by Ron Kurtus (revised 10 December 2012)
Electromagnets are used in a number of everyday devices.
One useful characteristic of an electromagnet is the fact that you can vary its magnetic force by changing the amount and direction of the current going through the coils or windings around it. Loudspeakers and tape recorders are devices that apply this effect.
Some electromagnets can be very strong and its power can be readily turned off and on. Junk yard electromagnets, common doorbells and electromagnetic locks are examples.
Electromagnets can also be used to create continual motion when opposed by other electromagnets or permanent magnets. Examples include electric motors and maglev trains.
Questions you may have include:
- What devices use variations of electromagnetic force?
- How can turning the magnetic field on and off be used?
- What devices create continual motion?
This lesson will answer those questions. Useful tool: Units Conversion
Devices using varying field
You can change the strength of an electromagnet's magnetic field by varying the electrical current that passes through the wires wrapped around it. If you change the direction of the electrical current the polarity of the magnetic field reverses.
These effects can be used to move a loudspeaker cone back and forth, creating sound according to the electrical current through the wire. They also can be used to create magnetic fields in a magnetic tape or computer hard drive, such that it stores information.
The loudspeakers in your radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone.
By varying the electric current through the wires around the electromagnet, the electromagnet and the speaker cone can be made to back and forth. If the variation of the electric current is at the same frequencies of sound waves, the resulting vibration of the speaker cone will create sound waves, including that from voice and music.
Cutout of a loudspeaker
If you examine the back area of a loudspeaker, you should be able to see the permanent magnet and coil of wire for the electromagnet. Some loudspeakers use an electromagnet without the iron core, which is called a solenoid.
When a Mylar tape covered with fine iron dust passes near a small electromagnet that has a varying magnetic field, according to an electrical signal, the dust become magnetized in different directions. The electrical signal could be from a radio or microphone.
The tape then is a record of the electrical signal. When it passes by another small electromagnet, it creates an electrical signal, duplicating that of the original signal. This signal can be amplified and played back through loudspeakers.
Devices using turning magnetic field on and off
The magnetic strength of an electromagnet depends on the number of turns of wire around the electromagnet's core, the current through the wire and the size of the iron core. Increasing these factors can result in an electromagnet that is much larger and stronger than a natural magnet. For example, there is no known natural magnet that is able to pick up a large steel object such as a car, but industrial electromagnets are capable of such a task.
Also, if the core of the electromagnet is made of soft iron, its magnetic force can be turned off by turning off the electricity to the electromagnet.
Picking up and dropped junk cars
Thus, an electromagnet can be used to pick up a piece of iron and then drop it someplace else.
Crane uses electromagnet to pick up junked car
Strong electromagnets are often used in areas of heavy industry to move large pieces of iron or steel. They are commonly employed in junkyards, where a crane with a huge electromagnet is used to pick up, move and drop old, junked cars.
An electromagnetic lock be used to lock a door by creating a strong field in an electromagnet that is in contact with a magnetic plate. As long as there is current through the electromagnet, the door remains closed and locked.
Another type of electromagnetic lock uses an electromagnet to extend a plunger between the doors, making it nearly impossible to open the door until the electromagnet releases the plunger.
An old-fashioned doorbell used an electromagnet that was rapidly turned on and off to pull a clangor against a bell.
Devices creating steady motion
Clever use of electromagnetic forces can create steady motion.
An electric motor is another application of electromagnets. Suppose you put some electromagnets on a wheel and put some permanent magnets around the wheel. The electromagnets could be made to attract and repel the surrounding magnets, causing the wheel to turn. By varying the current, the speed of the motor can be made to vary.
Look at an electric motor and see the internal wheel made of electromagnets and the outer shell made of permanent magnetic material.
A maglev (magnetic levitation) train works without wheels and is propelled by electromagnetic forces.
This type of train usually consists of a set of magnets along the bottom of the train and a series of electromagnets on the tracks or guide-way for the train. The electromagnets are adjusted to have the same polarity as the train's magnets, though complex computer controls. Since the magnetic poles repel, the train is levitated or floats slightly above the track. Guides on the sides prevent the train from sliding off.
Depending on the position of the train, the polarity of the electromagnets is adjusted, causing the train to move forward. Maglev trains can reach speeds over 260 mile per hour or 430 kilometers per hour.
Electromagnets are used in a number of devices, such as loudspeakers and tape recorders. Some electromagnets can be very strong and its power can be readily turned off and on, such as in junk yard electromagnets and electromagnetic locks.
Electromagnets can also be used to create continual motion such as with electric motors and maglev trains.
Use your imagination to create new applications
Resources and references
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