The transformer and the oscilloscope

Last time

Before we start: last time, in The d.c. motor, relays and loudspeakers, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why the grid uses 400 000 volts

Power stations send electricity along cables at hundreds of thousands of volts. Your home gets 230 volts, and your phone charger only 5. A transformer changes the voltage in between, and without it, cheap electricity for everyone would not be possible.

How a transformer works

A transformer has two coils of wire, the primary and the secondary, wound on a core of soft iron. An alternating current in the primary makes a changing magnetic field, and the iron core carries that field to the secondary. The changing field induces an alternating e.m.f. in the secondary coil. The two coils are not connected electrically.

Why a.c.?

A transformer only works with alternating current. Direct current gives a steady field, and a steady field induces nothing. The core is soft iron because it magnetises and demagnetises easily and carries the field to the secondary. Iron is not chosen for being a good electrical conductor.

The turns ratio

The voltages are in the same ratio as the numbers of turns. V s over V p equals N s over N p. More turns on the secondary: step-up transformer, a higher voltage. Fewer turns on the secondary: step-down, a lower voltage. A step-up transformer with a 1 to 10 ratio gives ten times the voltage.