Electromagnetism
Many historians of science believe that the compass, which uses a magnetic needle, was used in China as early as the 13th century B.C., its invention being of Arabic or Indian origin. The early Greeks knew about magnetism as early as 800 B.C. They discovered that the stone magnetite attracts pieces of iron.
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Hans
Christian Oersted
Danish physicist and chemist (1777–1851)
Oersted is best known for observing that a compass needle deflects when placed near a wire carrying a current. This important discovery was the first evidence of the connection between electric and magnetic phenomena. Oersted was also the first to prepare pure aluminum.
n 1269 a Frenchman named Pierre de Maricourt found that the directions of a needle near a spherical natural magnet formed lines that encircled the sphere and passed through two points diametrically opposite each other, which he called the poles of the magnet. Experiments showed that every magnet, regardless of its shape, has two poles, called north (N) and south (S) poles, that exert forces on other magnetic poles similar to the way that electric charges exert forces on one another. That is, like poles (N–N or S–S) repel each other, and opposite poles (N–S) attract each other.
The poles received their names because of the way a magnet, such as that in a compass, behaves in the presence of the Earth’s magnetic field. If a bar magnet is suspended from its midpoint and can swing freely in a horizontal plane, it will rotate until its north pole points to the Earth’s geographic North Pole and its south pole points to the Earth’s geographic South Pole.
In 1600 William Gilbert extended de Maricourt’s experiments to a variety of materials. Using the fact that a compass needle orients in preferred directions, he suggested that the Earth itself is a large permanent magnet. In 1750 experimenters used a torsion balance to show that magnetic poles exert attractive or repulsive forces on each other and that these forces vary as the inverse square of the distance between interacting poles. Although the force between two magnetic poles is otherwise similar to the force between two electric charges, electric charges can be isolated (the electron and proton) whereas a single magnetic pole has never been isolated. That is, magnetic poles are always found in pairs. All attempts thus far to detect an isolated magnetic pole have been unsuccessful. No matter how many times a permanent magnet is cut in two, each piece always has a north and a south pole.
The relationship between magnetism and electricity was discovered in 1819 when, during a lecture demonstration, the Danish scientist Hans Christian Oersted found that an electric current in a wire deflected a nearby compass needle.
In the 1820s, further connections between electricity and magnetism were demonstrated independently by Michael Faraday and Joseph Henry. They showed that an electric current can be produced in a circuit either by moving a magnet near the circuit or by changing the current in a nearby circuit. These observations demonstrate that a changing magnetic field creates an electric field. Later theoretical work by Maxwell showed that the reverse is also true: a changing electric field creates a magnetic field.
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Fig.5. The left-hand rule.
etailed experiments showed that the magnetic field produced by an electric current is always oriented perpendicular to the direction of flow. A simple method of showing this relationship is called the left-hand rule. The left-hand rule states that the magnetic flux lines produced by a current-carrying wire will be oriented the same direction as the curled fingers of a person's left hand, with the thumb pointing in the direction of electron flow (Fig. 5).The magnetic field surrounding a current-carrying wire is quite weak for common amounts of current, it is able to deflect a compass needle and not much more. To create a stronger magnetic field force (and more field flux) with the same amount of electric current, we can wrap the wire i
Fig.
6.
An electromagnet is a piece of wire which generates a magnetic field with the passage of electric current through it. Though all current-carrying conductors produce magnetic fields, an electromagnet is usually constructed in such a way as to maximize the strength of the magnetic field it produces for a special purpose. Perhaps the most obvious example of such a machine is the electric motor. Another example is the relay, an electrically-controlled switch. The magnetic field force produced by an electromagnet (called magnetomotive force) is proportional to the product of the current through the electromagnet and the number of complete coil "turns" formed by the wire.