Turns and voltage

Figure 13.26 shows a simple form of transformer: two coils of insulated copper wire wound on a soft iron ring.

Usually an AC source is connected to one coil and the other coil to a load (a resistor or a device that runs on AC). The first coil, which supplies energy to the transformer, is the primary coil; the coil that takes energy out is the secondary coil. We call the AC voltage supplied to the primary V_P and the voltage from the secondary V_S.

The AC voltage V_P drives an alternating current in the primary coil, which sets up an alternating magnetic field. The soft iron core guides this field to the secondary coil, and the changing field induces an alternating voltage V_S in the secondary.

So by changing the ratio of the primary turns N_P to the secondary turns N_S, the AC voltage on the primary can be made smaller or larger at the secondary.

Why the turns set the voltage

Every turn of wire on the core sits in the same changing field, so every turn has the same small emf induced in it — whichever coil it belongs to. The turns of a coil add up like cells in series. A secondary with ten times the turns of the primary therefore gives ten times the voltage; one with a tenth of the turns gives a tenth. That is the whole of the turns rule, and it is why transformer questions come down to a ratio.