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SPH3U: Electricity
Intro to Magnetism
Magnetic Poles

Every magnet has a north and south pole
N
S
S
N
 Like Poles Repel
N
S
N
S
 Opposite Poles Attract
Magnetic Field

The magnetic field around a magnet is
strongest at the poles.

The north pole and the south pole of the same
bar magnet are in general, equally strong.

If you cut a magnet in two pieces, each piece
will have a north and south pole. You can keep
cutting to make smaller and smaller magnets –
but each one will be weaker in strength.
Magnetic Field

Physicists are currently doing research to see
if it is possible to get a monopole magnet –
one in which there is only a north pole, or only
a south pole. Do you think this is possible?

A magnetic field is the space around a
magnet where a magnetic force can be felt.
It is very similar in theory to an electric field.
Properties of the Magnetic Field

Most materials will respond at least a little bit
to a magnetic field, but only a very small bit.
Materials that respond very strongly to a
magnetic field are called ferromagnetic.

Some examples of ferromagnetic materials
are:
- Iron
- Nickel
- Cobalt
Lines of Magnetic Force


Draw in the
pattern you see
with the iron
filings 
These are the
lines of
magnetic force
N
S
Lines of Magenetic Force

Lines of magnetic
force show how
the magnetic
force acts around
the magnet. The
force is strongest
at the poles
(where the lines
are close together)
and weaker the
further out you go.
Lines of Magenetic Force

When a compass
is places upon a
line of force, the
compass needle
always points
along the line.
The north pole of
the compass is
directed toward
the south pole of
the magnet.
Lines of Magenetic Force

The direction of a
line of force is
defined as the
direction in which
the north pole of a
compass points
when placed along
that line.
Lines of Magenetic Force

Even though we
draw the lines on
a flat sheet, it is
important to note
that the lines exist
in 3-dimensions.
Lines of Magnetic Force

A horseshoe magnet
Lines of Magnetic Force

When drawing Magnetic Field lines,
remember:
Lines go from North to South outside the
magnet
 Lines go from South to North inside the
magnet
 Lines never cross

Practice

Draw the magnetic field lines
Predicting Magnetic Forces

Lines of force that act in the same direction will
make the magnetic force stronger.
Predicting Magnetic Forces

Lines of force that act in the opposite direction
will make the magnetic force weaker.
Predicting Magnetic Forces

Parallel fields from two different magnets
show us that there is a repulsion.
Predicting Magnetic Forces

Opposite fields from two different magents
show us that there is an attraction.
The Earth

The Earth has a magnetic field, which can
be detected by a compass.

The magnetic field around the earth is
actually not the way we would think of it.
The geographical North pole actually
works the same way as the south pole of
a magnet.
The Earth
Homework
Read section 12.1 (pages 548-552).
 Make your own summarized notes of the
practical applications of magnets in
industry, and magnetism in the natural
world (pg. 550-551). Make sure you can
describe in detail at least 3 applications
of magnets.
 Answer questions #1-6 on page 552
