ALTERNATING-CURRENT TYPES
المؤلف:
S. Gibilisco
المصدر:
Physics Demystified
الجزء والصفحة:
356
10-10-2020
1358
ALTERNATING-CURRENT TYPES
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. 1). 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. 1. Polarity change in an ac electromagnet.
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