Efficiency of a Transformer

bleedblue1234

1. The problem statement, all variables and given/known data

The specifications of a transformer for a small appliance read "Input: 110 V, 6.4 W" and "Output: 8.9 V, 270 mA". What is the efficiency of this transformer?

2. Relevant equations

P = I * V

V = I * R

3. The attempt at a solution

6.4 W / (8.9*(270*.001)) ---> I think this is wrong

I am just not sure how the voltage drop of the transformer would affect the power output...

shungun

Your reasoning is absolutely correct, voltage drop theoretically shouldn't affect the power output, but in theory we assume that there is no losses, no heat generated etc.
Thus when such a question pops up (power out)*100/(power in ) will give you the efficiency.