Transformers physics question?

Question

A videotape rewinder uses a plug in transformer to convert 120 V to 12.0 V, with maximum current output of 200 mA. What is the current input? What is the power input?

Answer 1

That's nice and easy.

Assuming 100% efficiency:

V * A in = V * A out

So current in = 200 * 12 / 120 = 20mA

Power input = V * A = 120 * 0.02 = 2.4W

Answer 2

I1V1 = I2V2

I1 = I2V2 / V1

= (12.0 V x 0.200 A) / 120 V

= 0.020 A

Power = Current x Voltage

= 0.020 A x 120 V

= 2.4 W

Answer 3

power in equals power out less the losses in the transformer.