Why doesn't the transformer voltage formula work with a DC battery?

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pez
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Hey everyone, I had a question on a question about transformers:

The primary coil of a transformer contains 100 turns; the secondary has 200 turns. The primary coil is connected to a size AA battery that supplies a constant voltage of 1.5 volts. What voltage would be measured across the secondary coil?

So, I figured V_2/V_1 = N_2/N_1, where N = the number of coils and V = the volt. This led me to an answer of 3V, which is incorrect. Can someone explain why this is so? The question says the voltage is constant, so shouldn't the formula work?
 

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pez said:
Hey everyone, I had a question on a question about transformers:
The primary coil of a transformer contains 100 turns; the secondary has 200 turns. The primary coil is connected to a size AA battery that supplies a constant voltage of 1.5 volts. What voltage would be measured across the secondary coil?
So, I figured V_2/V_1 = N_2/N_1, where N = the number of coils and V = the volt. This led me to an answer of 3V, which is incorrect. Can someone explain why this is so? The question says the voltage is constant, so shouldn't the formula work?
The answer to give depends on the level of sophistication that you need.
Simple answer is of course that a transformer doesn't transform DC values. But depending on the sophistication, one might want the *transient* response of the transformer when you *connect* the battery. That's slightly more involved...
 
I recognize this question. This is a basic situation in which the student is supposed to recognize the *type* of electric current that must be used for transformation. So the the initial induction (as the switch is closed) is probably not what is asked for here.

So, what kind of current is required for a transformet to step-up or step-down voltage?