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Magnetic field relativistic effect

  1. May 29, 2013 #1
    Hi Forum,

    It is said that the Magnetic field is a relativistic effect. I went to the Feynmann lectures on Physics and found an example based on the force a charged has when moving at speed v next to a conductor with a current density J. I understand that the framework at rest uses a magnetic field to explain the force that makes the particle to move closer to the conductor, when the framework is on the particle there is unbalance of charge density that produces an electric field, which in turn, induces a force on the particle.

    So far so good, but what about the magnetic field produced by a magnetic material? There are no current there. We use the concept of magnetic momentum and some free currents to explain its origin. How is relativity used to explain its origin?

    Last but not least, can we conclude that magnetic fields are not real, just a relativistic effect of electric fields?
     
  2. jcsd
  3. May 29, 2013 #2
    There is a current there too. Its just at a very small scale. All individual matter particles found around us have an intrinsic magnetic moment associated with them. But they are, in general randomly oriented and in non-magnetic substances, more or less cancelled out. In magnets ( for example bar magnets) these moments are aligned creating a non-zero magnetic field. The phenomenon then goes into the quantum realm and you need to couple relativity and quantum mechanics. Now, that is a field of research.

    Magnetic fields are very much real. They just appear zero in a particular reference frame. In the example you cited, the asymmetrical (or rather anti-symmetrical) length change in the lengths of the conductor carrying positive and negative charge carriers causes a non-zero "electrostatic" force from the the frame of reference of the charge. When observed from a rest frame, we still see forces on the charge. And hence, we conclude that a certain other field exists. This is the magnetic field. Again, it is very much real and is one of the two fields involved in electromagnetic waves.
     
    Last edited: May 29, 2013
  4. May 29, 2013 #3

    WannabeNewton

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    Magnetic fields coming from spin alignment arise out of relativistic quantum mechanics-it is not a purely classical effect like the magnetic field due to a current in a wire. As for your second question, magnetic fields are certainly real. You can find frames where magnetic fields vanish sure but since there exist inertial frames where magnetic fields don't vanish, you cannot claim the fields aren't "real".
     
  5. May 29, 2013 #4

    Vanadium 50

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    Why do you think this makes the magnetic field any less real?
     
  6. May 29, 2013 #5
    Vanadium 50,

    I assumed it was a relativistic effect(the magnetic field) from the electric field. As I said, the force on a charge moving at speed v induced by a current density, J, in a wire could be explained either by a magnetic field, if the frame is at rest, or an electric field if the frame is on the charge. Fo both situations, same result, although the force has a different source (one magnetic and the other electric). So I thought that the Magnetic Field is just a relativistic effect of the electric field.

    I am going through the answers
     
  7. May 29, 2013 #6
    Sudu,

    But the force that the charge "feels" is from a electric field. It is us who decide where the frame is. But as far as the charge is concerned there is only an electric field. Same problem can be solved with different physics depending of where the framework is, which I found no less than interesting
     
  8. May 29, 2013 #7

    pervect

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  9. May 29, 2013 #8

    PeterDonis

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    That depends on the field. There are magnetic fields that are non-vanishing in every inertial frame: the most obvious example is the magnetic field associated with an electromagnetic wave.
     
  10. May 29, 2013 #9

    DEvens

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    Wait till you start studying the Aharonov-Bohm effect. In that case, the electric field is zero. The magnetic field is zero. But the electromagnetic field can still have an effect on an electron.

    http://en.wikipedia.org/wiki/Aharonov–Bohm_effect

    That makes one suspect that what is real is the vector potential Aμ. After all, what appears in QED is Aμ, not Ei or Bi.

    Yet, it is a potential. So we can't directly measure it, only certain special values about it. And to some extent it is arbitrary due to gauge invariance.

    Then you get into studying connections on principal fibre bundles, and life becomes very mathematical. I can still remember the question asked by one of my co-students in that class: Sir, what do you mean by "Lie algebra valued 1-form?"
     
  11. May 29, 2013 #10

    WannabeNewton

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    Yes indeed but I was commenting on his *possible* thought process that since we can have situations where Lorentz boosts make the magnetic field vanish, that they cannot be real; I didn't mean for it to be a general statement.
     
  12. May 30, 2013 #11

    BruceW

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    yeah, as people are saying, electric fields and magnetic fields are 'components' of the electromagnetic tensor. So in relativity, they are physically not separate concepts. (in the sense that only a combination of both fields gives a relativistic 'concept'). But there is no reason to say that the magnetic field is more real than the electric field. I think this is often said because there are only electric charges (no magnetic 'charges', or 'monopoles' to use the proper word). So some people like to think of the electric field as being the more important field, but both fields are important really.
     
  13. May 30, 2013 #12

    WannabeNewton

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    Lol field discrimination.
     
  14. May 30, 2013 #13

    BruceW

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    hehe, they say B fields are from venus, E fields are from mars.
     
  15. May 30, 2013 #14

    PAllen

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    I like to look at it more like this: Without relativity, you could conceive of a pure coulomb theory of charge, similar to Newtonian gravity. Then, it seems that magnetism is an optional feature, added separately, to match observation. With relativity + charge, you are forced to have electromagnetism; you simply have a relativistic theory of charge that necessarily entails a frame dependent combination of electric and magnetic fields.
     
  16. Jun 7, 2013 #15
    WannabeNewton, what is a Lorentz boost?
     
  17. Jun 7, 2013 #16

    WannabeNewton

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    It's just a Lorentz transformation in a certain direction.
     
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