1.10 · The Invisible Force: A Short Lecture on Magnetism

The Invisible Force: A Short Lecture on Magnetism

Lecturer: Good morning, everyone. Today’s topic is a force you can’t see, but you use it every day: magnetism. I’m going to do three things. First, what a magnet is. Then, two short stories from history. And finally, where magnets work for us today. So, what is a magnet? A magnet pulls on iron, and on most kinds of steel. Every magnet has two ends, and we call them poles: a north pole and a south pole. Here’s the key rule, so write this down: opposite poles attract, and like poles repel. And if you cut a magnet in half, you don’t get a north piece and a south piece. You get two smaller magnets, and each one has two poles. Around every magnet, there’s an invisible area where you can feel its force. This area is called a magnetic field. OK, history. By about 1200, sailors in China and Europe were using compasses. A compass is a magnetic needle that turns freely, and one end always points north. But for hundreds of years, nobody knew why. Some people said the North Star pulled the needle. Others believed in a huge magnetic mountain in the north. Then, in 1600, a book was published that changed everything. It was written by an English doctor, William Gilbert, and it was called De Magnete: “On the Magnet.” Gilbert made a little model of the Earth, a ball of magnetic rock, and moved a tiny compass needle over it. The needle behaved like a compass on a ship. His conclusion: the Earth itself is a giant magnet. And notice: he tested his idea instead of just arguing about it.

Student: Sorry, can I ask something?

Lecturer: Sure, go ahead.

Student: If the Earth is a giant magnet, why doesn’t everything made of iron stick to the ground?

Lecturer: Good question. The Earth’s field is weak at the surface. It’s strong enough to turn a light compass needle, but far too weak to pull a nail across a table. If it were much stronger, we’d have a lot of problems in the kitchen! Now, my second story. In 1820, a Danish scientist called Hans Christian Ørsted was working with an electric current in a wire. He noticed something strange: a compass needle near the wire moved every time he switched the current on. That was the first clear proof that electricity and magnetism are connected. And it’s the idea behind the electromagnet. Wrap a wire around a piece of iron, send an electric current through the wire, and the iron becomes a magnet. Switch the current off, and the iron stops being a magnet. Electromagnets are used everywhere. Here are three examples: the speakers in your phone, electric motors, and the cranes that lift old cars in a junkyard. Finally, one application that saves lives: the hospital scanner we call an MRI. It lets doctors see inside the body without surgery. The big magnet inside is an electromagnet. A typical scanner’s magnetic field is about 30,000 times stronger than the Earth’s. That’s why patients take off anything made of metal before they go in. So, the main idea is this: first we used magnets, then we understood them, and now they work for us everywhere.