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Magnetism and Electromagnetism
The ancient Greeks knew that a type of rock with magnetic properties known as lodestone or
magnetite attracted iron. The compass, an important device for navigation, has a suspended magnet
which aligns parallel to the magnetic field produced by the Earth and as a result points to the North
(seeking) pole.
The compass was documented as early as 1040. It is known that the Vikings used Lodestone to
navigate. By the end of the twelfth century, Europeans were using this simple compass to aid
navigation.
The earth's south magnetic pole is located near the geographic north pole and is called the
geomagnetic north or north seeking pole.
Properties of Magnets
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A magnet will always have two poles which we call arbitrarily North and South
If the magnet is broken in two this will create two new magnets with North and South poles.
Like poles repel each other. If a north pole is brought close to the north pole of a second
magnet a repulsive force will be felt. Similarly if a south pole is brought close to the south pole
of another magnet, the two magnets will repel each other.
Unlike poles attract and will stick together.
Magnets attract iron rich materials
The Domain Theory of Magnetism
The domain theory states that inside a magnet there are small regions in which the magnetic
alignments of all the atoms are aligned in the same directions.
The Magnetic Field
The image shows the field lines produced by a bar magnet. Iron fillings are sprinkled on a piece of
paper and the bar-magnet is placed under the paper. The iron-fillings line up and show the intensity
and direction of the magnetic field.
Magnetic flux density is given the symbol B and has the unit Tesla.
The strength of the magnetic field is determined by the number of field lines passing a unit area. The
more field lines the stronger the magnetic field. The direction of the field line can be determined by
using a compass needle.
By convention the arrow tip on magnetic field lines points towards the south magnetic pole and away
from the north magnetic pole.
Source
A hair dryer
Sunlight
Colour TV
A small bar magnet will produce
MRI body scanner magnet
Reseach Physics Labs produce up to
The field at the surface of a neutron star
Magnitude of B (T)
10-3
10-6
10-6
10-2
2
50
108
Magnetic fields obey the inverse field law - field strength decreases inversely with distance.
Note on representing three dimensions on diagrams
In electromagnetism the pattern of a magnetic field in the space around a conductor, or the connection
between directions for forces, fields and currents, are often important.
We have to consider three dimensions.
In art, the illusion of three dimensions is produced by the use of perspective.
In physics, diagrams are drawn flat and symbols used to show vectors which have directions into and
out of the page, that is:
a circle with a dot inside it (the tip of an arrow coming towards you) shows a vector out of the page
and
,
a circle with a cross inside it (the tail of an arrow going away from you) shows a vector into the page
.
The strength and direction of the magnetic field at any point is defined in terms of the force on a
moving charged particle such as an electron. The force created the magnetic field comes from the
Lorentz:
(This simplified equation only works when the motion of the charged particles is at right angles to the
magnetic field).
Magnetism and electricity
Magnetism and electricity were originally studied as separate areas.
Last century it was found that the two are intimately connected. Those connections are central to the
study of electric power.
Around a current flowing in a long straight wire there is a circular magnetic field. Its direction can be
found from the right hand grip rule as illustrated:
This is known as the Right Hand Grip Rule.
When current flows in a wire, a magnetic field is created around the wire. To visualize the magnetic
field, take your right hand, curl the fingers, and stick the thumb straight out. Point your thumb in the
direction of the current flowing in the wire (using conventional current).
The direction your fingers are curved around the wire is the direction of the magnetic field around the
wire. For example, if the current was going straight into this page your thumb would be pointing away
from you and your fingers would indicate a clockwise direction to the magnetic field around the wire.
The field strength decreases inversely with distance from the current carrying wire.
When two wires are placed close to each other with the current flowing in opposite directions, a more
complex magnetic field pattern is created.
A solenoid (a current carrying coil of wire) creates a magnetic field that looks much like a bar magnet:
For a wire wound into a flat coil or over the surface of a cylinder, the magnetic field direction can be
found by applying the RH grip rule or using the S N rule. Wire wound over the surface of a cylinder
forms a solenoid. Its magnetic field is of importance. Inside the solenoid the field is uniform (the same
everywhere), parallel to the solenoid axis. The right hand grip rule for solenoids gives the direction of
the magnetic field inside the solenoid: