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PH 122 (46)
Lecture

Faraday's Law and Transformers

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Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
Ferromagnetism The magnetic field produced by a coil of wire with a current through it is generally quite small. In order to increase the strength of the field, we can use a ferromagnetic material. Ferromagnetic materials have regions of strong local magnetic fields (domains) that are generally randomly oriented. Apply a small field to the material → lines up the domains Domains that are oriented in the same sense as the applied field will grow at the expense of domains that are not favorably oriented The net result is that the material will now be strongly magnetized; it will produce a field in the same sense as the applied field that may be up to 1000x as strong as the applied field. There are two basic kinds of ferromagnetic materials: “soft” and “hard.” The hard materials are used in permanent magnets; they are hard to magnetize, but once magnetized, they will maintain their magnetization. Soft materials are easy to magnetize, but when the external field is removed, they lose most of their magnetization. These materials are used in transformer cores; we want the materials to respond quickly to changes in the applied field. Solenoid A solenoid, or a coil of wire, has the strongest field in its center. If a piece of ferromagnetic material is placed near a solenoid and a current is passed through the solenoid, a field is produced. The ferromagnetic materials is magnetized in the same sense as the solenoid field. If you consider the fields of the solenoid and the material, you can see that the material will feel a force toward the region of greatest field strength: i.e., the center of the solenoid. This means that the material will be pulled into the solenoid. Solenoids with iron cores are used in this fashion on automatic door locks on cars, “buzzers” that people use to let others into an apartment buildi
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