The little boy on the left is pressing the doorbell on his playhouse. The doorbell is connected to a battery, so it actually rings when he pushes the button. The little girl on the right is using a hand-held fan to cool off on a hot day. The fan is also battery-operated, and the blades of the fan turn in a blur of motion. What do the doorbell and fan have in common? Both of them work because they contain electromagnets.
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, we’ll 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.
How a Doorbell Works
The Figure below 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. You can see an animation of a doorbell at this URL: http://www.schoolphysics.co.uk/animations/Electric_bell/index.html
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.
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 below. 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. You can make a very simple electric motor by following the instructions at this URL: http://www.youtube.com/watch?v=VhaYLnjkf1E
Why does the motor’s 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.
You can see an animation of an electric motor in action at this URL: http://www.schoolphysics.co.uk/animations/Electric_motor/index.html
- Electromagnetic devices are devices that contain electromagnets. Examples of electromagnetic devices include doorbells and any devices that have electric motors, such as electric fans.
- The electromagnet in a doorbell attracts the clapper, which hits the bell and makes it ring.
- An electric motor is a device that uses an electromagnet to change electrical energy to kinetic energy. When current flows through the motor, the electromagnet rotates, causing a shaft to rotate as well. The rotating shaft moves other parts of the device.
electromagnet: Magnet created by electric current flowing through a coil of wire that is wrapped around a bar of iron or other ferromagnetic material.
electric motor: Device that uses an electromagnet to change electrical energy to kinetic energy.
Explore the electric motor animation at the following URL. Then describe two ways you can make the electromagnet in the motor rotate more quickly. http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/electricityworld/thecostofelectricityrev1.shtml
- Describe an electromagnet.
- What are some common devices that contain electromagnets?
- Describe the role of the electromagnet in a doorbell.
- What is an electric motor?
- Explain how an electric motor turns the blades of an electric fan.