Magnetism

Electric currents and magnetic fields are closely related. Whenever an electric current flows—that is, when charge carriers move—a magnetic field accompanies the current. In a straight wire that carries electrical current, magnetic lines of flux surround the wire in circles, with the wire at the center, as shown in below Fig. (The lines of flux aren’t physical objects; this is just a convenient way to rep- resent the magnetic field.) You’ll sometimes hear or read about a certain number of flux lines per unit cross-sectional area, such as 100 lines per square centimeter. This is a relative way of talking about the intensity of the magnetic field.

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Magnetic flux lines around a straight, current-carrying wire. The arrows indicate current flow.

Magnetic fields are produced when the atoms of certain materials align themselves. Iron is the most common metal that has this property. The atoms of iron in the core of the earth have become aligned to some extent; this is a complex interaction caused by the rotation of our planet and its motion with respect to the magnetic field of the sun. The magnetic field surrounding the earth is responsible for various effects, such as the concentration of charged particles that you see as the aurora borealis just after a solar eruption.

When a wire is coiled up, the resulting magnetic flux takes a shape similar to the flux field surrounding the earth, or the flux field around a bar magnet. Two well-defined magnetic poles develop, as shown in following Figure.

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Magnetic flux lines around a current-carrying coil of wire. The flux lines converge at the magnetic poles.

The intensity of a magnetic field can be greatly increased by placing a special core inside of a coil. The core should be of iron or some other material that can be readily magnetized. Such substances are called ferromagnetic. A core of this kind cannot actually increase the total quantity of magnetism in and around a coil, but it will cause the lines of flux to be much closer together inside the material. This is the principle by which an electromagnet works. It also makes possible the operation of electrical transformers for utility current.

Magnetic lines of flux are said to emerge from the magnetic north pole, and to run inward toward the magnetic south pole.