That's how many electrostatic machines work such as the Wimshurst.
There are 3 different things, voltage, field strength and charge imbalance, in this case the Voltage goes up, however the field strength goes down (though is still considerable and covers more area) and the charge imbalance obviously remains the same. It is also possible to increase the electric field density without changing the voltage or net charge imbalance by use of a point. On 2/6/07, Michel Jullian <[EMAIL PROTECTED]> wrote:
Nice! It's pretty obvious why voltage increases when you think of it as variable gap capacitor with a constant charge, q=C*V, if you lower C by increasing the gap then V necessarily increases. > Does gluing a needle to the small pan make it possible to pull a longer > spark? The answer is yes obviously (the needle is sharper than your knuckle) but there is no need to glue it to the small pan, you can hold the needle in your discharging hand (whichever side of the discharge gap you put the pointed end is fine) Also I don't see why it would make a difference whether the large pan is grounded or not. Simplest is to rest it on the table without further ado. You didn't say, but I imagine the bottom larger pan must be upside down :) Michel ----- Original Message ----- From: "Stephen A. Lawrence" <[EMAIL PROTECTED]> To: <vortex-l@eskimo.com> Sent: Monday, February 05, 2007 8:57 PM Subject: Re: [Vo]: Voltage versus field, and the electrophorus > > > Stephen A. Lawrence wrote: >> There's been a little confusion over field strength versus voltage >> recently on this list. Here's a cool gadget that helps to illustrate >> the difference. Most likely most readers are already familiar with it, >> but perhaps it will be new to some. (General description obtained from >> "Models for Experiments in Physics", by A. D. Bulman, no doubt out of >> print for 30 years at this point.) >> >> (BTW I've been planning to build one of these for the last few decades >> and haven't _quite_ gotten around to it just yet -- maybe before this >> winter's over...) >> >> Start with two shallow tin trays, baking pan size or a bit smaller, with >> rolled edges. >> >> Glue an insulated handle to the smaller one. Recommended: An empty >> detergent bottle, with cap glued to pan, so the handle may be unscrewed. >> (Remove label first, he sez; maybe paper isn't so hot in the >> "insulator" department?) >> >> Glue a sheet of "good insulator" to the larger pan. Recommended >> sources: Bottom of an old plastic dishpan, or a cylindrical polyethelene >> bottle, top and bottom cut off, cylinder slitted and "unrolled" so it's >> flat; heat it to flatten it, he sez. Not too hot or it will stink, sez I. >> >> Now warm everything up (why warm it? dunno) and then charge the >> insulating sheet by "flicking" it with a piece of flannel, or rubbing it >> with a "dry dust cloth" or piece of fur. >> >> You're done. (Does the large pan have to be grounded? Book doesn't >> say; I think the answer is no, it must float, but I'm not sure.) >> >> Now, to use it, holding the smaller pan by its handle, place it on the >> insulator on the larger pan (insulator must be bigger than smaller pan, >> did I neglect to mention that?). Touch the small pan with a finger to >> ground it. Now lift it up by the handle, being careful not to touch it. >> Finally, bring your knuckle near the small pan, which you are holding >> by the handle; supposedly, if all goes well, you can pull a spark an >> inch long from it ... despite the fact that it was at exactly the same >> potential as your finger mere moments before, and no charge has been >> added or removed since. >> >> You can do it again, as often as you like, _without_ recharging the >> insulator, until the charge on the insulator finally bleeds off through >> the air (this is presumably best done in the winter). >> >> Anyhow so says the text. As I said, I keep meaning to build one of >> these, but we use our old tin baking pans for baking things, I don't >> have any cylindrical polyethylene bottles lying around the house, and I >> just never got around to scaring up alternatives. >> >> Now, the point of this is that the _field_ between the small and large >> pans is strongest when they're closest together, yet the _voltage_ >> between the small pan and ground is largest when it's farthest from the >> large pan. The voltage is boosted mightily by the work done in pulling >> the pans apart, while the actual field between them is either unchanged >> or actually decreases in intensity. > > Minor addendum/correction: The field is small, _UNTIL_ you bring your > finger or other grounded pointy thing near the pan; at that point the > field between the point and the pan will get very large, which is what > finally breaks down the air to form the spark. > > This suggests that you must lift the gadget straight up (don't tilt it), > to avoid having the charge on the pan "slosh" to one edge due to the > (approximately vertical) field from the insulator, thus forming a "hot > spot" in the charge density which would break down the air around it and > short out before you got them far enough apart to be interesting. > > Does gluing a needle to the small pan make it possible to pull a longer > spark? Dunno. Sigh... I've gotta finally spend the time to build one > of these things... > > >> >> Anybody cares to find out if this gadget actually works, I'd love to >> hear about the results -- or, as I said, maybe I'll finally get around >> to building one this year, before the weather gets warm again and the >> humidity goes up... >> >
