http://www.ck12.org Chapter 25. Electromagnetism
electromagnet again attracts the clapper, which hits the bell once more. This sequence of events keeps repeating as
long as the button by the front door is being pressed.
FIGURE 25.7
A doorbell uses an electromagnet to move
the clapper of a bell.Electric Motors
Anelectric motoris a device that uses an electromagnet to change electrical energy to kinetic energy.Figure25.8
shows a simple diagram of an electric motor. The motor contains an electromagnet that is connected to a shaft.
When current flows through the motor, the electromagnet turns, causing the shaft to turn as well. The rotating shaft
moves other parts of the device.
Why does the motor’s electromagnet turn? Notice that the electromagnet is located between the north and south
poles of two permanent magnets. When current flows through the electromagnet, it becomes magnetized, and its
poles are repelled by the like poles of the permanent magnets. This causes the electromagnet to turn toward the
unlike poles of the permanent magnets. A device called a commutator then changes the direction of the current
so the poles of the electromagnet are reversed. The reversed poles are once again repelled by the like poles of
the permanent magnets. This causes the electromagnet to continue to turn. These events keep repeating, so the
electromagnet rotates continuously. You can make a very simple electric motor with a battery, wire, and magnet
following instructions at this URL: http://www.youtube.com/watch?v=VhaYLnjkf1E.
Lesson Summary
- A solenoid is a coil of wire with electric current flowing through it. It produces a strong magnetic field with
north and south poles like a bar magnet. - An electromagnet is a solenoid wrapped around a bar of iron or other ferromagnetic material. The magnetic
field of the coil magnetizes the bar, which adds to the strength of the magnetic field. Electromagnets are the
strongest magnets made. - Many common electric devices, such as doorbells, contain electromagnets. If they have moving parts, they are
likely to have an electric motor. An electric motor is a device that uses an electromagnet to change electrical