Speakers and Motors We ve come a long way with this magnetism thing and hopefully you re feeling pretty good about how magnetism works and what it does. This lesson, we re going to use what we ve learned to make simple versions of two gadgets that you use every day. Speaker Let s start with the slightly simpler gadget. Let s make a speaker! Now to understand what s happening here, we need to take a trip down memory lane to the last lesson, Magnetism and Electricity. Remember, that electricity is moving electrons and what do moving electrons create...a magnetic field, right! Also, remember that a magnetic field, when moved near a metal that can conduct electricity, will create an electric current in that metal. (You did this in the last lesson when you did the Ins and Outs of Electricity experiment.) Magnetism can create electricity and electricity can create magnetism. Now, let s go way back in time and try to remember the work we did with sound. Sound is vibrations. If something vibrates between a frequency of 20 20,000 Hz our ears can detect it as sound. To make a speaker, we need to somehow make something vibrate. Hmmm, I wonder if this magnetism/electricity could somehow be useful here. Well, hold on to your hats young genius and let s make a speaker! Experiment 1 Speaker of the House You need: Three feet of magnet wire to make a coil (you can reuse any of the coils you made in the last lesson if you wish) A toilet paper tube (if you re making a new coil) Speakers and Motors 1
A strong and preferably round magnet (it needs to fit into the above coil of wire). Those little donut magnets work great. A thin plastic cup or a disposable metal pie tin Masking tape An old set of headphones (you will need to sacrifice a set of headphones if you d like to do this experiment) A radio that has a headphone jack (Use one that you don t care about much. I haven t seen this mess up a radio yet but it s possible.) 1. Take your wire and (if you re not reusing one from the last lesson) wrap it around a toilet paper tube until you have nice tight coil with about 4 inches of wire on either side of it. 2. Remove the coil from the toilet paper tube. Remove at least an inch of the insulation from each end of the coil. 3. Tape the coil to the bottom of the cup on the outside of the cup. Tape it securely and feel free to have the tape go over the coil. 4. Now, place the magnet on top of the tape that s on top of the wire. Tape it securely as well. 5. Take the old set of headphones and cut the input jack off. Leave at least 6 inches of wire coming off the jack. 6. Strip at least one inch of insulation off the end of each wire. 7. Twist one end of headphone jack s wires tightly around one end of the coil s wires Speakers and Motors 2
and twist the other end of the headphone jack s wires around the other coil wire. Wrap tape around the twists to keep them in place. 8. There s your speaker! Now plug it into a radio and give it a try. You will probably need to turn the radio up quite a bit and the sound you hear will still be fairly soft, but it s still pretty cool. Remember to use a cheap radio that you don t care about much. Just in case something goes wrong. Congratulations! You ve made your first speaker. So what s going on with your speaker? What makes it work? Okay, here s the deal. The radio provides the electricity that gets pumped through the wires. The radio very quickly pumps electricity in one direction and then switches to pump it in the other direction. This movement of electrons back and forth creates a...you guessed it, magnetic field in the coil of wire. Since the electricity keeps reversing, the magnetic field keeps reversing. Basically, the poles on the electromagnet formed by the coil go from north to south and back again. Since the poles keep reversing, the permanent magnet you have taped to the cup keeps getting attracted, then repelled, attracted, then repelled. This causes vibrations. The cup that you ve strapped your coil and magnet to, acts as a sound cone. The magnet causes the sound cone to vibrate and since it s relatively large, it causes air to vibrate. This is the sound that you hear. Almost all speakers work just like the one you created. They just use fancier materials so that the sound is louder and clearer. Now that you ve given that a try, take a look at this! Speakers and Motors 3
Experiment 2 A Simple Motor You need: About 2 feet of wire (yes, again!) Plastic or foam cup D-cell battery A strong magnet 2 Paper Clips Masking tape Permanent marker 1. This time you need to make a coil that s a bit smaller then the last ones you ve made. I ve had good luck wrapping it around a D-cell battery. Just do about 4 or 5 loops. One of the keys to this gadget is to make sure your coil is very tightly wrapped and as circular as possible. Have about 2 inches of wire coming off each end of the coil. Wrap the ends of the wire around the coil a couple of times to hold the coil together. 2. Another key (probably the most important one) is to make sure that the coil is nicely balanced. The coil is going to spin rapidly so the weight must be evenly distributed on both sides of the wires that stick out. 3. Strip the insulation off of both wires that stick out off the coil. Speakers and Motors 4
4. Take a marker and carefully put a line on half of one side of the wires that come off of the coil. This seems kind of weird but it will make sense later. 5. Now that you ve got your perfect coil, take your paper clips and bend them like you see in the picture. 6. Tape the paperclips to both sides of the battery. Make sure the paper clips make a nice tight contact with the poles of the battery. 7. Attach the magnet to the battery. It should be strong enough to hold itself to the battery. You might want to use a compass to make sure that the north or south pole of the magnet is facing out from the battery. 8. Tape the battery to the top of an upside down foam or plastic cup. Make sure the paperclips are positioned like they are in the picture (see the first picture). 9. Place the coil into the cradle formed by the two paper clips. 10. Give the coil a little flick to get it spinning. If all goes well, it should continue to spin on its own. 11. If it didn t spin on its own, don t worry about it. It rarely works right away. Here s some tips. Make sure the coil spins freely. It should be balanced well and it shouldn t hit anything as it spins. Feel free to adjust the coil or the paper clips as necessary. Try to get the coil as close to the magnet as possible. Speakers and Motors 5
Another thing to check, is to see if electricity is flowing. When you put the coil on the cradle you may see the coil wiggling on its own. You may also find that the wire is getting hot. If this is happening you ve got power. If you re not sure you re getting power, you may want to check the connections of the paper clips to the battery. Hold them tight against the battery and see if anything happens. You may want to take them off and try again. If you re not getting power, you want to make sure the coils wires are nice and striped. You may want to take them off and restrip them to make sure. Take your time. This can be frustrating but can work quite well if you re patient and careful. Pretty cool huh? So what happened there? Well first, you created a circuit by allowing current to flow from the battery, through the paper clip, through the coil, through the second paper clip and then back to the battery. This electrical current is also known as moving electrons and moving electrons create, all together now, a magnetic field! So as the current flowed through the coil, it caused the coil to become an electromagnet. The coil is attracted to the magnet so it is pulled toward the magnet. Now can you see why we put the marker on half of one of the wires? If the current flows all the time the wire will always be pulled to the magnet and will stop spinning. But, if the current turns off, then the coil will spin freely due to its own inertia. The marker acts like insulation so when the marker side of the wire touches the paper clip the current is turned off. This causes to one side of the coil to always get pulled towards the battery causing the coil to spin. This motor that you just created is the basic form of just about every electric motor out there. It s the motor that turns the CD in your stereo, the motor that turns your hard drive in your computer, the motor that turns the wheels on your toys. Just about all electric motors work using the exact same principles of the little fellow you just made. There you have it. Magnetism and electricity. One creates the other and by using that knowledge you ve created two of the most useful inventions in today s world. Pretty powerful stuff huh? Gotta love physics!!! Speakers and Motors 6
In a Nutshell Electricity is a flow of electrons. A flow of electrons creates a magnetic field. Magnetic fields can cause a flow of electrons. Magnetic fields can cause electricity. Speakers and Motors 7
Did You Get It 1. When you made the motor and the speaker, why did you hook the coil of wire to an electrical source (a battery for the motor and the radio for the speaker)? 2. Why did the permanent magnet vibrate in the speaker? 3. Why did the electromagnet in the speaker keep reversing poles? 4. In the motor, why did the coil become attracted to the permanent magnet? 5. Why did the coil spin, instead of just being constantly pulled towards the permanent magnet? Speakers and Motors 8
Answers 1. By hooking the coil of wire to the battery, you caused electrons to move. Since electrons were moving through the coil, it became an electromagnet. 2. The permanent magnet vibrated because the poles of the electromagnet kept reversing. This causes the permanent magnet to be attracted, then repelled, attracted, then repelled. (Kind of like some people I know!) 3. The radio caused the current running through the coil of wire to go in one direction, then the other. Since the flow of electrons kept changing directions, the magnetic field did too. 4. The coil became attracted to the permanent magnet because the coil became an electromagnet when current from the battery was going through it. 5. The current wasn t always flowing through the coil, so the coil s electromagnetism was being turned on and off. The current was turned off when the marker side of the wire touched the paper clip. Electricity couldn t flow, so the magnetic field stopped. Speakers and Motors 9