http://www.ck12.org Chapter 25. Electromagnetism
Solenoids
Asolenoidis a coil of wire with electric current flowing through it, giving it a magnetic field (seeFigure25.5).
Recall that current flowing through a straight wire produces a weak electromagnetic field that circles around the
wire. Current flowing through a coil of wire, in contrast, produces a magnetic field that has north and south poles
like a bar magnet. The magnetic field around a coiled wire is also stronger than the magnetic field around a straight
wire because each turn of the wire has its own magnetic field. Adding more turns increases the strength of the field,
as does increasing the amount of current flowing through the coil. You can see an actual solenoid with a compass
showing its magnetic north pole at this URL: http://www.youtube.com/watch?v=AgZHqfIBkUI.
FIGURE 25.5
How does a solenoid resemble a bar magnet?Electromagnets
Solenoids are the basis of electromagnets. Anelectromagnetis a solenoid wrapped around a bar of iron or other
ferromagnetic material (seeFigure25.6). The electromagnetic field of the solenoid magnetizes the iron bar by
aligning its magnetic domains. The combined magnetic force of the magnetized iron bar and the wire coil makes an
electromagnet very strong. In fact, electromagnets are the strongest magnets made. Some of them are strong enough
to lift a train. The maglev train described earlier, in the lesson "Electricity and Magnetism," contains permanent
magnets. Strong electromagnets in the track repel the train magnets, causing the train to levitate above the track.
Like a solenoid, an electromagnet is stronger if there are more turns in the coil or more current is flowing through it.
A bigger bar or one made of material that is easier to magnetize also increases an electromagnet’s strength. You can
see how to make a simple electromagnet at this URL: http://www.youtube.com/watch?v=emlzh9XXWgQ (4:57).
MEDIA
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