Amazing Rings (Magnetism) Demo

Where have you seen magnets before? (refrigerator, speakers). Do you guys know what happens when you put two magnets together? (they attract or repel) Magnets have two different ends called north and south poles. Just like positive and negative charges, when you put two like poles together, they repel, and if you put two opposite poles together, they attract. (show with permanent magnets). These magnets are called permanent magnets because they’re always magnets: you can’t turn them off. There’s another kind of magnet that you can turn on and off, and were going to show you a few of those.

Has anybody ever done this experiment where you take a nail, wrap wire around the nail and then connect a battery between the ends of the wire? (show homemade electromagnet). Do you know what happens to the nail? (it becomes a magnet). We can show that the nail becomes a magnet because we can use it to pick up paperclips (pick up paperclips with nail). We can also see if I disconnect the battery, the nail isn't a magnet anymore (futilely attempt to pick up paperclips with disconnected nail). This type of magnet is called an electromagnet because the nail is only magnetic when the battery is connected to the wire to make electricity run around the nail. In the last demo you learned about electric charges and what happens when they stand still. Here we have an example of what happens when the charges move. When the battery is connected a complete circuit is formed and charges move in circles in the coils around the nail. This is pretty amazing because by making electricity move in circles around the nail we made a magnet.

What we have here is also an electromagnet (show big jumping rings magnet). You might be able to see this is just a huge nail with a whole bunch of wires wrapped around it at the bottom, and we plug it into the wall to get electricity. This is a much stronger magnet than the first one. How can we prove that this is also a magnet? (try to stick some paperclips to it. Actually use the paperclips to prove it is an electromagnet).

Now that we have a magnet that we can turn on and off, we can do some interesting experiments. What we have here is a light bulb connected to a bunch of coils of wire, just like the coils we used to make the magnet (show apparatus). What do you think will happen if we turn the magnet on and put the ring of wire over the magnet? (the light bulb lights up).* (Do the experiment). Wow! What would make the light bulb light up? (electricity). But the wires aren’t touching, so the light bulb isn’t getting electricity from the wall outlet (Do the experiment again, and move your hand around in between the coils and the magnet to show that they are not touching ). You remember with the nail we used electricity moving around the wires to make a magnet. Here the situation is reversed. We already had a magnet and when we put the wires over the magnet, it makes electricity go around the wires to light up the bulb. So far we've used electricity to make a magnet and a magnet to make electricity.

Here is a plain aluminum ring (show ring). What do you think will happen when we put it on the magnet? (get a few guesses, then show the ring jumping off). Wow! What happened to the ring to make it jump off the magnet like that? (the ring also became a magnet). Is the aluminum ring naturally magnetic? (Try sticking the ring to the side of the electromagnet) Nope. Do you remember how the electromagnet works? The electricity moving around the bar makes a magnet. And with the light bulb we saw that the magnet makes electricity go around in the loops of wire attached to the bulb. So the electromagnet is making electricity go around the aluminum ring, and we know that electricity going around in circles like that makes a magnet, so it repels from the electromagnet*.

Here is a ring that is just like the last ring, but it has a slit cut in it (show ring). What do you think will happen when we put this ring on the magnet? (wait for the answers and show that the ring does not jump off). The first ring we used jumped off because the magnet made electricity move around the ring to turn it into a magnet, so why doesn’t this ring jump off? (electricity can’t go around the ring because of the slit). Cutting the slit in the ring is like cutting the power cord to your tv. It doesn’t make a complete circuit anymore, so the ring doesn’t become a magnet.

Here is another ring made of crumpled up aluminum foil, so it’s the same material as the last two (show ring). What do you think might happen with this ring? (wait for answers and show that the foil doesn’t jump very high). It looks like it’s jumping a little bit, so it must be making a very weak magnet. Why is it only a very weak magnet? (get answers). In the other ring the electricity going around made it into a magnet. In this ring, because the foil is all crumpled up, the electrons have to go around in a crooked path, so it’s harder to get around. There must be less electricity going around, making it a weaker magnet, so it barely jumps.

[this last experiment requires liquid nitrogen. Either do it after the liquid nitrogen demo or just tell the students that you are going to make the ring very cold and you’ll tell them about the cold stuff later. Also make sure you have a high ceiling, or just skip this experiment.] We have one last thing to show you about magnetism. What do you think will happen if we make the metal rings very cold? (dunk the rings into liquid nitrogen for a couple of minutes. Do this ahead of time so it doesn’t hold up the show. Then show the rings jump higher, you may even get the foil ring to levitate). Why do the rings jump higher now? It is easier for charges to move around the ring when it’s cold, so there’s more electricity to make a stronger magnet? But why is it easier for charges to move around the ring? All of the things (atoms) in the metal that can get in the electron’s way shake a lot when they’re hot. When those things are shaking around, it’s hard for the electrons to get by. When we cool down the ring, those things stop shaking and freeze up. Once they’ve stopped moving, it’s easier for the electrons to get by.

*Technical notes: to be entirely accurate, we should say that the magnetic field of the electromagnet has to be changing to induce a current in the wire-and-light bulb apparatus. The electricity from the wall outlet is alternating current, so it works. If we powered the electromagnet with a large battery, however, the magnetic field would be constant and it would not induce a current in the wire-and-light bulb apparatus. The same is true for the rings.

When we induce a current in the aluminum ring to make a magnet, the magnetic force is always repulsive according to Lenz’ law. I don’t know an intuitive way to explain Lenz’ law at this level, so we just say that we created another magnet and leave it at that.