6.4 Magnetism


Figure 6-16 Magnetic lines of force.

A magnetic field is described as invisible lines of force which come out of the North Pole and enter the South Pole of a magnet. For example, if iron filings were sprinkled on a piece of glass on top of a bar magnet, the filings would form themselves in curved lines (Figure 6-16). These curved lines, extending from the two poles of the magnet, follow the magnetic lines of force surrounding the magnet. Lines of force rules are as follows:

The lines of force (outside the magnet) pass from the North Pole to the South Pole of the magnet.

The lines of force act somewhat as rubber bands and try to shorten to minimum length.

The lines of force repel each other along their entire length and try to push each other apart.

The rubber band characteristic opposes the push-apart characteristic.

The lines of force never cross each other.

The magnetic lines of force, taken together, are referred to as the magnetic field of the magnet.

The magnetic fields of a bar and of a horseshoe magnet are shown in Figure 6-17. In each, note how the lines of force curve and pass from the North Pole to the South Pole.


Figure 6-17 Magnetic fields.

 


Figure 6-18 Effects between magnetic poles.

Effects between magnetic poles are depicted in Figure 6-18. When two unlike magnetic poles are brought together, they attract. When like magnetic poles are brought together, they repel. These actions can be explained in terms of the rubber band and the push apart characteristics. When unlike poles are brought close to each other, the magnetic lines of force pass from the North Pole to the South Pole. They try to shorten (like rubber bands) and, therefore, try to pull the two poles together. On the other hand, if like poles are brought close to each other, lines of force going in the same direction are brought near each other. Because these lines of force attempt to push apart, a repelling effect results between the like poles.

Electromagnetism

An electric current (flow of electrons) always creates a magnetic field. In the wire shown in Figure 6-19, current flow causes lines of force to circle the wire. It is thought that these lines of force result from the movement of the electrons along the wire. As they move, the electrons send out the lines of force. When many electrons move, there are many lines of force (the magnetic field is strong). Few electrons in motion means a weak magnetic field or few lines of force.


Figure 6-19 Electromagnetism.

Electron movement as the basis of magnetism in bar and horseshoe magnets can be explained by assuming that the atoms of iron are so lined up in the magnets that the electrons are circling in the same direction and their individual magnetic lines of force add to produce the magnetic field.

The magnetic field is produced by current flowing in a single loop of wire (Figure 6-20). The magnetic lines of force circle the wire, but here they must follow the curve of the wire. If two loops are made in the conductor, the lines of force will circle the two loops.


Figure 6-20 Electromagnetism in a wire loop.

In the area between the adjacent loops, the magnetic lines are going in opposite directions. In such a case, because they are of the same strength (from same amount of current traveling in both loops), they cancel each other out. The lines of force, therefore, circle the two loops almost as though they were a single loop. However, the magnetic field will be twice as strong because the lines of force of the two loops combine.

 When many loops of wire are formed into a coil, the lines of force of all loops combine into a pattern that greatly resembles the magnetic field surrounding a bar magnet (Figure 6-21). A coil of this type is known as an electromagnet or a solenoid. Electromagnets can be in many shapes. The field coils of generators and starters, the primary winding in an ignition coil, the coils in electric gauges, and even the windings in a starter armature can be considered to be electromagnets. All of these components produce magnetism by electrical means.


Figure 6-21 Electromagnetism in a coil of wire.

The North Pole of an electromagnet can be determined, if the direction of current flow (from negative to positive) is known, by use of the left-hand rule (Figure 6-22). The left hand is wrapped around the coil with the fingers pointing in the direction of current flow. The thumb will point to the North Pole of the electromagnet. This rule is based on current, or electron, flow from negative to positive. If the direction of current is known, the left-hand rule also can be used to determine the direction that the lines of force circle a wire carrying current. This is done by circling the wire with the left hand with the thumb pointing in the direction of current flow (negative to positive). The fingers will then point in the direction that the magnetic field circles the wire.


Figure 6-22 Left-hand rule.

The strength of an electromagnet can be increased greatly by wrapping the loops of wire around an iron core. The iron core passes the lines of force with much greater ease than air. This effect of permitting lines of force to pass through easily is called permeability. Wrought iron is 3,000 times more permeable than air. In other words, it allows 3,000 times as many lines of force to get through. With this great increase in the number of lines of force, the magnetic strength of the electromagnet is greatly increased, even though no more current flows through it. Practically all electromagnets use an iron core of some type.

Electromagnetic Induction

Current can be induced to flow in a conductor if it is moved through a magnetic field at 900 or perpendicular to the lines of force. In Figure 6-23, the wire is moved downward through the magnetic field between the two magnetic poles. As it moves downward cutting into the lines of force, current is induced in it. The reason for this is that the line of force resists cutting and tends to wrap around the wire as shown. With lines of force wrapping around the wire, current is induced. The wire movement through the magnetic field produces a magnetic whirl around the wire, which pushes the electrons along the wire.


Figure 6-23 Electromagnetism.

If the wire is held stationary and the magnetic field is moved, the effect is the same. All that is required is that there be relative movement between the conductor and the magnetic lines of force to produce enough voltage to move the electrons along the conductor.

Moving the magnet can move the magnetic field or, if it is a magnetic field from an electromagnet, starting and stopping the current flow in the electromagnet can move it. Suppose an electromagnet, such as the one shown in Figure 7-21, has a wire held close to it. When the electromagnet is connected to a battery, current will start to flow through it. This current, as it starts to flow, builds up a magnetic field.

A magnetic field forms because of the current flow. This magnetic field might be considered to expand and move out from the electromagnet. As the lines of force move out, the wire will cut them. This wire will therefore have current induced in it. If the electromagnet is disconnected from the battery, these lines of force will disappear and current will stop flowing in the wire.

It can be seen now that current can be induced in a wire by three methods:

The wire can be moved through the stationary magnetic field (this principle applied in a DC [Direct Current] generator).

The wire can be held stationary and the magnet can be moved so the field is carried past the wire (this principle applied in an AC [Alternating Current] generator).

The wire and electromagnet can both be held stationary and the current turned on and off to cause the magnetic field to build up and collapse so the magnetic field moves one way or the other across the wire (the principle applied in an ignition coil).

Summary

Knowledge of the electrical theories presented in this chapter is essential for the safe conduct and completion of your job as a construction mechanic. Your ability to apply this knowledge will help you when you deal with direct current and electrical circuits and when you are called upon to troubleshoot electrical circuits. During your career as a construction mechanic, you will apply these and other theories in your everyday job.