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First law of Maxwell

  1. May 2, 2010 #1
    I know that the first law of Maxwell states that every moving charge creates a magnetic field.

    But, is there an explanation of WHY this happens?

    Why a moving charge creates a magnetic field?

    Or this is the kind of law that nobody knows why exist?

    Thank you
  2. jcsd
  3. May 2, 2010 #2
    I assume you're referring to ampere's law. (generally Gauss's law is written as the "first" of Maxwell's equations.)

    My physics professor told us it is actually a relativistic effect due to the electrons traveling near the speed of light. I found this thread which might elaborate on it more than I could.

  4. May 2, 2010 #3
    Ah that's the whole beauty of electricity and magnetism!

    Turns out that Maxwell (the guy the equations are named after) discovered that electricity and magnetism are one and the same and that's why we now call the force the "electromagnetic" force.

    Maxwell actually used 4 different laws, and collectively they are known today as Maxwell's laws. When Maxwell tried to combine the four equations he realized that they describe a wave that is a mixture of both electric and magnetic waves.

    What this means is that electricity and magnetism are exactly the same. The reason a moving charge looks like it "creates" a magnetic field is because as stated by Nu naught has to do with relativity.

    If you know anything about relativity you know about the Lorentz transformation. Well turns out that if you do a Lorentz transformation on a moving electric charge's electric field you get a magnetic field. This is due to the fact that moving things relativity to us appear to act differently according to the laws of relativity.

    In summary electric and magnetic fields are one in the same. A moving charge "creates" a magnetic field by moving which is then subject to a Lorentz transformation which shows that all that happens is the electric field transforms into a magnetic field.
  5. May 2, 2010 #4
    Thank you for the reply.

    So, when a charge starts to move it's electric field is transformed into a magnetic field? So a moving charge has no electric field?

    I'm not very good at relativity, but I have notions of the concepts. Is it possible to explain me without using complex equations what happens in relativity to the magnetic field to appear at a moving charge?
  6. May 2, 2010 #5


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    Staff: Mentor

    No, a moving charge has both an electric field and a magnetic field.
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