Specification References

6.17 recall the structure of a transformer and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides.
6.18 explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy.
6.19 recall and use the relationship between input(primary) and output(secondary) voltages and the turns ratio for a transformer: Primary voltage/secondary voltage = primary turns/secondary turns.

Starter

The direction of the magnetic field is determined by the direction of the current and the direction of the turns around the iron core. If you change the direction of the current, the north and south poles of the electromagnet will switch.
With DC electricity, you must physically change the wires to change the direction of the current. With AC electricity, the direction changes with each cycle.
Thus, one end of an AC electromagnet is switching from north to south and back again 60 times per second in the U.S. or 50 times per second in some other countries.
A transformer can be a long piece of iron with wire having with AC current going through it and wrapped around the piece of iron near one end. It also has wire that creates electrical current wrapped around it at the other end. A more common configuration is a square or donut shaped iron core with the wire wrapping on both sides.
The strength of the magnetic field is proportion to the input voltage and the number of turns around the core (called the primary coil). By reversing the rule, the output voltage is proportional to the strength of the changing magnetic field and the number of turns (called the secondary coil).
For example, if you wanted to increase your house voltage from 110 volts (110V) to 220V in order to power your electric stove, you could use a transformer with twice the turns in the secondary coil as in the primary coil.

The relationship is written as: input volts / input turns = output volts / output turns 110V / 5 turns = 22 = 220V / 10 turns

transformers don't work with d.c., only with a.c.!
for a 100% efficient transformer, Power supplied in Primary = Power in Secondary
Vp x Ip = Vs x Is