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Law of transformers

  1. Dec 19, 2013 #1
    the law of the transformer states that
    in primary coil , the EMF = -n multiplied by the rate of change of flux with respect to time
    but that law applies to induced EMFs , and since the voltage in the primary coil is not actually induced but rather produced by an external source of A.C , then how can we apply the rule to such a thing ? in the secondary the voltage is purely induced so the law does apply , but in the primary coil the potential difference is not induced .
    Last edited: Dec 19, 2013
  2. jcsd
  3. Dec 19, 2013 #2


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    It's the same thing as in the other thread. The voltage across the primary is going to be the sum of the induced voltage and the voltage needed to drive the given current through the wire. The induced voltage works exactly the same way in both coils, but in primary it's the back EMF, while in secondary, it's EMF for whatever that coil is driving.

    If it makes it simpler for you, you can always think of a real coil as a perfect, zero resistance inductor and a resistor in series. Resistor is just ohmic, giving voltage drop equal to IR, while the inductor portion gives just the induced voltage proportional to the rate of change of magnetic flux.
  4. Dec 19, 2013 #3
    oh , so the inductor voltage just acts as a voltage drop , i never knew that a voltage drop was opposing the current :D thanks alot , made lots of things clear
  5. Dec 19, 2013 #4
    there is voltage drop on resistor for example and it doesn't produce a opposite current flow.It just opposes current , but that is a different thing, in a resistor the current flow produces interactions at the subatomic level which manifest themselves as heat , in the case of a back EMF a field works against the charged particle flow ,current.

    voltage drop isn't responsible for current reversal , in an inductor or transformer and also in electric motors , there is a phenomenon called back EMF , in other words when you add a potential difference to a coil , it causes current to flow , that current has a magnetic field around it which when changing in both magnitude and direction creates a changing electric field which opposes the original current flow magnetic field that created it.

    actually member K2 answers this in your other post quite nicely.
    Last edited: Dec 19, 2013
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