Your parentheseed comment--"(or Vs changes with frequency)"--likely contains the vast majority of any discrepancies between what you're measuring and what you're expecting. Vs depends critically on the impedance of the circuit that your voltage source is driving into. If that circuit contains resonances in the frequency range of interest, you can see quite dramatic changes in the voltage at the output of your function generator (or whatever you're using to generate the signal).
If you have a 'scope that has a long timebase capability, you can set the function generator to chirp from a start to end frequency (say over 10 seconds), and then set the scope to capture the resulting signal, both on the input and output to the transformer. If you have access to a network analyzer, that would be an even better way to do it. Failing those, you can build a simple envelope-detector circuit for about $2, and find the DC signal levels. Any of these, or even stepping through of the frequencies by hand, and reading off RMS values from the scope would be a good undergraduate exercise.
Takeaway lesson for undergraduates: We usually speak as though the voltage we're applying to a circuit is constant, but we very definitely must measure it, or have a good theoretical understanding of it, to do the kinds of ratio-based calculations implied by the ideal transformer equation!
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Down with categorical imperative!
flutzpah@yahoo.com
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________________________________
From: Mangala Joshua <mjoshua@mesacc.edu>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Thursday, August 13, 2009 1:36:10 PM
Subject: [Phys-l] transformer question
The transformer equation in undergraduate texts is
Vp/Vs=Np/Ns
However if we set up an experiment we observe that
Vp/Vs depends on the frequency of the primary source( or Vs changes with frequency)
What is the cause of this frequency dependence?
Mangala Joshua
Physical Science Department
Mesa Community College
480-461-7053