Electromagnetism

When current flows through a conductor a magnetic field is
produced around the conductor. The magnetic field is made up
of lines of flux, just like a natural magnet. The size and strength
of the magnetic field will increase and decrease as the current
flow strength increases and decreases

Left-Hand Rule for Conductors

A definite relationship exists between the direction of current
flow and the direction of the magnetic field. The left-hand rule
for conductors demonstrates this relationship. If a current carrying
conductor is grasped with the left hand with the thumb
pointing in the direction of electron flow, the fingers will point in
the direction of the magnetic lines of flux

In the following illustration it can be seen that when the
electron flow is away from the viewer (indicated by the plus
sign) the lines of flux flow in a counter clock wise direction
around the conductor. When the electron flow reverses and
current flow is towards the viewer (indicated by the dot) the
lines of flux reverse direction and flow in a clockwise direction

Electromagnet

An electromagnet can be made by winding the conductor into
a coil and applying a DC voltage. The lines of flux, formed by
current flow through the conductor, combine to produce a larger
and stronger magnetic field. The center of the coil is known as
the core. In this simple electromagnet the core is air

Adding an Iron Core

Iron is a better conductor of flux than air. The air core of an
electromagnet can be replaced by a piece of soft iron. When a
piece of iron is placed in the centre of the coil more lines of flux
can flow and the magnetic field is strengthened

Number of Turns

The strength of the magnetic field in the DC electromagnet can
be increased by increasing the number of turns in the coil. The
greater the number of turns the stronger the magnetic field

Changing Polarity

The magnetic field of an electromagnet has the same
characteristics as a natural magnet, including a north and south
pole. However, when the direction of current flow through
the electromagnet changes, the polarity of the electromagnet
changes. The polarity of an electromagnet connected to an AC
source will change at the same frequency as the frequency
of the AC source. This can be demonstrated in the following
illustration.At Time 1 current flow is at zero. There is no magnetic field produced around the electromagnet At Time 2 current is
flowing in a positive direction. A magnetic field builds up around
the electromagnet. The electromagnet assumes a polarity with
the south pole on the top and the north pole on the bottom. At
Time 3 current flow is at its peak positive value. The strength
of the electromagnetic field is at its greatest value. At Time
4 current flow decreases and the magnetic field begins to
collapse, until Time 5 when current flow and magnetic field are
at zero. Current immediately begins to increase in the opposite
direction. At Time 6 current is increasing in a negative direction.
The polarity of the electromagnetic field has changed. The
north pole is now on top and the south pole is on the bottom.
The negative half of the cycle continues through Times 7 and 8,
returning to zero at Time 9. This process will repeat 60 times a
second with a 60 Hz AC power supply