The connection develops from the fact that an
electric current (the flow of electrons in a metal)
produces a magnetic field
Faraday shows that a changing electric field
produces a magnetic field and, vice-versus, a
changing magnetic field produces an electric
Maxwell completes the theory with a full
mathematical description of the relationship
between electric and magnetic fields =
The electric motor
Electric current flowing around the
coil of the electric motor produces
oppositely directed forces on each
side of the coil.
These forces cause the coil to
Every half revolution the split ring
commutator causes the current in
the coil to reverse otherwise the
coil would stop in the vertical
Model Electric Motor
What do you need?
Stationary magnets - rotating magnets - electromagnets
If an electrical conductor cuts
through magnetic field lines, a
voltage is induced across the ends
of the conductor.
If the wire is part of a complete
circuit, a current is induced in the
This is called electromagnetic
induction and is sometimes
called the generator effect.
If a magnet is moved into a
coil of wire, a voltage is
induced across the ends of the
If the direction of motion, or
the polarity of the magnet, is
reversed, then the direction of
the induced voltage and the
induced current are also
also occurs if the magnetic
field is stationary and the coil
A transformer is a device
that is used to change
one alternating voltage
level to another.
Structure of a transformer
A transformer consists of at least two coils of wire
wrapped around a laminated iron core.
of Np turns
of Ns turns
laminated iron core
The transformer equation
The voltages or potential differences across the
primary and secondary coils of a transformer are
related by the equation:
Calculate the secondary voltage of a transformer that has a
primary coil of 1200 turns and a secondary of 150 turns if the
primary is supplied with 230V.
Vp = Np
230 / Vs = 1200 / 150
230 / Vs = 8
230 = 8 x Vs
230 / 8 = Vs
Secondary voltage = 28.8 V
Calculate the number of turns required for the primary coil of
a transformer if secondary has 400 turns and the primary
voltage is stepped up from 12V to a secondary voltage of 48V.
Vp = Np
12 / 48 = Np / 400
0.25 = Np / 400
0.25 x 400 = Np
Primary has 100 turns
In a step-up transformer the
voltage across the secondary coil is
greater than the voltage across the
The secondary turns must be
greater than the primary turns.
Use: To increase the voltage output
from a power station from 25 kV
(25 000 V) to up to 400 kV.
In a step-down transformer the voltage
across the secondary coil is smaller than
the voltage across the primary coil.
The secondary turns must be smaller
than the primary turns.
Use: To decrease the voltage output from
the mains supply from 230V to 18V to
power and recharge a lap-top computer.
1. A transformer has a primary voltage of 480 volts
and a secondary voltage of 120 volts. If the
primary windings have 700 turns, how many turns
are in the secondary windings? (Voltage and
number of turns are directly proportional.)