Question

A transformer connected to a 120--V ac line is to supply 12.0V (rms) to a portable electronic device. The load resistance in the secondary is 4.70 ohms
1) What should the ratio of primary to secondary turns of the transformer be?

N1/N2 = _________

2) What rms current must the secondary supply?

I = _________ A

3) What average power is delivered to the load?

P = ________ W

4) What resistance connected directly across the source line (which has a voltage of 120?V ) would draw the same power as the transformer?

R = ________ ohms

Answer 1

We don't know anything about the structure or materials of the real transformer, so all we can do is assume that it's ideal, with no losses.

1). N1/N2 = 120v/12v = 10:1

2). I = V/R = 12/4.7 = 2.55 Amperes

3). Power = V²/R = (12)² / 4.7  =  30.64 watts

4). Power = V² / R

R = V² / power

R = (120)² / (30.64 watts)

R =  470 ohms