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Magnetic attraction v/s induction ?

  1. Mar 18, 2010 #1
    Which comes first , magnetic attraction or magnetic induction and Why?
  2. jcsd
  3. Mar 18, 2010 #2


    Staff: Mentor

    That is a very chicken-or-egg question. As near as we can tell they happen at the same time. All of Maxwell's equations and the Lorentz force equation are valid at all times.
  4. Mar 18, 2010 #3
    Neither. If you have a changing current which is a source for both a magnetic field B and magnetic induction which creates an E field in space (at the same spot) one finds that BOTH proceed outward from the source current at the speed of light. (for that matter A, the magnetic vector potential, is also "retarded" as well which means delayed by the speed of light.)

    Usually nobody likes to deal with retardation in Maxwell's equations because it's a major pain in the butt. But it is real and many times it is also important. An important fact that comes out of this observation is that since B and E at a given point are happening at the same time (same distance and same speed of light from source) it logically follows that Resnick and Halliday's statement that a changing B field creates an induced E field is clearly wrong. Things that occur at the same time cannot cause each other. Truth is that a changing current creates BOTH a B field and the E field (and an A field) which propagate away together. Now since both have the same source one can certainly use the values of the B field to calculate the value of the E field. This is why Maxwell said that the induced E field is "measured by" the B field rather than "created by" it. It's a fine point, but an important one.
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