Basic Magnetic Theory
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Magnetic Fields
Permanent magnets like the one illustrated in the figure below have a
magnetic field surrounding them. The field is composed of lines of force
that radiate from the North pole to the South pole and back through the
magnet to the North pole. The "Lines of Force" are imaginary,
lines which are used to describe and calculate magnetic fields. The shape
of the field can be made visible if a piece of paper is placed over the
magnet and iron filings are sprinkled on the paper. The iron particles
align with the magnetic field, showing its shape.
Iron filings tracing out the magnetic field of a horseshoe magnet.
Attraction and Repulsion of Magnetic Poles
If two magnets are brought together with unlike pole adjacent to each
other the magnets will attract each other. If one magnet is rotated so
that two like faces are near each other the magnets will repel each other.
Magnetic Flux
The group of force lines that surround the magnet are called the magnetic
flux. This is represented by the lower case letter phi from the Greek
alphabet. The Unit of flux is called the Weber. Since one Weber is a fairly
large value of flux, most of the time values are in micro-Webers.
Magnetic Flux Density
The flux per unit area of a magnetic field is referred to as the Flux
Density. It's symbol is B. The unit of measure is the Tesla. This unit
used to be called a Gauss.
Electromagnetism
A magnetic field is created around a wire if a current is passed through
the wire. The magnetic field takes a circular form around the entire length
of the wire. Again if the wire is passed through a piece of paper iron
filings can trace out the circular lines of force.
Direction of the lines of force the left hand rule
If the wire passes through a piece of paper the lines of force will be
clockwise if the current is flowing towards you. If the current flows
in the opposite direction the field will be counter clockwise. Using your
left hand the thumb points in the direction of current flow and your fingers
indicate the lines of force
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