ELECTROMAGNETISM DEFINITION AND BASIC INFORMATION

Electromagnetism is the production of magnetism by an electric current. An electric current is a flow of electrons; we can compare the flow of electrons in a wire to the flow of water in a pipe. Today we can read about electronics and electrons in newspapers, magazines, and many schoolbooks.

However, the practical electrician needs to know more about electrons than a mechanic or machinist does. Therefore, let us see how electric current flows in a wire. An atom is the smallest particle of any substance; thus, the smallest particle of copper is a copper atom.We often hear about splitting the atom.

If a copper atom is split or broken down into smaller particles, we can almost say that it is built from extremely small particles of electricity. In other words, all substances, such as copper, iron, and wood, have the same building blocks, and these building blocks are particles of electricity. (This is technically not an entirely true statement, but it is close enough for our use here.)

Copper and wood are different substances simply because these particles of electricity are arranged differently in their atoms. An atom can be compared to our solar system in which the planets revolve in orbits around the sun. For example, a copper atom has a nucleus, which consists of positive particles of electricity; electrons (negative particles of electricity revolve in orbits around the nucleus.

Figure 1-13 shows three atoms in a metal wire. An electron in one atom can be transferred to the next atom under suitable conditions, and this movement of electrons from one end of the wire to the other
end is called an electric current.


Electric current is electron flow. To make electrons flow in a wire, an electrical pressure must be applied to the ends of the wire. This electrical pressure is a force called electromotive force, or voltage. For example, an ordinary dry cell is a source of electromotive force.

A dry cell produces electromotive force by chemical action. If we connect a voltmeter across a dry cell, as shown in Figure 1-14, electrons flow through the voltmeter, which indicates the voltage of the cell.



We measure electromotive force (emf) in volts (V). If a dry cell is in good condition, it will have an emf of about 1.5 V. Note that a dry cell acts as a charge separator. In other words, the chemical action in the cell takes electrons away from the carbon rod and adds electrons to the zinc cylinder.

Therefore, there is an electron pressure or emf at the zinc cylinder. When a voltmeter is connected across a dry cell (see Figure 1-14), this electron pressure forces electrons to flow in the connecting wire, as shown in Figure 1-13. We observe that electrons flow from the negative terminal of the dry cell, around the wire circuit, and back to the positive terminal of the dry cell.


Next, we will find that an electric current produces a magnetic field. For example, if a compass needle is brought near a current carrying wire, the compass needle turns, as shown in Figure 1-15.


Since a compass needle is acted upon by a magnetic field, this experiment shows that the electric current is producing a magnetic field. This is the principle of electromagnetism. The magnetic lines of force surrounding a current-carrying wire can be demonstrated as shown in Figure 1-16.


When iron filings are sprinkled over the cardboard, the filings arrange themselves in circles around the wire. Although a current-carrying wire acts as a magnet, it is a form of temporary magnet. The magnetic field is not present until the wire is connected to the dry cell.

Magnetic force lines are produced only while there is current in the wire. As soon as the circuit is opened (disconnected from the dry cell), the magnetic force lines disappear. Let us observe the polarities of the compass needles in Figure 1-16.

The magnetic lines of force are directed clockwise, looking down upon the cardboard. This experiment leads us to a basic rule of electricity called the left-hand rule. Figure 1-17 illustrates the lefthand rule; if a conductor is grasped with the left hand, with your thumb pointing in the direction of electron flow, then your fingers will point in the direction of the magnetic lines of force.



Experiments show that the magnetic field around the wire in Figure 1-16 is weak, and we will now ask how the strength of an electromagnetic field can be increased. The magnetic field around a straight wire is comparatively weak because it is produced over a large volume of space.

To reduce the space occupied by the magnetic field, a straight wire can be bent in the form of a loop, as shown in Figure 1-18. Now the magnetic flux lines are concentrated in the area enclosed by the loop.

Therefore, the magnetic field strength is comparatively great inside the loop. This is an elementary form of electromagnet.


Next, to make an electromagnet with a much stronger magnetic field, we can wind a straight wire in the form of a helix with a number of turns, as shown in Figure 1-19. Since the field of one loop adds to the field of the next loop, the total field strength of the electromagnet is much greater than if a single turn were used.

Note that if we use the same wire shown in Figure 1-16 to form the electromagnet in Figure 1-19, the current is the same in both circuits. That is, we have not changed the amount of current; we have merely concentrated the magnetic flux by winding the wire into a spiral. Electricians often call an electromagnet of this type a solenoid.

The name solenoid is applied to electromagnets that have an air core. For example, we might wind the coil in Figure 1-19 on a wooden spool. Since wood is not a magnetic substance, the electromagnet is essentially an air-core magnet. Note the polarity of the magnetic field in Figure 1-19 with respect to the direction of current flow.

The left-hand rule applies to electromagnets, just as to straight wire. Thus, if we grasp an electromagnet as shown in Figure 1-20, with the fingers of the left hand in the direction of electron flow, then the thumb will point to the north pole of the electromagnet.

Since iron is a magnetic substance, the strength of an electromagnet can be greatly increased by placing an iron core inside a solenoid. For example, if we place a soft-iron bar inside the wooden spool in Figure 1-19, we will find that the magnetic field strength becomes much greater. 

Let us see why this is so (with reference to Figure 1-10). The molecular magnets in the soft-iron bar, or core, are originally oriented in random directions. However, under the influence of the flux lines inside the electromagnet, these molecular magnets line up in the same direction. 

Therefore, the magnetic field of the molecular magnets is added to the magnetic field produced by the electric current, and the strength of the electromagnet is greatly increased.

Note that we have not changed the amount of current in the wire by placing an iron core in the solenoid. The magnetic field produced by the electric current remains unchanged. 

However, the electromagnet has a much greater field strength when an iron core is used because we then have two sources of magnetic field, which add up to produce the total field strength. 

If we open the circuit (shut off the current) of an iron-core electromagnet, both sources of magnetic field disappear. In other words, both solenoids and iron core electromagnets are temporary magnets.


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