A level revision

Transformers

Transformers increase (step up) or decrease (step down) the voltage in a circuit. This is done using electromagnets made from coils of wire and iron. A basic transformer setup is displayed below:

Transformers only work with alternating current because this provides a constant change of flux and thus a constant induced voltage. (See: Electromagnetic induction).

An input voltage is supplied to the left, primary coil. This causes flux to flow around the iron. This causes a voltage to be induced in the right of the transformer. The voltage induced on this, the secondary coil, depends on the number of turns in the wire of the coil. To increase the output voltage from the input voltage you increase the amount of coils on right compared to the left. The opposite causes the voltage to decrease. This gives rise to the formula:

Where V is the voltage and N is the number of turns on the coil. Using this formula we can caluclate the number of turns required to give a certain output voltage from a known input voltage

Examples

A socket supplies a TV with 240V however the TV circuit only requires 12V.
The TV uses a transformer with 400 turns on the primary coil to step down the voltage.
How many turns are required on the secondary coil?

Using the above formula, fill out what you know - the two voltages and the turns on the primary coil:

Calculate the ratio of the voltages:

Then simply solve for :