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Confused about transformers formula derivation

  1. Apr 7, 2010 #1

    I've been a bit confused with how the equation Vp/Vs = Np/Ns came about. From reading a physics textbook, I understand that the voltages came from the emfs ε (primary) and ε (secondary). The primary voltage through the primary coils causes a change in magnetic flux which induces an emf in the secondary coils. From this, I can see why ε (secondary) is equal to Vs, but what I don't understand is why the ε (primary) can be replaced with Vp when the formula for ε (primary) is equal to -N (primary) * delta (flux)/delta (time). I thought this equation is for the induced emf, not the applied voltage from the primary source. Or is the primary voltage equal to an induced emf in the primary coils from iron core ie. if a voltage is applied to a solenoid with N(primary) coils, then would there result in a delta (flux)/delta (time)?

    If apply voltage to solenoid, then would applied V= N* (delta flux)/ (delta time)?
    which would essentially be the same backwards : induced emf = N* (delta flux)/ (delta time)? Is this true or am I totally wrong?

    (ideal wires assumed)

    Thank you.
  2. jcsd
  3. Apr 7, 2010 #2
    It is just the Faraday law, which connects the magnetic flux, voltage and turns.
    Magnetic flux rate is same for both coils.
    Last edited: Apr 7, 2010
  4. Apr 7, 2010 #3

    Hi Rajini, thanks for replying.

    So I'm just wondering, does the change in magnetic flux through the primary coils cause an induced emf equal to the applied primary voltage through the primary coils themselves as well? Would that counteract the voltage from the primary source?
  5. Apr 7, 2010 #4
    Actually when you apply a voltage to primary coil it induces a voltage in secondary according to faraday induction law. Now the magnetic flux is same for both coils..Keeping this in mind you can find the relation for secondary coil..The same applies to primary..equating both the relations you get the famous transformer law..
    look in to wiki
    there you will find explanation with picture.
  6. Apr 8, 2010 #5
    I think I understand now, thanks a lot!

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