Elementary theory of the Magnetism
Magnetism and electricity
Nowadays, the physicists believe that all of the magnetic phenomena result of forces among electric charges in movement, and in the days today great amounts of electric power are generated by the relative movement between electric drivers and magnetic fields.
On the other hand, the electric power is transformed in mechanical energy also for systems that use this relative movement between electric currents and magnetic fields. The function of many electric instruments of measurement depend on the relationship between the electricity and the magnetism.
Before we undertake the study of the magnetic effects of the electric currents, we will examine the magnetic properties of the substances and we will learn some thing on the nature of the magnetism and of the magnetic fields.
Magnetic substances
Deposits of ore magnetic iron were discovered for the Greek, in an area of Turkey, there are many centuries. The area was known then as Magnesia and, like this, the ore was called magnetite. Other magnetite deposits are found in other areas of the world, and the magnetite pieces are known as natural magnets.
One of those pieces, hung in a thread, he/she joins with the magnetic field of the Earth. About the century XII, the men began to use those natural magnets the one that gave the stone-magnet name, as the first magnetic compasses.
Some materials, especially the iron and the steel, they are strongly atraídos for the magnets; the cobalt and the nickel are atraídos in smaller degree. It is said that those substances have properties ferromagnéticas.
Special leagues, as the permalloy and the alnico, have extraordinary properties ferromagnéticas. The physicists have been demonstrating a lot of interest for the structure of the materials endowed with the property of the ferromagnetismo.
Nowadays, very strong and versatile artificial magnets are manufactured, with substances ferromagnéticas. The magnets of current alnico (Al, Ni and Co) support a weight of more than 1 000 times the one of the own magnets. The substances ferromagnéticas are called "magnetic substances" commonly.
No-magnetic substances
The materials are commonly classified as magnetic or no-magnetic. It is said that the ones that don't demonstrate the fort ferromagnetismo of the Family of the Iron of the metals are "no-magnetic." However, if those materials be put in the field of a very strong magnet, it is observed that some of them are repelled lightly by the magnet, while others are lightly atraídos.
The zinc, the bismuth, the chloride of sodium, the gold and the healthy mercury some of the substances lightly repelled and it is said that are diamagnéticas. The property of the diamagnetismo is an important concept in the modern theory of the magnetism, as we will further on see.
Wood, aluminum, platinum, oxygen and copper (II) sulfate are examples of substances lightly atraídas for a strong magnet. It is said that those materials are paramagnéticos and that type of magnetic behavior is called paramagnetismo.
The theory of the domains of the magnetism
That theme was already approached, with prominence, in this Room 13. For reading to click here: Magnetic domains.
The force among magnetic poles
The fact that the filings of iron if they arrest mainly in the extremities of a bar magnet indicates that the magnetic force acts on the filings basically in those areas or poles; that doesn't mean that the intermediate area of the magnet is desmagnetizada THE pole that appears for the North, when the magnet is free to rotate on a vertical axis, it is called north pole commonly or simply pole N. THE opposite pole, that it appears for the South, it is called south pole or pole S.
Questions and Answers
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