Re: funny capacitor (EUREKA ?)

[brian whatcott <inet@INTELLISYS.NET>]

At 14:32 3/18/01 -0500, John Denker wrote:
> ....
> An electrical engineer will tell you that at DC, capacitors don't even
> exist. ....


Let me tell you how a person, as it might be an engineer or a
physicist, can use the DC properties of a series
string of capacitors as an illustration of how the charge applied
to these components cannot necessarily be deduced from a knowledge
of the DC volts then applied to the ends.

One prepares a series string of three capacitors large enough
to hold a reasonable charge for a reasonable time.
 They could be placed in a black box.
One charges cap1 to plus 10 volts, one charges cap2 to -10 volts.
One checks  cap3 is uncharged.
(At DC, the capacitors are holding a charge. This is the fundamental
definition of a capacitor, I suppose, so yes! caps exist at DC.)

Demonstrate that there is zero volts across the end of the caps,
then connecting them to a 10 volt source though a 1 megohm resistor,
note the time for the charging current to fall to 1/e of its
 initial value.
 Ask if the charge stored on the capacitor string can now be deduced.

(This was something like the example I mentioned some years ago
on the list, during one of the famous capacitor dissention threads).




brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!