An electromagnet is a device that uses electric current for the purpose of generating a
magnetic field. Such devices are based upon Ampere's Law
(or equivalently the Biot-Savart Law), which says basically that a flowing current will
generate a magnetic field around it. For a straight wire, this field goes in concentric
circles around the wire. Usually, if you make an electromagnet, you want to get a strong
field aligned in one direction, so this is normally done by coiling the wire up in such a
way that these circular fields around each small length of wire
reinforce each other.
If you make the wire into a loop, then at the center the field due to each piece points
in the same direction and they all add up to make a stronger field. Unfortunately, this
gets weak very quickly as you move away in the direction perpendicular to the plane of the
loop. You can coil the wire up in a long coil like a spring (forming a helix), so that
it's like a series of loops, one on top of another. This means the field is even stronger
and more uniform over the inside of the coil. This configuration is called
a solenoid. It is usually made by wrapping wire around a cylinder. Still, even in this
configuration, the field spreads out and gets weak quickly outside the ends of the coil, so
you can improve it by taking the two ends of the coil and bending the coil until the two
ends meet (so that it is now like the wire is wrapped around a torus). This is called a
toroidal solenoid. In this configuration the fields from what used to be the ends
reinforce one another. This is now just a closed doughnut shaped coil of wire that
has a very strong field, which is completely uniform in the azimuthal direction. Also,
if you left a little space between the ends when you bent the solenoid (so that it
formed a doughnut with a bite out of it), in that region the field would be much stronger
and more uniform than at the end of a cylindrical solenoid of equal number of turns per unit length. There
are of course, even more complicated schemes, but these two form the basis of the vast majority
of all electromagnets. Which one you use depends on what your materials are, where it
needs to fit, and what properties are most important for the magnetic field to have.
One more way to increase the strength of electromagnets that is usually used is to put in a
core, made out of some material that will amplify the magnetic field, inside the coils of the
electromagnet. This core is made out of a material that has the property that its
molecules have magnetic moments (are like little bar magnets) that tend to
line up along the direction of an external magnetic field. These materials are either
paramagnetic or ferromagnetic, though ferromagnets are
in general much stronger and, therefore, a better choice. With a core, when current flows
through the wires of the electromagnet it creates a magnetic field that
magnetizes the core (temporarily or permanently), thus increasing the magnetic
field. With a ferromagnetic core this can be a huge increase in strength.
The main limitations on an electromagnet are set by the properties of your core, your
wire, and your power source. We already discussed the core, and I don't want to get into
power sources (mainly because I know nothing about them, really), so let's talk briefly
about wiring. You want to be able to wrap your wire tightly enough and in a well
enough organized way to get the field you want, and you want to be able to pass enough
current through it to get the strength you need without burning or melting the wire. This
often means that you need to cool the wire, or get rid of the resistance. The former can
be done with air in some cases but may require more elaborate cooling systems in others.
The latter can be done using superconducting wire (which requires
cooling for a different reason).
Finally, I just wanted to say that electromagnets are really, really
important. They are integral parts of a ton of electrical devices. Some of those
include:
More examples, thanks mricicle:
...and, well, almost any time you want to use electricity to move something (unless it's
very highly charged, in which case electric fields can be used
successfully). Well, there are a lot more examples, because they're in practically everything electronic.
Please /msg me or write-up more examples if you think of them.
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