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, the magnetic lines of flux surround the wire in circles, with the wire at the center (Fig. 1). Actually, these aren’t really lines or circles; this is just a convenient way to represent the magnetic field.
You might sometimes hear of 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.

Fig. 1: Magnetic flux lines around a straight, current-carrying wire. The arrows indicate current flow.

Magnetic fields can be produced when the atoms of certain materials align themselves. Iron is the most common metal that has this property. The iron in the core of the earth has 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 Fig. 2.
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

Fig. 2: Magnetic flux lines around a coil of wire. The fines converge at the magnetic poles.

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. But this is just a semantical thing, about which theoretical physicists might speak. It doesn’t need to concern you for ordinary electrical and electronics applications.