Have you ever put a picture on the refrigerator? Have you used a key card to unlock a hotel room door, listened to music through speakers or saved information on a computer? All of these tasks, and many, many more, are made possible by magnets. Magnets come in different shapes and sizes. The magnets in a computer hard drive are only about 200 nanometers wide. That’s thinner than a human hair! The magnets on industrial cranes can be several feet across. But all magnets, regardless of their size, have the same basic properties. A magnet is something that has a magnetic field. The magnetic field is the area around the magnet that exerts magnetic force, which can attract or repel other magnets or objects made of metal. The stronger the magnet, the more force the magnetic field exerts.

Which of these involves the strongest magnetic force?