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Magnetism
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.
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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.
