Any electric current, or movement of charge carriers, produces a magnetic field. This field can become intense in a tightly coiled wire having many turns and carrying a large electric current. When a ferromagnetic rod, called a core , is placed inside the coil, the magnetic lines of flux are concentrated in the core, and the field strength in and near the core becomes tremendous. This is the principle of an electromagnet (Fig. 14-4).
Fig. 14-4 . A simple electromagnet.
Electromagnets are almost always cylindrical in shape. Sometimes the cylinder is long and thin; in other cases it is short and fat. Whatever the ratio of diameter to length for the core, however, the principle is always the same: The flux produced by the current temporarily magnetizes the core.
You can build a dc electromagnet by taking a large iron or steel bolt (such as a stove bolt) and wrapping a couple of hundred turns of wire around it. These items are available in almost any hardware store. Be sure the bolt is made of ferromagnetic material. (If a permanent magnet “sticks” to the bolt, the bolt is ferromagnetic.) Ideally, the bolt should be at least inch in diameter and several inches long. You must use insulated or enameled wire, preferably made of solid, soft copper. “Bell wire” works well.
Be sure that all the wire turns go in the same direction. A large 6-V “lantern battery” can provide plenty of dc to operate the electromagnet. Never leave the coil connected to the battery for more than a few seconds at a time. And do not—repeat, do not —use an automotive battery for this experiment. The near-short-circuit produced by an electromagnet can cause the acid from such a battery to violently boil out, and this acid is dangerous stuff.
Direct-current electromagnets have defined north and south poles, just like permanent magnets. The main difference is that an electromagnet can get much stronger than any permanent magnet. You should see evidence of this if you do the preceding experiment with a large enough bolt and enough turns of wire. Another difference between an electromagnet and a permanent magnet is the fact that in an electromagnet, the magnetic field exists only as long as the coil carries current. When the power source is removed, the magnetic field collapses. In some cases, a small amount of residual magnetism remains in the core, but this is much weaker than the magnetism generated when current flows in the coil.
You might get the idea that the electromagnet can be made far stronger if, rather than using a lantern battery for the current source, you plug the wires into a wall outlet. In theory, this is true. In practice, you’ll blow the fuse or circuit breaker. Do not try this. The electrical circuits in some buildings are not adequately protected, and a short circuit can create a fire hazard. Also, you can get a lethal shock from the 117-V utility mains. (Do this experiment in your mind, and leave it at that.)
Some electromagnets use 60-Hz ac. These magnets “stick” to ferromagnetic objects. The polarity of the magnetic field reverses every time the direction of the current reverses; there are 120 fluctuations, or 60 complete north-to-south-to-north polarity changes, every second (Fig. 14-5). If a permanent magnet is brought near either “pole” of an ac electromagnet of the same strength, there is no net force resulting from the ac electromagnetism because there is an equal amount of attractive and repulsive force between the alternating magnetic field and the steady external field. However, there is an attractive force between the core material and the nearby magnet produced independently of the alternating magnetic field resulting from the ac in the coil.
Fig. 14-5 . Polarity change in an ac electromagnet.
Electromagnets Practice Problem
Suppose that the frequency of the ac applied to an electromagnet is 600 Hz instead of 60 Hz. What will happen to the interaction between the alternating magnetic field and a nearby permanent magnet of the same strength?
Assuming that no change occurs in the behavior of the core material, the situation will be the same as is the case at 60 Hz or at any other ac frequency.
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