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In Your Classroom: Dr. Megavolt
by The demonstrations that Dr. Megavolt gives in this episode are an ideal way to get your high school kids "charged up" about physics. I used this clip as a catch to get kids interested in the topic of magnetism and electricity. After twenty years of teaching physics I can safely say that the Tesla coil is the number one science demo kids want to see. So I want to give a few pointers, and suggestions about how to use these safely in the classroom.
The Tesla coil is often confused with the Van DeGraff generator. Both can give high voltage (close to a million volts), but the Van DeGraff is a static source of charge while the Tesla coil is an alternating potential of charge. Since both excite kids, lets start with the Van DeGraff Generator and lessons you can do to introduce static charge.
I introduce static charge with the familiar rabbit fur and plastic. It helps to bring in a chunk of Amber, which in Greek means "Electron". Apparently Amber was used as a material to rub and transfer charge way back before our plastic generation. If you are crafty, you ought to hang two pieces of plastic from the ceiling and charge one negative and the other positive; you can do this by experimenting with different types of plastic. Bring one charged strip to each and you will see one repel and the other attract. Usually I then hang a piece of Aluminum foil from a string and bring a charged rod close to it. You can have kids talk about induced charge. Another favorite is to bring a charged rod close to a two by four lumber that is carefully balanced on a watch glass. You can move it quite well back and forth through induced charge in the wood.
The best static demo comes from the San Francisco Exploratorium. Check out their Snack Book of Ideas. Here you can find a way of charging by induction that produces a sizable charge. Have your kids charge up a pie pan and then levitate a small loop of Christmas tree tinsel. It is an exciting hands-on lab, and it is even cooler to watch as the tinsel becomes charged and suddenly repels from the plate. There are lots of other cool physics ideas there.
Some cool demo's you can do with a Van DeGraff generator are presented at The Science House. I would like to add a few suggestions to these standard demos. It is very important to clean the ball. This removes small points that carry off the charge. I use a dry erase cleaner and that seems to do a good job. Also, to store lots of charge and release it, consider purchasing an oblong discharger, it concentrates the charge by capacitance. We use this to discharge through a Television set. If you can find an old TV tube, bring the generator close to the back where the wires attach to the filament inside the tube. You will get some amazing colors as the three filaments fire electrons randomly toward the screen. You can check out a photo of this below.
Other fun things to do with the Van DeGraff Generator are making chains of people holding hands, turning on a fluorescent bulb, or giving each student a Neon bulb to light as they bring it close to the generator.
Once a teacher has covered static electricity, it is normal to move on to magnetism. All magnetic fields come from moving charges, even the magnetic moments produced by the electron itself! There are many demo's on magnetism, but lets cut directly to the Tesla coil. The coil itself is nothing more than a transformer. So by all means have the kids build a simple transformer. To do this, have each student wrap an iron nail with #26 gauge wire (this takes lots of wire!), energize one coil with a function generator, and bring it close to another coil. Hook the second coil to an LED and you will see it go on. I like to bring out my Jacob's ladder at this point and show them how you can make large sparks with a larger transformer. I made mine from the igniter of an oil burner and simply attached two coat hangers to the end. A time exposure of a Jacob's ladder in action is shown to the right.
Once students understand a Tesla Coil you should bring out the smaller demonstration ones available from science supply companies. This handheld one works very well and produces sparks a few inches long. You can have fun making long sparks and you can get a few lessons across. I like to produce ozone with it, as the temperature of the sparks are high enough to slam Oxygen molecules together to make Ozone, and this has a unique odor that kids can sense. I use this in a lesson about the ozone hole from my time spent in Antarctica launching ozone balloons. A strong WARNING though. It is often fun to hold onto the end of the Tesla coil and become the electrode yourself. It does give a small burn to the skin if you are not careful in doing this, so I do not recommend you ask kids to do it. If you are confident you might get a colleague to come in, dim the lights and you can run sparks down the persons back with your fingers. Kids like this. Also, a BIG WARNING, and I have not seen this published anywhere. I once started a fire on a student's sweater by sparking them with a Tesla coil. The small fibers that stand up on a sweater are an ideal kindling for these sparks. So be careful!
Finally, your students will want to build their own Tesla Coil and do all the cool things the Dr MegaVolt does. There are so many plans on the internet for this. You can check out this instructional video.
I had my students build one out of a neon transformer and a large tube with a hand wound secondary coil. They used glass plates with foil as their capacitor. We actually cracked the quarter inch glass from the heat generated by the sparks. A really cool thing to do is to turn on an AM radio while the Tesla coil is in operation, you will get a lot of crackle from the sparks going off at radio frequencies. This is a nice way to demonstrate that the sparks are different from the Van DeGraff generator. I am told that you can touch this high voltage at radio frequencies because the current will traverse over your body instead of through it. However, I have not had the guts to try that, and you might note that Dr MegaVolt has a well grounded suit to make sure that it goes over his body and not through it. So be careful and have fun.
Additional WIRED Science Video Segments
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