Magnetism
Electric currents and magnetic fields are closely related to each other. Whenever an electric current flows, when charge carriers move-a magnetic field accompanies the current. In a straight wire, the magnetic lines of flux surround wire in circles, with the wire at the center. Actually, these are not really lines or circles; this is a convenient way to represent magnetic field. You might at times hear of a certain number of flux lines per unit cross-sectional area, like 100 lines per square centimeter. This is a way of talking about the intensity of magnetic field.
Figure-- Magnetic flux lines around the straight, current-carrying wire. The arrows are indicating current flow.
Magnetic fields can be produced when atoms of certain materials align themselves. Iron is the most common metal which posses this property. The iron in the core of the earth has become aligned to some extent; this is a complex interaction because of rotation of our planet and its motion with respect to the magnetic field of the sun. The magnetic field surrounding earth is responsible for various effects, like the concentration of charged particles which you see as the aurora borealis after a solar eruption.
When the wire is coiled up, the resulting magnetic flux takes a shape similar to flux field surrounding earth, or flux field around a bar magnet. The two well-defined magnetic poles develop, as shown in the figure drawn below.
The intensity of a magnetic field can be increased by placing the special core inside of a coil. The core should be of iron or other material which can be readily
Figure--Magnetic flux lines around a coil of wire. The fines converge at the magnetic poles.
magnetized. This type of substances is called as ferromagnetic. A core of this kind cannot increase the total quantity of magnetism in and around the coil, but it will cause lines of flux to be closer inside the material. This is the principle by which the electromagnet works. It also makes possible the operation of electrical transformers for the utility of current.
Magnetic lines of flux are said to emerge from magnetic north pole, and to run inward toward magnetic South Pole. It does not need to concern you for normal electrical and electronics applications.
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