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So I've been confused about transformers lately. In searching for some

  1. Jun 21, 2010 #1
    So I've been confused about transformers lately. In searching for some information regarding the basic transformer, I came across this: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html#c1. While this page was informative, I am still left confused in regards to one point in particular.

    Assuming the two windings are not touching and are simply magnetically coupled, shouldn't the current in the secondary loop be given by the induced voltage divided by the resistance in the loop? Thus, an increase in voltage, such as in a step-up transformer, should generate a larger current in the secondary loop. This line of reasoning, however, comes into conflict with the link's energy conservation logic, which claims that an increase in voltage should result in a lower current in the secondary loop. I see the validity in their reasoning, but I fail to see the flaw in mine.

    I would greatly appreciate any and all help. Thanks in advance.
  2. jcsd
  3. Jun 21, 2010 #2


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    Re: Transformers?

    It's not resistance in the loop you have to worry about, it's the load.

    Lets say you have a generator attached to your first loop applying some voltage V1 to the first coil. That induces voltage V2 on the second coil. Now, lets say that this second coil is attached to resistor R. The current in second coil, I2 is just V2/R. Does increasing V2 increase that current? Of course. But we should be comparing it to the current in first coil, I1. And that current works out to be I1 = V22/(V1R). So while current in second coil increases with increase in V2, the current in first coil increases even more, and regardless of the two voltages I1/I2 = V2/V1. So when transformer steps up voltage, it steps down current by the same ratio.

    All of the above assumes no resistance in coils. With real transformer, you have to factors these in, but qualitative results are the same.
  4. Jun 21, 2010 #3
    Re: Transformers?

    I'm sorry if this is an elementary question, but could you define load for me? What exactly is the load in this situation? The internal resistance of the generator...?

    Additionally, I see how you can derive I1/I2 = V2/V1 from I1 = V22/(V1R) by substituting I2 = V2/R. But how did you get I1 = V22/(V1R)?
  5. Jun 21, 2010 #4
    Re: Transformers?

    The load is the resistor (usually) furthest away from the generator for a simple case like this.

    The equation comes from equating the input power I1*V1 to the output power V2^2/R.
  6. Jun 21, 2010 #5
    Re: Transformers?

    Well, K^2's post begins by saying that I must worry about the load, not the resistor. But by using your logic, the load is the resistor in the second loop. Could you clarify what the load actually is in this example?

    Also, while I understand your argument, I would like to stay away from energy conservation as a solution, seeing as I could have simply jumped straight to the equation I wanted to derive using that method.

    Thanks in advance.

    EDIT: What I mean by my second statement, is that if I substitute V2 = I2R in that equation, then I simply arrive at equation I wanted by energy conservation methods, which I already knew to be a valid method.
  7. Jun 22, 2010 #6


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    Re: Transformers?

    I'm considering a circuit like this Σ===3||ε===(~).

    So you have a resistor, transformer, and the generator. Resistance in generator and transformer are assumed to be zero. Resistor is the sole load.

    In general, the load might have complex impedance, but I'm ignoring all the subtleties in this example. Just showing how the current changes from generator to resistor.
  8. Jun 22, 2010 #7
    Re: Transformers?

    Is there method to arrive at the equation I1 = V22/(V1R) excluding energy conservation (due to reasons outlined above) ?

    Thanks for being patient with me. I am just having a hard time with this concept.
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