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B Lenz's Law and Induced Magnetic Fields

  1. Jul 9, 2017 #1
    In a transformer, the primary coil has a current running through it which creates a magnetic field. Then this field induces a current in the secondary coil. That induced current creates a magnetic field exactly opposite the inducing magnetic field.

    This might be an obvious question, but that means there is effectively zero magnetic field around the second coil, correct? What about the wires that are carrying that current away? Do they now have an associated magnetic field because they are not submerged in the primary coils initial magnetic field to cancel it out?

    The reason I ask this is I'm curious how this is managed in electrical shielding circuits. Only part of the shielding is submerged in the EM noise at a given moment. So that part of the shielding will cancel the field, but the wires carrying that current off to ground will still have a magnetic field. Are those wires just put in places furthest from any sensitive electronics?

  2. jcsd
  3. Jul 9, 2017 #2


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  4. Jul 9, 2017 #3
    Yes, that's right.
    No. It does not perfectly cancel out. This is because the reverse magnetic field acts to appose the current in the primary coil. When a load is attached to the secondary coil, the resistance of the load is reflected back onto the primary winding through this effect.
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