# Homework Help: Power in transformers

1. Mar 21, 2005

### myself

Let's say that there is a transformer with 10 loops on one side (side A) and 100 loops on the other (side B). If ther is a rms 10V potential difference produce by a generator on side A, then there would be a rms 100V potential difference on side B. It would seem like the resistance through the wire on side A is constant. Since rms voltage is constant, the rms current should be constant as well. Therefore, it would seem like the rms power supplied by the generator is constant. However, different resistances on side B would cause different amounts of power to be needed by side B. Because energy is conserved, power supplied by the generator is equal the power wanted by side B. Therefore, the rms power supplied by the generator is not constant. So how does side B "tell" side A how much power it wants?

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2. Mar 22, 2005

### Davorak

Short answer is through the magnetic field.

Current on side A creates a magnetic field that induces a current in side B.

The AC power supply on side A is fixed at 10V rms but will then provide what ever current is necessary to provide 10V rms on side A and 100 rms on side B.

Too elicit more responses you may want to explain what you have already figured out or talk through what you have found to work.

3. Mar 22, 2005

### myself

I guess I wasn't very clear at first. First of all, this isn't really a homework problem, but I found a section about transformers in the textbook, and I didn't find the explanation comopletely helpful.

I couldn't figure out how side A knew how much current to "send" to side B. I know how much current is sent, but I cannot figure out how side B "tells" side A how much power is needed.

I think that this is probably something to do with inductance and back EMF, and that side B would only take a portion of the energy in the magnetic field. Then, there would be some back EMF in side A, which means side A would send less current, until a balance is reached. However, I am very unsure about this, and I am still confused.