What do a wrecking yard, a rock concert and your front door have in common? They each use electromagnets, devices that create a magnetic field through the application of electricity. Wrecking yards employ extremely powerful electromagnets to move heavy pieces of scrap metal or even entire cars from one place to another. Your favorite band uses electromagnets to amplify the sound coming out of its speakers. And when someone rings your doorbell, a tiny electromagnet pulls a metal clapper against a bell.
Mechanically, an electromagnet is pretty simple. It consists of a length of conductive wire, usually copper, wrapped around a piece of metal. Like Frankenstein's monster, this seems like little more than a loose collection of parts until electricity comes into the picture. But you don't have to wait for a storm to bring an electromagnet to life. A current is introduced, either from a battery or another source of electricity, and flows through the wire. This creates a magnetic field around the coiled wire, magnetizing the metal as if it were a permanent magnet. Electromagnets are useful because you can turn the magnet on and off by completing or interrupting the circuit, respectively.
Before we go too much farther, we should discuss how electromagnets differ from your run-of-the-mill "permanent" magnets, like the ones holding your Popsicle art to the fridge. As you know, magnets have two poles, "north" and "south," and attract things made of steel, iron or some combination thereof. Like poles repel and opposites attract (ah, the intersection of romance and physics). For example, if you have two bar magnets with their ends marked "north" and "south," the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). An electromagnet is the same way, except it is "temporary" -- the magnetic field only exists when electric current is flowing.
The doorbell is a good example of how electromagnets can be used in applications where permanent magnets just wouldn't make any sense. When a guest pushes the button on your front door, the electronic circuitry inside the doorbell closes an electrical loop, meaning the circuit is completed and "turned on." The closed circuit allows electricity to flow, creating a magnetic field and causing the clapper to become magnetized. The hardware of most traditional doorbells consists of a metal bell and metal clapper that, when the magnetic pull causes them to clang together, you hear the chime inside. The bell rings, the guest releases the button, the circuit opens and the doorbell stops its infernal ringing. This on-demand magnetism is what makes the electromagnet so useful.
In this article, we'll take a closer look at electromagnets and discover how these devices take some pretty cool science and apply it to gizmos all around us that make our lives easier.