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Andy Resnick
May27-09, 07:33 AM
Sci Advisor
P: 5,523
Quote Quote by HarryWertM View Post
Reading Feynman, there are various examples of finding voltages, forces, et cetera, when you know the charge[s] involved. But going other way? Almost nothing. To be specific, how do you answer the following question [[NOT a homework question; self invented question]]:

A vacuum tube plate is connected to a 500 volt plate bias power supply. The plate is 1 sq cm [[1 cm high ; 1 cm circumference]], 5 mils thick. When there are no other influences on the plate, how many electrons , in Coulombs, are missing? IE, what is the charge?

-Harry Wertmuller
I think there are some conceptual issues here. First, charge is a property of ponderable matter- AFAIK,there is no massless charge. Charges can be free or bound as well- the particles can move or not. Voltage, however, is a mass-free concept. Voltage is a way to describe potential energy for charged matter, just as gravity is a way to describe the potential energy of ponderable matter. More specifically, the absolute value of a voltage is not particularly meaningful, it's the voltage drop or gradient that is considered useful.

So, while an isolated bare wire has lots of charged particles, the two ends of the wire are not at different voltages. Connecting the wire across a battery (for example) to induce a voltage drop from one end to the other will cause free charges to move (current). As another example, the voltage drop across a cell membrane is about 60 mV. This voltage is maintained to allow the *controlled* flow of ions into and out of the cell.