Stephen A. Lawrence wrote:
> Assuming you live in the Northland, where the humidity drops into one's > shoes time time of year....) > Hang a paper snowflake from the ceiling, using tape and a thread. (Or, > better yet, hang up several.) > Blow up a latex balloon (the old-fashioned rubbery kind, not a Mylar > one). Then, either using a reasonably clean head of hair or a natural > fiber sweater, charge up the balloon. For no other reason than to be punctilious, those metallized balloons aren't Mylar, they're Nylon. > Bring it near the snowflake. The flake is strongly attracted, of > course. Typically one point heads straight for the balloon (or, anyway, > that's what I was seeing this evening). > Let the snowflake touch the balloon with one point. Let it stick > there. Hold the balloon still. > A few seconds later the flake pops off the balloon, and is repelled. > Chase it around a bit with the balloon, and notice that, repelled or > not, it continues to aim the same point at the balloon -- it rotates to > keep the same orientation relative to the balloon. > Now, most of this behavior is obvious, though fun, but there were a > couple questions I had some trouble answering. I can't believe Bill hasn't bitten into this. He must be out of town. Since he's not answering, I'll have a stab at it. > 1) The flake picks up a charge from the balloon ... obviously ... and > then is repelled. But why doesn't it happen _right_ _away_? What's > going on during the delay? The flake pops off all at once when it > finally leaves, and seems strongly repelled afterwards -- why doesn't it > come loose when it's just about neutral relative to the balloon? (The > flakes were cut from plain-paper copier paper, by the way. Sizes varied > from about 1.5" to perhaps 4".) Paper is a high resistance conductor at the voltages you've generated, probably in the 10kV to 30kV range. As a consequence, it takes a while for the charge to spread over the whole surface of the paper. > 2) Last I heard, dry paper was a pretty bad conductor. The charge > draining off the balloon must be going into the point of the flake which > is touching the balloon. One would expect that point to end up with the > largest charge; then, the flake would spin around and turn the other > way. But that doesn't happen. Once the flake stops being attracted to > the balloon, the point which was _touching_ the balloon is the one which > remains pointing toward the balloon, even as the flake as a whole > skitters away. How come? That' just it. Paper is a bad conductor. As the charge slowly accumulates in the mass of the paper, the highest voltage exists at the point farthest away from contact with the balloon. When enough total charges exist on the paper to make it lose contact with the balloon, this voltage gradient remains and the orientation of the paper to the balloon remains. > If you happen to be an LP afficionado, you can try this, too: > Stick a balloon or two or three to the ceiling with static. > Zap them with a Zerostat. > They go right on sticking -- they don't come loose! Why not? Why is > the Zerostat so bad at bleeding charge off a balloon? It used to work > like gangbusters getting dust off my LPs. This one appears to still > work; I can still feel the breeze from the nozzle when I work the > trigger. The room was starting to smell like ozone by the time I gave > up trying to shoot down a balloon from the ceiling with it. The Zerostat is bleeding the charge from the balloon, just not at the point where it's attached to the (conductive) ceiling. The reason for this is that when you charged the balloon it had a pretty high voltage. When you stuck it to the ceiling, the voltage near the point of contact dropped to a level where it is not easily conducted away through the air. When you bring a charged object into close proximity to a grounded conductor the coulomb-volts remain constant, but as the coulombs rise, the voltage drops. IOW, if you remove the balloon from the ceiling and zap it with the Zerostat, it will no longer be attracted to the ceiling. > ************************ > If you're really bored and are sick of discussing religion on Vortex, or > you need to entertain someone young, you can also put a well-charged > balloon or two on a tabletop and bring another charged balloon near > them. No surprises there but it's fun none the less... Oh, I don't have to be bored to play around with frictionally generated charges. I mess around with this stuff constantly and compulsively. I find it far more amusing than a grown man should. My best party tricks are generating 15 inch sparks with nothing but a glass top table and a piece of wax paper, or making a full 2 liter pop bottle roll back and forth across a level surface with the charge on a PVC pipe. Just in case you haven't already seen it, my more or less abandoned and pathetically underdeveloped web page can be seen at: http://www.pcpages.com/chv1 Here you can see a nice big spark generated for low cost. M. _______________________________________________ Join Excite! - http://www.excite.com The most personalized portal on the Web!
