Build a Motor. Materials

Transcription

1 Build a Motor Building a motor in class is an interesting extension of the standard electromagnet activities. Motors, it turns out, basically work using well timed electromagnets. Outlined below is one of the simpler motors, from both a construction and conceptual standpoint. Even so, this is an intermediate concept, requiring the students understand magnets and electromagnets and that they have some fine motor skills. This experiment is a nice culmination of a unit on electricity and magnetism. If you show the students the procedure ahead of time, this is an activity that pairs or small groups could work on. The tasks, such as building the coil and holder, can be divided to give each student responsibility for part of the device. Materials Battery (9V or D work well) and matching battery-holder or wires and clips Two paperclips Small magnets; ceramic works fine but the stronger the better About a foot of magnet wire. o A roll of this is reasonably priced and available at common stores such as Radio Shack. o You want a wire thick enough to hold its shape, but thin enough that you can bend it easily. If the packaging says something similar to 22 gauge, you are on the right track. One large or several small books Sandpaper Scissors Dry erase marker, or some cylinder of a similar diameter Directions There are two parts that you need to prepare: a coil and a holder. Once we ve built our motor correctly, you ll be able to place the coil into the holder, and it will turn rapidly.

2 To build the coil, have the students wrap the wire around the marker a handful of times. Leave a straight section of the wire free on the starting end, and after coiling all the wire, end with a straight section out the other side. These should be opposite each other and as centered as possible. Pull your coil off the marker. If the wire springs out of shape and won t stay flat, an easy solution is to wind the two tail ends of wire through the hole of the loop and out again. A couple such wrappings will keep everything in a nice package. Once you hook this up to a battery, the wire will be our electromagnet. To get a good electrical connection, though, you will need to remove some of the insulation from the wire. Magnet wire, even though it looks like bare wire, actually has a thin, clear layer of insulation around it. You will use sandpaper to remove this layer in the right places. You do not want an electromagnet that stays on all the time, so you are going to remove just some of the insulation. On one of the arms, use sandpaper to remove all the insulation. The metal should be noticeably shinier, and you should do this all the way up to where the arm meets the coil. For the second arm you should remove just half the insulation. Hold the coil near the edge of a table covered with some surface you won t mind scratching with sandpaper such as a notebook. Set the second arm onto that surface. Sand off the top half of the insulation. This is important, so make sure your coil matches the picture. Again, do this all the way to where the arm meets the coil. Now, as the coil turns the current will be turned off and on. To make the holder, take a thick book or stack of books and place them flat on the table with the binding facing away. Make two hooks out of the paperclips. To do this, bend the clips as in the picture. The goal is to slip one curve of the

3 paperclip flat into the pages of the book and to create a j shaped hook with the other end. (When it comes to this shape, a picture is indeed worth a thousand words, so show your students an example). Place the two shaped paperclips into the pages of the book at the same height and a few inches apart. Using the battery and the appropriate battery holder and/or wires, hook one paperclip up to the positive terminal and the other up to the negative. Be sure the paperclips are not touching in the book. Now put a magnet (or stack of magnets) underneath where the coil will go. In order to keep the magnets still, I found it easiest to stack them on the blades of closed scissors.

4 When you are ready, place the coil onto the paperclips. Give it a little spin with your hands. If one direction doesn t work, try the other. If everything is set up correctly, it should take off on its own and keep spinning. You might need to change your device a bit after your first attempt to get it to work better.

5 If the coil hits the magnet as it turns or is too far away you may need to adjust your paperclips slightly. By moving them up and down pages and sliding them back and forth, they are easy to change. The coil might also be unbalanced, making it difficult to rotate. In that case, make sure the two arms of the coil are indeed as opposite one another they can be. Adjust to get it to balance and spin freely. If it still doesn t work, check the exposed metal on the arms of the coil. Make sure one arm is completely clear. Make sure the other arm is half blocked. Sand off slightly more if you need to, always leaving at least some insulation on the second arm. Be sure that the magnet has its north or south pole facing up. You can tell this easily by stacking two magnets together. A vertical stack means the poles are facing vertically, too. Explanation The motor works because the electromagnet is ON half the time and OFF the other half. When the electromagnet is on it has a north and south pole, and the magnetic force will turn it to face the permanent magnet. (Opposite poles, of course, attract.) As the coil rotates the insulating section starts to touch the paperclip, and the electromagnet turns off. So instead of stopping at the preferred direction, the coil keeps spinning with the momentum that it has. Once the coil has turned far enough, the circuit is closed again. The opposite poles again attract, beginning the whole process once more. Basically, when it is on, the coil turns to face the right way, when it is off, the coil is allowed to keep turning back to its original way.

6 Notes for Classroom Use How you approach this experiment of course depends upon the students familiarity with the material. You might start with an example motor where both arms have been completely stripped by sandpaper. This means the electromagnet will always be on. The coil will turn to the preferred direction and stop, just as you d expect. This makes the mental jump to a continuously spinning coil a bit easier. Try changing variables, and see how they affect the speed of rotation: number of turns in the coil, distance from the magnet, number of magnets, perhaps even different strength batteries. This experiment has a long history, and variants of it have been described in many different ways by many different people. For other resources, search online for Beakman s Motor, so named because this type of motor once appeared on that show. Safety Tips Since you are connecting the two ends of a battery with just wire and paperclips, you will basically be short circuiting the battery. If you leave it on for long enough, the paperclips may get hot. Test out how your materials work, and advise the kids to avoid touching the paperclips during or right after the experiment.