[SI-LIST] Re: FEM/BEM/MoM
- From: "Pratt, Gary" <gary_pratt@xxxxxxxxxxx>
- To: arafi001@xxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 28 Aug 2003 14:07:50 -0400
The quarter-Joule turns into heat when the switch is vaporized into a
cloud of smoke.
Whats the paradox?
Gary L. Pratt, P.E.
Product Manager
High-Speed Design Kits
Mentor Graphics
(503) 685-1177
gary_pratt@xxxxxxxxxx
-----Original Message-----
From: Abdulrahman Rafiq [ mailto:arafi001@xxxxxxx
<mailto:arafi001@xxxxxxx> ]
Sent: Thursday, August 28, 2003 12:36 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] FEM/BEM/MoM
Geoff,
Could you ellaborate on this a little more, as I am afraid i
can't quite see what happened to the 1/4 Joule of energy.
Perhaps if you could do a quick back of the envelope
calculation as an example.
-----------------
Just a little diversion to show the error in a schematic:
An old example often quoted is the problem of connecting two
one farad
capacitors together by a switch; one is at 1V potential, the
other zero.
The initial energy is 1/2 CV**2 =3D 0.5 joule. After closing
the switch th=
ere
is charge distribution, and energy =3D 1/8 +1/8 =3D 0.25
joule. Where did =
the
missing 0.25 joule go? If you did an electrical degree,
you'd see the
paradox. If you studied high frequencies, you'd know the
answer. (I did a
Physics degree, then built RF circuits, so for me a capacitor
is not a
capacitor.)
The answer is
1) it's not physically possible to put two capacitors
together at one point
2) therefore they are separated by a distance
3) therefore on closing the switch, the discharge current
travels a distance
4) the conductors have finite conductivity
5) therefore there is a varying electromagnetic field and
energy is
dissipated and radiated
6) please don't talk about too much about inductance because
it's only an
approximation.
-------------------
---------------------------------------
Abdulrahman Rafiq
Department of Physics
University of California
Riverside, Ca. 92521
Email: arafi001@xxxxxxx
URL: www.geocities.com/arafiq786
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