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Direction of magnetic field and direction of current

  1. Nov 6, 2008 #1
    it may sound very stupid question but please correct my doubt--
    when a current flows into a straight conductor , magnetic lines are said to be flowing in the plane perpendicular to the conductor,or when say current is flowing in a circular loop, direction of magnetic field is said to be perpendicular to that plane(circular loop),
    now my question is---- why magnetic lines are flowing perpendicular to the direction of counductor,what gives magnictic field this direction???
  2. jcsd
  3. Nov 6, 2008 #2


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    Not quite correct. The magnetic field lines curl around the straight conductor. If you look in a plane perpendicular to the wire axis, they look like concentric circles. Similarly for a circular loop, the lines are not everywhere perpendicular but curl around as shown in the following link:


    Hope this helps.
  4. Nov 7, 2008 #3
    thnks for the reply marcus;
    however my question is---- why magnetic lines are flowing perpendicular to the direction of counductor,or rather what gives magnetic field this direction only??

    is it something related to change in Electric field in some direction which gives magnetic field the direction perpendicular to flow of current ???if yes how a changing electric field distorts the space around to make another magnet to move in a particular direction only.???

    thanks in advance
  5. Nov 7, 2008 #4


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    The answer to this question takes one beyond elementary physics. It turns out that what we call a magnetic force is not really a separate entity, but rather is the electric force as seen according to the transformations of special relativity by a moving charge. In other words, electricity and magnetism are just velocity-transformed versions of the same thing--the electric interaction. In fact, in a relativistic formulation all electric and magnetic interactions are contained in a single object called the electromagnetic tensor. Mel Schwartz gave a beautiful presentation of this in his undergrad book "Principles of Electrodynamics." Well, the existence of the vector potential, which gives rise to the "perpendicular" nature of magnetic field lines that you are asking about, comes out of this.

    I am hesitant to proceed with details without knowing the level of your mathematical and physics background, so we'll leave it there for now.
  6. Nov 8, 2008 #5
    I assume you are familiar with the "right hand rule" routinely discussed in introductory physics texts. Precisely why it works is a different issue.

    Precisely why the electric field is the way it is can be described mathematically but why it manifests as it does is another of those many, many "unknowns". Maybe its like asking "why is the speed of light "c" and not c plus 1 meter/sec. There is an unknown underlying physical principle we just haven't uncovered yet.

    While electrically F = qQ/4(pi)er^2 (Coulombs law) and the gravitational force is
    F = GmM/r^2, with charge q and (Q) and mass m and (M) playing apparently similar roles, it turns out the gravitational field has an added level complexity relative to the electromagnetic. Both vary with the inverse of distance, r, but their natures are quite different as well as their relative strengths. The EM force is about 10^40 times stronger than the gravitational. Nobody knows why.

    In fact the EM and gravitational fields are so different, nobody has been able to unify (combine mathematically) gravity with the other forces. Gravity requires a tensor description uncovered by Einstein. From the perspective of unification, it may be that the electromagnetic force is the way it is because of the initial environmental conditions that existed when a phase transition moved the very early universe from a high energy unstable state to a lower energy more stable one where the unified force separated into four distinct ones we see today. But nobody knows that for certain.
  7. Nov 9, 2008 #6

    There is also an interesting phenomena to do with electrons and magnets. If you send an electron past a magnetic pole the electron deflects - like in tv electron guns. The same thing happens with metal of course. Magnetic poles make metals and opposite poles deflect towards them; or repel same poles away from them. But things are slightly different for electrons.

    Electrons do deflect; but neither towards one of the poles nor away from one of the poles. Instead they deflect to the left or to the right depending upon which pole it is. In other words the deflection is 90º to the magnetic force; not towards or away from it.

    Rather strange behaviour.

    The only other time we really see this sort of behaviour is in gyroscopic motion. If you spin a wheel while suspending it by one side the wheel appears to stay upright defying gravity rather than hanging loosely. It seems to twist the gravitational force 90º sideways. Same with the floor waxers that spin while you push them. Push them forwards and they move sideways.

    These examples from nature would lend me to think that electrons must be spinning to create this similar sort of behaviour. Something to do with 'equivalence' I think...

    The funny thing is that Einstein recognises spin in his general relativity and our quantum physicist's attribute spin as an attribute of particles; but no-one seems to link this spin with the 90º relationship between electricity and magnetism. I have to wonder why.

    But anyhow I thought you might find the electron with magnets an interesting phenomena. The rest is just my babble. However I'm afraid you will have to get the more complex answer from the more knowledgeable fellows here to be aware how current scientists explain it. Remember, the simplest answers are apparently not always the right answers.
    Last edited: Nov 10, 2008
  8. Nov 10, 2008 #7

    The gravitational force is NOT being twisted at all.
  9. Nov 10, 2008 #8
    And with regards to floor waxers, you don't just "push them" to create sideways motion.
    You lift-up from the extended lever handle to go left, and press down to go right( or vice-versa depending on the wheel rotation)
    And that happens simply because one is forcing a direction with the edge of a flat rotating body IN CONTACT with the floor. This would not happen in space without a floor, or a floor waxer suspended by a rope.
    Last edited: Nov 10, 2008
  10. Nov 10, 2008 #9
    And to be even more accurate, a floor waxer(in contact with the floor) lifted or pressed down from the extended lever handle will not just move left or right, but will "arc" to subscribe a wide circle if not controlled.
    Last edited: Nov 10, 2008
  11. Nov 10, 2008 #10
    A spinning toy gyroscope "suspended" sideways on a string does not twist gravity. It's total weight is fully expressed on the string. There is no 90 degree alteration of the gravitational force. The force vector STILL points downward, but is shifted laterally to be expressed fully on the string.

    I hope this makes sense to you and, remember, that energy IS REQUIRED TO BE EXPENDED in order for this effect to occur.
  12. Nov 10, 2008 #11
    Hi Pallidin. That's a lot of posts.

    Q1. Are you saying that the spinning wheel moves the centre of weight of the wheel sideways somehow so that it is balancing on the string?
    Q2. If you hit a billiard ball at another straight on then the 2nd one will then move off in the same line. If you hit a billiard ball straight at a spinning top the top moves off towards a sidewards direction; not straight. Is that not so?
    Q3. If you simply try to push a floor waxer forwards like a shopping trolley; it will refuse to go forwards will it not; and will instead twist your movement towards the left or right?

    I can't remember when, where or how but I'm sure I've tried pushing one of those things in my past and that was my experience. You had to apply pressure in a direction 90º to where you wanted to go to get it to go in the desired direction ie twist right to go forwards; twist left to go backwards. As you say it was kind of arc'y. I don't remember anything about pushing handles down or up but it was a long time ago. I'll ask the cleaner guy I know; maybe he uses one.

    Q4. If its not impolite; where does you information come from?
  13. Nov 10, 2008 #12
    Thanks a lot people: i was looking for answers now i have even more questions; however every single response is highly appreciated;
    thanks again.
  14. Nov 10, 2008 #13
    Q1: This can be called "lateral vector-shifting", but requires a loss of energy and, as far as I know, can only be done using gyroscopic techniques.
    The actual center of mass does not change; it's kind of an illusion, if you will. The gyroscope resists being tilted, so the tilting force must either be absorbed or translated.
    In this case, the tilting force is laterally translated to whatever point is holding the gyro end axis(the string)
  15. Nov 10, 2008 #14
    Q2: True.
    But, also consider what happens to the trajectory of the first ball after hitting a spinning ball straight-on.

    Q3: When you "push" or pull a floor waxer you are actually tilting it at the same time, because the handle is at an off-set axis. But yes, you are absolutely correct that minor movements(twisting/tilting) on the handle will cause specific directional arc movements.
  16. Nov 10, 2008 #15
    Further clarification: If you apply pressure(or a weight) to the end handle of a rope-suspended floor waxer that is energized, the machine will start to rotate around that axis(if I'm thinking clearly)
  17. Nov 10, 2008 #16
    Q4: You are very polite. Since informational references were not asked for I did not do this, thus expressing just from myself what I hope is correct.
    If it is not I humbly stand corrected.
  18. Nov 11, 2008 #17
    Why is the induction tilted against the current?

    After years spent using and engineering electromagnetic things (I created the inductive RFID technology at 13.56MHz for instance), I can't give any better answer than "because that's their nature". So sorry for that!

    Sure you may check that equations for EM fields want it. But fields do not obey to equations - equations are fitted to explain and sometimes predict Nature. It's even a wonder that Nature can be modelled and that we Humans are somewhat capable of it.

    Maybe a more general theory (electroweak) can give you an answer, but I doubt it will be the kind of answer you hope.

    And then, to the kind of questions like "what is an EM field" or "how does it propagate" I have no better answer than "here are the equations". Again so sorry for that as well. After years, I still can't taste nor smell EM fields - though their consequences can be perceived that way under some circumstances.
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