Electromagnets

• When electric current flows through a wire it generates a magnetic field. A coil of wire called a solenoid makes a stronger magnetic field. The magnetic field of a solenoid can be strengthened by placing an iron bar inside the solenoid to form an electromagnet. When the wires are connected to a battery, electric current flows through the coil to create a magnetic field. The magnetic field of the solenoid aligns the domains in the iron core and the iron core becomes magnetized. When the power that generates the electric current is switched off, the magnetic field of the electromagnet is also turned off. Electromagnets are useful because their magnetism is not permanent.
  
  

Electric Motors

• When an electric current flows through a wire placed between the two poles of an electromagnet, the electromagnet exerts a magnetic force on the wire and causes it to rotate. The rotation of the wire starts the motor and causes the electric current to change directions as the wire rotates. The motor keeps running because the current continuously changes direction. When the current is interrupted, the wire stops rotating and the motor stops running.
  
  

Speakers and Microphones

• Speakers and microphones use both an electromagnet and a permanent magnet. In a speaker, a membrane with a small permanent magnet attached covers an electromagnet. When electricity flows through the electromagnet, the membrane vibrates and moves back and forth because the permanent magnet is attracted or repulsed by the magnetic field of the electromagnet. When the electric current that produces the electromagnet's magnetic field is carefully controlled, the vibrations of the membrane produce sound waves. A microphone uses a similar mechanism, but in reverse. Sound waves produced by a voice cause the membrane to vibrate and move the permanent magnet back and forth. The electromagnet changes and causes electric current to flow through the wire and into a speaker.
  
  

Doorbells

• A doorbell uses an electromagnet to turn off and on; if doorbells used a permanent magnet, they would ring continuously. The doorbell button is a switch that opens and closes an electrical circuit. When the button is pushed, electric current flows through the solenoid and turns on the electromagnet. The electromagnet attracts a metal striker, which hits a bell to make the doorbell sound. When the button is released, the circuit closes and the electromagnet turns off. A spring pulls the striker back to its original position.