Electromagnets

There are actually 2 types of magnetism:

1.) Temporary
2.) Permanent

Soft iron can be easily magnetized by placing it inside a magnetic field. However, as soon as the iron is removed from the field, most of its magnetism fades away. A negligible amount of magnetism is, however, retained. This type of magnet is called a TEMPORARY MAGNET . The small amount of magnetism that does remain is called RESIDUAL MAGNETISM .

Steel or hard iron, which is difficult to magnetize, retains the majority of its magnetism long after it has been removed from the magnetic field. This type of magnet is called a PERMANENT MAGNET . Permanent magnets are generally made in the shape of a bar or a horseshoe. Of the two shapes, the horseshoe type has the stronger magnetic field because the magnetic poles are closer to each other. Horseshoe magnets are used in the construction of headphones. Loudspeakers, on the other hand, generally use a type of Bar magnet.

It has been found that when a compass is placed in close proximity to a wire, and an electrical current flows through the wire, the compass needle will turn until it is at a right angle to the conductor. Since a compass needle lines up in the direction of a magnetic field, there must be a magnetic field around the wire, which is at right angles with the conductor! Science has discovered then, that wires which carry current have the same type of magnetic field that exists around a magnet! We say that an electric current INDUCES a magnetic field.

If you closely examine the picture on the right, you will find that there are "rings" circling about the wire. These rings represent the magnetic lines of force which exist around a wire which carries an electric current. They are strongest directly around the wire, and extend outward from the wire, gradually decreasing in intensity. You will also note that the compass needle is steady, and not spinning. This indicates that the magnetic field goes in a ring around the wire. It also travels in a specific direction.

The direction of the magnetic field can be predicted by use of what we call the LEFT HAND RULE . According to the left hand rule, if you wrap your left hand around the wire that is carrying the current, with your thumb following the direction of current flow (thumb points positive), your fingers will show you what direction the magnetic field will turn. Note that when the current flows from negative to positive, it induces a magnetic field in a specific direction, such that the north pole is ALWAYS at right angles with the electrical current flow.

No matter which way we turn or twist the wire, the left hand rule applies. But what happens if we put a loop in the wire? When the wire is looped, as you will see from the picture on the right, the little magnetic fields that wrap around the wire cross through each other's path. If you use the left hand rule, and follow around the coils of the wire, you will find that the magnetic field acts as if it is running through the hole inside of the loop. (If the loop were a donut, the magnetic field would go through the hole in the donut). Thinking along these lines... if we put a dozen donuts side to side, with a stick going through the holes, the magnetic field would follow the stick.

Through experimentation, it was found that if a wire is wound in the form of a coil (coiled up), the total strength of the magnetic field around the coil will be magnified. This is because the magnetic fields of each turn add up to make one large resulting magnetic field. Furthermore, it was found that the direction of the magnetic field could be predicted. The POSITIVE end of the battery is ALWAYS connected to the NORTH POLE of the coil, regardless of whether the coil is wound clockwise or counterclockwise. The coil of wire, because of their properties and capabilities, makes up one of the main components in electronics. For this reason, it has taken on many names, to include:
ELECTROMAGNET
INDUCTOR
SOLENOID
COIL

Coils have been given their own schematic symbol. So far we have discussed the schematic symbol for the resistor, lamp and battery. The schematic symbol for the coil is on the left. Note that there can be many variations of this, which will be discussed in more detail later.

There are several factors which determine the strength of a given electromagnet. They are:

1). The amount of current - the greater the current, the greater the field.
2). The number of turns - the greater the number of turns in a coil, the greater the field.
3). The PERMEABILITY of the core.
The core of a coil is the material that the coil is wrapped around. It can be glass, wood, metal, air, or even a vacuum. If the coil is wound upon an iron core, the strength of the electromagnet is increased several hundred times over what it would be with an air core. We say that iron is more permeable than air. Permeability is the ability of a given substance to conduct magnetic lines of force. It is similar to the effect of conductance with respect to electrical current flow. The standard for permeability is air, which is given a permeability of one. All other substances are compared to air. Some examples of substances with high permeability are permalloy and iron.

To the right is a picture of a " variable " air core coil. This particular coil is adjustable in value, based on a moving " tap " in the coil, which rolls along the outside of the coil as the spindle is turned. Sometimes this is called a " roller inductor ". As the spindle is turned, the coil itself rotates, and the tap moves along the length of the coil, changing its " electrical length ". Of course this is just one example of the many types and shapes of coils that exist. The key thing to remember is that any length of wire that is wrapped up into a coil, has the same electrical properties as a coil.

Just as conductance has an opposite - resistance; permeability also has an opposite - reluctance. RELUCTANCE is mathematically the reciprocal of PERMEABILITY. The unit of measurement for reluctance is the REL or OERSTED , and its symbol is Ö .

Voltage is the measurement for Amplitude of an electrical circuit. Magnetism also has a counterpart for this, which is called MAGNETOMOTIVE FORCE . Magnetomotive force is the force which produces the magnetic lines of force or FLUX . The unit of magnetomotive force is the GILBERT , and its symbol is G . The formula for finding the value of G is as follows:         

G = N x I x 1.26
Where:
N = the number of turns in the coil
I = the current flowing through the coil in Amperes
There is a catch phrase for N x I which is AMPERE-TURNS