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.
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
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