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Electricity and Magnetism
motor effect, magnetic lines of force, right-hand rule, inverse square law, induced current
Many similarities exist between electric and magnetic phenomena. A magnet has two opposite poles, referred to as north and south. Opposite magnetic poles attract each other, and similar magnetic poles repel each other, exactly as happens with electric charges.
The force with which magnetic poles attract or repel each other depends on the strength of the poles and the distance between them. This relationship is similar to the Coulombís inverse square law for electric charges. See also Magnetism.
The similarities between electric and magnetic phenomena indicate that electricity and magnetism are related. Electricity produces magnetic effects and magnetism produces electric effects. The relationship between electricity and magnetism is called electromagnetism.
Magnet Effects of Electricity
It has been noted that an electric field exists around any electric charge. If electric charges are moving, they constitute an electric current. The magnetic effect of electricity is demonstrated by the fact that a magnetic field exists around any electric current. The field can be detected when a magnet is brought close to the current-carrying conductor.
The magnetic field around an electric current can be thought of as lines of magnetic force that form closed circular loops around the wire that carries the current. The direction of the magnetic field can be determined by a convenient rule called the right-hand rule. To apply this rule, the thumb of the right hand is pointed in the direction in which the current is flowing and the fingers are curled around the wire. The direction of the fingers then indicates the direction of the lines of magnetic force. (The right-hand rule assumes that current flows from positive to negative.)
As already stated, a magnetic field exists around a wire carrying an electric current, and a magnetic field exists between the two poles of a magnet. If the wire is placed between the poles, the magnetic fields interact to produce a force that tends to push the wire out of the field. This phenomenon, known as the motor effect, is used in electric motors.
If a wire is bent into many continuous loops to form a long spiral coil, then the magnetic lines of force tend to go through the center of the coil from one end to the other rather than around the individual loops of wire. Such a coil, called a solenoid, behaves in the same way as a magnet and is the basis for all electromagnets. The end from which the lines exit is the north pole and the end into which the lines reenter is the south pole. The polarity of the coil can be determined by applying the left-hand coil rule. If the left hand grasps the coil in such a way that the fingers curl around in the direction of the electron current, then the thumb points in the direction of the north pole.
Electric Effects of Magnetism
If a wire is moved through a magnetic field in such a way that it cuts the magnetic lines of force, a voltage is created across the wire. An electric current will flow through the wire if the two ends of the wire are connected by a conductor to form a circuit. This current is called an induced current, and the induction of a current in this manner is called electromagnetic induction.
It does not matter whether the wire moves or the magnetic field moves, provided that the wire cuts through lines of force. If a magnet is moved near a stationary wire, the lines of magnetic force are cut by the wire and an electric current is induced in the wire.
Like any electric current, an induced current generates a magnetic field around it. Lenzís law expresses an important fact concerning this magnetic field: The motion of an induced current is always in such a direction that its magnetic field opposes the magnetic field that is causing the current.
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