Everything Science Grade 11

18.4 CHAPTER 18. ELECTROMAGNETISM

DEFINITION: Transformer

A transformer is an electrical device that uses the principle of inductionbetween the

primary coil and the secondary coil to either step-up or step-down the voltage.

The essential features ofa transformer are two coils of wire, called the primary coil and the secondary
coil, which are wound around different sectionsof the same iron core.

iron core

primary coil secondary coil

magnetic flux

When an alternating voltage is applied to the primary coil it creates an alternating current in thatcoil,
which induces an alternating magnetic field in the iron core. The changing magnetic flux through the
secondary coil inducesan emf, which creates acurrent in this secondary coil.

The circuit symbol for atransformer is:

T

By choosing the number of coils in the secondary solenoid relative to the number of coils in the
primary solenoid, we can choose how much bigger or smaller the induced secondary current isby
comparison to the current in the primary solenoid (so by how much thecurrent is stepped up or
down.)

This ability to transformvoltage and current levels according to a simpleratio, determined by theratio
of input and output coilturns is a very useful property of transformers and accounts for the name. We
can derive a mathematical relationship by usingFaraday’s law.

Assume that an alternating voltage Vpis applied to the primarycoil (which has Npturns) of a
transformer. The currentthat results from this voltage generates a changing magnetic flux φp. We can
then describe the emf inthe primary coil by:

Vp= Np

Δφp

Δt

Similarly, for the secondary coil,

Vs= Ns

Δφs

Δt

If we assume that the primary and secondary windings are perfectly coupled, then:

φp= φs

which means that:
Vp
Vs

=

Np

Ns