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Question on electromagnetism

  1. Sep 10, 2003 #1
    for example, there is an electron moving at some speed in a straight line. a circular magnetic field is produce(just like electron moving in a line of wire). if there's a n observer moving along with the electron, then the electron will appear to be at rest with the observer. does it mean that the magnetic field dissappear or it is still there? can someone explain to me please.
  2. jcsd
  3. Sep 10, 2003 #2


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    Don't forget to include Heisenberg's Uncertainty Principle into your calculation
  4. Sep 10, 2003 #3


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    It's not so much "Heisenberg" as "Einstein". It is that problem (that electromagnetic force is, according to Maxwell's equations, dependent upon speed) that implies an "absolute speed", contrary to Galilean relativity, and lead Michaelson and Morley to do their famous experiment. The null result of that experiment lead to Einstein's theory of relativity. Applying relativity, a stationary observer finds that there is a magnetic field (strength dependent upon the speed relative to the stationary observer) while an observer moving at the same velocity as the electron see no magnetic field.
  5. Sep 10, 2003 #4
    I'd just like to say that was a very interesting question. I have never thought of that before! Wow.
  6. Sep 10, 2003 #5

    Chi Meson

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    Yet it dissappears and No I can't explain it. Not in this space, and not without a lot of math. As Hallsofivy said, it's a conundrum that lead to one of the greatest leaps of all of physics. In the second or third year of college physics, you get to Maxwell's equations. It's at this point you will start being comfortable with this weirdness.

    For me, I think I had it conceptualized 20 years ago, but now all that math is gone, and I just accept it as true; a charge must be moving relative to the observer for the magnetic field to be detected. but I'll let someone else prove it.
  7. Sep 10, 2003 #6
    Re: Re: question on electromagnetism

    They go over Maxwell's equations in first year here.

    Doesn't mean I'm comfortable with them yet though, heh.
  8. Sep 10, 2003 #7


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    Simply put - neitgher the electric field nor the magnetic field has an existance which is indenpendant of the observer. An electric field in one frame becomes a combination of an electric and magnetic field in another frame. The two entities, i.e. electric and magnetic fields, become one thing in relativity - the EM field. It's considered to be one thing. It's what's known as a tensor quantity.

    Take a look at this page

    it might give you a vague idea of what this means. However I did not write it for that purpose. But it does show how an electric field can all of a sudden appear from just changing your frame of referance.

  9. Sep 11, 2003 #8

    Chi Meson

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    Re: Re: Re: question on electromagnetism

    I first remember Maxwell in my first year too, but we REALLY did Maxwell in the second year. Most of us needed to learn the calculus first, so it was kind of like "here's what's going to dirve you crazy next year!"

    "Del dot del dot what?"
  10. Sep 11, 2003 #9
    Re: Re: question on electromagnetism

    Pete, this is correct notion of observer frame dependence.

    I am interested in your linked site, as it clearly states the charge around a neutron Star, which is currently being investigated!

    Another important aspect, as Iam sure you know, is that the dependence on observation, can have differing effects, you may detect different effects in different frames, a basic example in simplistic terms is polarization.

    Now there is within the tensor field of Einsteins equations, a direct consequence for the detection of the speed of light or EM wave propergation, this relates its dependence on the fact that say for the normal term for lights constancy:The speed of light is Constant in a high Vaccum field.

    This as everyone knows has been tested in all manner of situations, but not everyone knows that the 'Constant' term itself is relative to the Vacuum, the Vacuum is the Constant! It is this very fact that is neglected when dealing with light and its speed of propergation.

    The 'limit' or its constancy, was present before a photon, EM wave, is placed in to the Vacuum!

    What I am saying is the Constant speed of light is governed by the Constant Vacuum it propergates through, and will always place this limit upon observers.

    A single photon travelling through a Constant Vacuum, disperses into the vacuum, which in turn reduces the Vaccum, by its very presence, the photon has reduced the Constant Vacuum, which reduces the observed speed, actually it produces the Limit of observation.

    A varying Vacuum can be seen as relative to the amount of EM waves within a extended Vacuum background. For example the farthest open space's, that lay at large distances from Galaxies, are of a High Vaccum density, these area's contain little observed matter, like Stars and such, but are Vacuum points of extreme Energy, or the reduced ElectroMagnetic field.

    This area, is actually governed by the amount of stress contained( a single photon entering a Vacuum point on Earth, inside a man-made vacuum creates a stress point across the total vacuum) and as our Galaxy is one such point, within a local group, our area of Vacuum, is quite distinct from say an area far, far away in the total Vaccum Field, and so within our Galaxy, we are constrained by the amount of Vacuum we can produce, just as a single photon is governed by the Vacuum we create, the Galaxy, by its very presence is governed by the Amount of local Vacuum it sits within.
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