-neutral (there is equal amount of both + and - particles, it's that only some move...).
    Last edited: Sep 25, 2006
  19. Sep 25, 2006 #18


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    It's not necessary for the source to be neutral in order to get a magnetic field. A single moving charged particle generates a magnetic field. It's not as simple as the field produced by a steady current, because it varies (at the observer's location) as the particle approaches, passes, and recedes. Or you can have a beam of many particles traveling through space, which produces a magnetic field that is (ideally) just like the field outside a current-carrying wire.

    The only difference with a neutral wire is that it doesn't produce an electric field around the wire, so it's easier to observe the effect of the magnetic field.
  20. Sep 25, 2006 #19
    Nor did I claim the opposite.

    I just said that the fact which is maybe more important than that geometrical ZapperZ talked about, is that in order for MAGNETIC field to exist it is not neccesary that the conductor is non-electroneutral (i.e. it can be electroneutral as a whole).

    (by the way, electrons have same charge - they should disperse - how come they form a spark (like e.g. a thunder); wouldn't it be logical that cloud electric discharges are actually a cloud-like beam of glowing mist?)
  21. Sep 25, 2006 #20
    I think the answer is that lightning follows a path of least resistence through the air. Such a path never exists in a straight, neat line. The lightning has to, opportunistically, follow the first twisted, complex route of low resistance that presents itself, just like someone bushwhacking through a jungle. Many twists and turns are necessary because plants don't grow wild in neat rows. The path through always ends up being confined and narrow. That's probably why you don't see spreading out into anything like a cloud.
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