# electromagnetic devices

## electric motor

An electric motor is a device that uses an electromagnet to change electrical energy to kinetic energy. You can see a simple diagram of an electric motor in the Figure 1.2. The motor contains an electromagnet that is connected to a shaft. When current flows through the motor, the electromagnet rotates, causing the shaft to rotate as well. The rotating shaft moves other parts of the device. For example, in an electric fan, the rotating shaft turns the blades of the fan. Why does the motors electromagnet rotate? 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 rotate 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 again repelled by the poles of the permanent magnets, which have not reversed. This causes the electromagnet to continue to rotate. These events keep repeating, so the electromagnet rotates continuously.

## how a doorbell works

The Figure 1.1 represents a simple doorbell. Like most doorbells, it has a button located by the front door. Pressing the button causes two electric contacts to come together and complete an electric circuit. In other words, the button is a switch. The circuit is also connected to a source of current, an electromagnet, and a clapper that strikes a bell. What happens when current flows through the doorbell circuit? The electromagnet turns on, and its magnetic field attracts the clapper. This causes the clapper to hit the bell, making it ring. Because the clapper is part of the circuit, when it moves to strike the bell, it breaks the circuit. Without current flowing through the circuit, the electromagnet turns off, and the clapper returns to its original position. When the clapper moves back to its original position, this closes the circuit again and turns the electromagnet back on. The electromagnet again attracts the clapper, which hits the bell once more. This sequence of events keeps repeating. Q: How can you stop the sequence of events so the doorbell will stop ringing? A: Stop pressing the button! This interrupts the circuit so no current can flow through it.

## devices with electromagnets

Many common electric devices contain electromagnets. An electromagnet is a coil of wire wrapped around a bar of iron or other ferromagnetic material. When electric current flows through the wire, it causes the coil and iron bar to become magnetized. An electromagnet has north and south magnetic poles and a magnetic field. Turning off the current turns off the electromagnet. To understand how electromagnets are used in electric devices, well focus on two common devices: doorbells and electric motors like the one that turns the blades of a fan. Q: Besides doorbells and fans, what are some other devices that contain electromagnets? A: Any device that has an electric motor contains electromagnets. Some other examples include hairdryers, CD players, power drills, electric saws, and electric mixers.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

devices that contain electromagnets include

``````a) doorbells.

b) electric fans.

c) hairdryers.

-->  d) all of the above
``````

an electromagnet has north and south magnetic poles and a magnetic field.

``````-->  a. true

b. false
``````

any device with an electric motor contains an electromagnet.

``````-->  a. true

b. false
``````

turning off the current to an electromagnet has no effect on its magnetic field.

``````a. true

-->  b. false
``````

a doorbell rings when an electromagnet turns off.

``````a. true

-->  b. false
``````

when the clapper of a doorbell moves to strike the bell, it

``````a) closes the doorbell circuit.

-->  b) breaks the doorbell circuit.

c) turns on the electromagnet.

d) two of the above
``````

## diagram questions

No diagram questions associated with this lesson