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Changes in magnetic fields and causality

  1. Nov 3, 2011 #1
    I was thinking about rapid changes in magnetic fields and the mechanics of cause and effect:

    Suppose you have a:
    Primary coil with switch and a battery
    and a separate Secondary coil perhaps hooked up to a oscilloscope.

    In the first femtosecond in which you flick the switch and dc current from battery runs through the primary coil a magnetic field must propagate out from the wires to fill space at the speed of light. A very short time later the expanding field must reach the secondary coil and induce current.

    Now in the second femtosecond after flicking a switch, once the expanding magnetic field from the primary has arrived at the secondary coil, surely the secondary coil begins to act like a resistor, now reducing the number of electrons moving in the primary coil. The battery is now doing some work inducing current in the second coil.

    Questions:
    1. If I shut off the battery at Femtosecond 2, before the propagating magnetic field reached the secondary coil what happens? I think in the first femtosecond I made the magnetic field with zero energy cost - because the field hadn't yet done any work no energy was spent? Does it still do work on the secondary coil when it finally arrives?

    2.How does cause and effect work here? I can explain my problem with this series of events: Electrons move in primary coil first, they cause a changing mag field, which expands ( at speed of light?), which then hits secondary coil, which then induces electrons to move in the secondary coil, which then causes resistance in the primary coil.

    So how fast does the "resistance information" from the secondary coil flow back to the primary.... is there some back flow of information perhaps at the speed of light? Was this sort of experiment ever proposed perhaps on a larger scale?

    Please help.
    Puzzled.
     
  2. jcsd
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