Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Magnetism 55
Magnetic Fields and Lines
of Force
A magnetic field (Figure 4-3) is made up of many
invisible lines of force, which are also called lines of
flux. Magnetic flux is another term applied to lines of
force, which can be compared to current in electricity:
They come out of one pole and enter the other
pole. The flux lines are concentrated at the poles and
spread out into the areas between the poles.
Magnetic Field Intensity
The magnetic field intensity refers to the magnetic
field strength (force) exerted by the magnet
and can be compared to voltage in electricity. The
magnetic field existing in the space around a magnet
can be demonstrated if a piece of cardboard is
placed over a magnet and iron filings are sprinkled
on top of the cardboard. The iron filings will be
arranged in a pattern showing the flux lines. A
weak magnet has relatively few flux lines; a strong
magnet has many. The number of flux lines is
sometimes described as flux density, as shown in
Figure 4-4.
Magnetism Summary
• Magnetic flux lines leave the north pole and
enter the south pole of the magnet.
• The more powerful the magnet, the higher
the flux density or concentration of the lines
of force.
• The greatest flux density occurs at the poles.
• Magnetic lines of force are always parallel
and never cross.
• Like poles repel and unlike poles attract.
Atomic Theory and Magnetism
Magnetism starts with the atom. Each atom has
electrons spinning around the nucleus in orbits, as
well as spinning on their own axis. It is this spinning
of the electrons that creates small permanent
magnets. In most elements, the electrons spin in
opposite directions and as a result do not form a
magnetic field. The iron atom has 26 electrons and
22 of these cancel themselves out because they
have an opposite spin direction. However, the four
in the next to last outer shell all spin in the same
direction, giving iron a magnetic characteristic.
ELECTROMAGNETISM
In 1820, scientists discovered that current-carrying
conductors are surrounded by a magnetic field. A
conductor, such as a copper wire, that is carrying an
electrical current creates a magnetic field around
the conductor and is called electromagnetism.
Figure 4-3.
Magnetic field/lines of force.
Figure 4-4. Flux density equals the number of lines
of force per unit area.