# AC Transformer

- Author:
- Tan Seng Kwang

The ratio of the secondary to primary voltages in a transformer equals the ratio of the number of loops in their coils, i.e. .

Assuming that resistance is negligible, the electrical power output of a transformer equals its input.
Hence, .
Rearranging, we have
Also,

## Check your understanding

A travel adapter has a step-up transformer, the 120 V input of which is transformed to a 240 V output. The primary coil has 18 loops and draws a current of 0.200 A when in use. What is the number of loops in the secondary coil?

What is the current output of the secondary coil?