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Battery with voltage V=12V is connected to the primary of a transform

  1. Dec 14, 2011 #1
    1. The problem statement, all variables and given/known data
    A battery with voltage V=12V is connected to the primary of a transformer. A resistor with resistance R =20 Ohms is connected to the transformer secondary. The transformer has an iron core with Np=200 turns on the primary and Ns=600 turns on the secondary. Assume the resistance of the wires is zero.

    What is Is, the current in the resistor?


    2. Relevant equations
    Vs= (Ns/Np)Vp

    3. The attempt at a solution
    Vs=(600/200) * 12 V
    Vs=36 V


    36V=I(20 ohms)
    I =1.8A, but the answer is 0? Could someone please show me where I have made my error or how to arrive at the answer of 0 A?
    Last edited: Dec 14, 2011
  2. jcsd
  3. Dec 14, 2011 #2


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    Staff: Mentor

    Re: Transformers

    Transformers require AC to operate. A DC voltage source on the primary will just run a DC current through the primary (limited by the primary's resistance). After any initial transient response when the voltage is first connected the current in the primary will be constant, so there will be no changing magnetic field to influence the secondary. So after an initial 'glitch' there will be no current in the secondary.
  4. Dec 14, 2011 #3


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    Homework Helper

    Re: Transformers

    The answer dies not lie in calculations.
    Do you know any of the electromagnetic theory behind a transformer?
    What is it that causes a current in the secondary winding?
    What is the nature of the current in the primary that causes that?
  5. Dec 14, 2011 #4
    Re: Transformers

    Thank you for the quick response and explanation. I now understand.
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