I'm trying to figure out how a couple of guys who are clearly better educated, and probably a lot smarter than I, can have gone so far wrong.
Stephen A. Lawrence wrote: > Frederick Sparber wrote: > Posted earlier: >> This Field Line Applet is cheaper than buying more VDGs. http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html >> If the force around a positively charged sphere is reduced by >> cladding it with a high dielectric constant material (K) > It won't be, not unless the cladding has a net charge. The > dielectric or other properties of the cladding aren't relevant > to its ability to "shield" something, which can be done only > by canceling the original field. (There's no such thing as a > "true" electric shield, of course. At least according to > conventional theory, the E field obeys the law of > superposition,and you can't actually "block" it; you can > only cancel it.) > If you surround a charged sphere with any spherically > symmetric material which does not, itself, carry a net charge, > the result will be no change in the external field (outside > the cladding) and no change in the net force acting on the > system (sphere+cladding) due to an external field. This is demonstrably not the case. If a polar dielectric is interposed between a charged conductor and a detector, the strength of the field will drop substantially, especially if the dielectric has a high K. Work is done by aligning the molecular dipoles with the result that the voltage drops while the coulombs increase. When the dielectric is removed the voltage increases again. As a practical matter, you can feel the strong attraction between the polar dielectric and the charged conductor, and as the dielectric approaches the charged object the attractive force between the charged conductor and other objects which may have been attracted initially is noticeably diminished. If you have an electrostatic voltmeter, the voltage drops as the dielectric approaches the charged conductor. This effect is not observed if the interposed insulator is a non-polar material, polyethylene for example. For a really unequivocal demonstration, you want a material with K > 100. Glass works, but its dielectric constant is only about 7. You'd like to use barium titanate, but a free-standing plate of this stuff is almost impossible to come by. I like to use the T.T. Brown standby, litharge in epoxy, K = approx 700. > If it did you could close the cladding, move a bit, open the > cladding, move back, close the cladding, and so forth, and > voila, you have a PMM. With uncharged cladding opening and > closing the cladding (by sliding it) should require > insignificant work. Well, no. The only time lateral transposition of the dielectric would require no work would be if the charged object was standing alone in a universe of no other charged or conductive objects. One of the old classic electrostatic demonstrations is the drawing in of a dielectric sheet between the plates of a charged air gap capacitor. Work in, work out. In Fred's proposed setup, however, as there would be a continous supply of current provided by the Van de Graaff generators, the voltage at the surface of the dielectric covered sphere would eventually be the same as a the three outlying bare metal spheres. It would just take longer to get there. A better solution would be to expel the positive charge at the juncture of the three Van de Graaff generators with a cluster of needle points, perhaps assisting the the hoped- for lift with an ion wind. Fred wrote: > Force = 1/K * 1/4(pi)eo * q* Q/R^2 > Hence a 3-point craft with a cladded positive center sphere > and three exposed negative spheres should repel the earth's > excess negative charge up to an altitude that requires charge > reversal for getting past the ionosphere. There is a fundamental problem with this idea. While the earth has a net negative charge of say, one megajoule, the tiny fraction of a joule per square meter just won't supply the repulsive force you need unless your Van de Graaff spacecraft is very large and already elevated. The earth is is quite conductive, especially at the voltages we are talking about here. Therefore, an object with a strong negative charge will charge the earth locally positive by induction and your spheres will be strongly attracted to the earth, not repelled. The earth's negative field will have been slightly shifted to the opposite side of the world. If you want to play around with a propulsive force, I suggest you try some Brown-Biefeld experiments. I've had numerous positive results, but haven't written about them because I'm not sure I've eliminated all the artifacts such as ion wind. Charge on. M. _______________________________________________ Join Excite! - http://www.excite.com The most personalized portal on the Web!
