----- Original Message ----- From: "Horace Heffner" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, August 14, 2007 8:27 AM Subject: Re: [Vo]:Re: Only potential differences matter
... > Michel has said on an abstract basis that only relative potential > between plates matters with regard to charge balance, i.e. electron > fugacity. I don't think that is so. But you will, do you want to bet? ;-) > Unless the experiment is > enclosed in a grounded Faraday cage and potential is measured > relative to the cage Good, you are now talking in terms of *relative* potentials :-) > it is not possible to know anything for sure > about electron fugacity, because of atmospheric fields and earth > surface charge. Within the cage a negative surface potential with > respect to the cage, especially if it is large, is in fact an > indication of high electron fugacity at that surface. Wrong (read below). > Outside the > cage, relative potentials are not necessarily an indication of excess > charge of either kind. What you are still missing Horace is that the two plates, provided they are closer together than their lateral extent, act as their own Faraday cage so their surface charges really only depend on their difference of potential, irrespective of their potential relative to nearby external objects including the external Faraday cage (whose use is desirable anyway as previously discussed). This follows as I said from Gauss's law (also mentioned by John, appropriately). I sent you a link earlier, here it is again: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gaulaw.html excerpt:"It is an important tool since it permits the assessment of the amount of enclosed charge by mapping the field on a surface outside the charge distribution." Once you will have come to terms with this powerful law and its implication which I already mentioned(*) that a conductor's surface charge density is uniquely determined by the field right above that surface, then this controversy will be solved, at last :-) Michel (*) I first mentioned Gauss's law/theorem a few days ago, you may remember I applied it to a judiciously chosen small box, I suggest you dig that post out (search for "Gauss"). In my derivation I had forgotten a permittivity factor I realize, sorry about that, surface charge density sigma is in fact permittivity epsilon (or epsilon0 if dielectric is vacuum or air) times the field strength E (=delta_V/d in the case of two parallel plates), and its sign is determined by the field direction (negative if the field is towards the plate, which is the case for the most negative of the two plates). Capacitor equations follow from the above, see: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html#c2 (note again that the capacitor charge equation q=C*Delta_V alone should have sufficed to convince you, surely you realize that what you have been preaching at length contradicts this simple elementary equation, isn't this a bit foolish?)
