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Translations of Encyclopedia about Physics

 

Magnetism

Since our beginnings, mankind has always known about magnets and knew that, based on their position, magnets can either attract or repel one another and that they attract objects made out of iron. However, beforehand, no one could explain why it was like this or could not think how to make practical use out of them.

Today however, we know that there are magnetic areas of microscopic size in many substances. If they are oriented identically, they become magnetic, creating around themselves a magnetic field. If we place such an object in close proximity to another magnetic object, they either attract or repel one another, depending on whether the respective magnetic fields are identically or oppositely oriented.

Magnetic fields are not in any major way influenced by their surroundings and, contrary to sound, work in a vacuum. Furthermore, the magnetic material within them can neither be removed nor changed.

A compass using a magnetic needle was the first and, for a long time, the only practical use of magnets. The Chinese apparently discovered the magnet several thousand years ago, the Europeans only until a few hundred years ago. A compass needle is a small magnet which easily rotates and which orients itself according to the magnetic field of the earth, in a north/south direction. Besides the position of the stars, compasses were for a long time the only navigational aid used by sailors.

The magnetic field of the earth is created by the convection (flow) of molten iron in the earth’s core. The geographical and magnetic poles are not exactly the same and the magnetic north pole is close to the earth’s geographic south pole.

At present, mostly only electromagnets are used in technical practice because they are more effective and their strength and orientation can be easily regulated.

An electromagnet is composed of an electrical conductor wound around a core, usually made of iron. If a current passes through this winding (electrical coil), a magnetic field is created relating to the iron core and electromagnetic fields are located on its ends. If the size of the current or its direction changes, the strength of the magnetic field changes as well, or its polarity is reversed. As soon as a current stops passing through the coil, the magnetic effect ceases.

Electric motors, electric relays and loud speakers all operate on this principle.

Artificially created magnetic fields are also used for storing data. Cassettes, diskettes and hard drives all store information magnetically. In these case, however, it is important that the information stored on them remain protected even once the electrical current is disconnected, in order that its data do not get erased. This is accomplished by using several protective layers made from special materials.

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