Electricity

C

Field Strength

The strength, or intensity, of a field at any point is defined as the force exerted on a charge of 1 coulomb placed at that point. For example, if a point charge of 1 coulomb is subjected to a force of 10 newtons, the electric field is 10 newtons per coulomb at that point. An object with a charge of 5 coulombs would be subjected to a force of 50 newtons at the same point.

Field strength is represented graphically by the closeness (density) of the lines of force. Where the lines are close together, the field is strong. Where they are far apart, the field is weak. Near a charge, the field is strong and the lines are close together. At greater distances from the charge, the field weakens and the lines are not as close together. The field strength values that the lines represent are relative, since a field can be drawn with as many lines as desired.

IX

Electricity and Magnetism

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 Coulomb’s inverse square law for electric charges. See also Magnetism.

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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. See also Quantum Electrodynamics.

A

Magnetic 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.)

B

Motor Effect

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. See also Electric Motors and Generators.

C

Solenoids

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.

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