[SI-LIST] Re: what is the conductivity of a dielectric?

Sainath,

I am not certain how to answer your question. Any time you have time 
varying electric fields you will have displacement current. Therefore any 
structure that is excited by time varying fields will be described, in 
part, by displacement current. Just like every electric circuit is 
described, in part, by voltage, or whatever other associated parameter you 
care to choose. A correct field solution depends on using the correct 
mathematical model. Displacement current is part of that model.

The best example I can give you is a capacitor connected to an AC 
generator. If you recall from static field theory:

int(H dot dl) = int(J dot ds) = I

What this says is that the line integral of H dotted with dl along the 
boundary of a surface S is equal to the current flowing through the surface 
S. The choice of the surface is arbitrary as long as it is bounded by the 
line integral path. Referring back to the AC generator with a capacitor 
across the terminals, I can choose the line integral path to circle around 
the wire connecting the generator to the capacitor but the surface I can 
choose either to cut the wire or to pass between the parallel plates of the 
capacitor. The line integral is the same regardless of the choice of 
surface and gives a value equal to the current in the wire, but clearly 
when the surface passes between the plates of the capacitor, the conduction 
current through the surface is zero. This is a contradiction that 
displacement current saves us from, because in dynamic fields the J term is 
really J = Jc + Jd. Between the plates of the capacitor, the conduction 
current density, "Jc", is zero but the displacement current density, "Jd", 
is not.

I suppose you could say that displacement current is a dominant feature in 
the performance of a capacitor but I still consider that an over 
simplification.

I hope this helps.

Regards,

Rob Hinz
Principal Engineer
SiQual Corporation
rob@xxxxxxxxxx
phone (503)885-1231
fax   (503)885-0550
http://www.siqual.com


At 11:50 AM 4/12/2002 -0800, Sainath Nimmagadda wrote:
>Hi Rob,
>
>After reading your nice explanation on displacement current, I wanted to
>ask for examples. Considering the vast chip/package/board development
>arena, which features(for example, vias) come to your mind whose
>function can be explained, wholly or partly, using displacement current?
>That would really help understand it.
>
>Thanks,
>Sainath

Rob Hinz
Principal Engineer
SiQual Corporation
rob@xxxxxxxxxx
phone (503)885-1231
fax   (503)885-0550
http://www.siqual.com

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