[SI-LIST] Re: Concept of Voltage

  • From: Albert Ruehli <albert.ruehli@xxxxxxxxx>
  • To: Shawn Hermite <shawn.hermite@xxxxxxxxx>
  • Date: Mon, 18 Jul 2011 09:20:15 -0400

Hi Shawn
What you are saying is that we have to be careful in using voltage which
is the difference between two potentials to infinity (at least in 3D) .  The
terminals cannot be too far apart, else we make an error.  This is correct.
So, it really is also like a port in 3D which we know works well.

The potential is consistent with Maxwell's equations as well as circuit
theory
as the PEEC method shows.  Some  think that circuit theory is limited
to low frequencies which is not correct.

Albert Ruehli

On Mon, Jul 18, 2011 at 2:06 AM, Shawn Hermite <shawn.hermite@xxxxxxxxx>wrote:

> To SI experts, especially those used Maxwell's equations in their books:
> The concept of voltage has been deeply engrained in our minds. It's the
> foundation of the circuit theory and electrical engineering at large.
> Recently, when collecting materials for a presentation advocating the
> importance of signal integrity, I realized that the concept of voltage is
> based on the assumption of the electric field being conservative. The
> general differential form of Maxwell's equations (the Faraday's Law part in
> particular) contains the dB/dt term that ruins the validity of curl {E} =
> 0.
>
> There are two categories of arguments:
> (1) With the integral form of the Faraday's Law, we can treat the
> time-varying magnetic flux term as an electromotive-force (like battery),
> hence fix the KVL, the general idea of voltage is still being the line
> integral of the electric field between two points.
> (2) Switching to the frequency-domain, the surface integral of the B-field
> is related to the characteristic dimension of the system (D) and the
> operating frequency. It goes with some hand-waving arguments, and the claim
> is that the unfriendly term is nearly zero when D << wavelength. This
> argument also goes hand-in-hand with validity of lumped vs. distributed
> element modeling.
>
> I have also observed that in a multi-conductor transmission-line (MTL)
> system where the TEM mode is propagating, the E-filed is conservative on
> each cross-section, thus voltage is well defined between a signal conductor
> and the common-reference conductor.  Here, the general electrodynamic
> Maxwell's equations are all satisfied. The MTL theory explains why
> RF/microwave testing has to be done using co-ax cables. It also makes sense
> why 'port' (instead of circuit node) voltage is used in constructing
> S-parameters.
>
> If the second explanation holds true, I am really worried about what the
> frequency limit is before we get into trouble with freely applying
> knowledge
> developed with DC or low-frequency circuits.
>
> Any way, looking for a better answer or proof on the validity of the
> concept
> of voltage.
>
> Thanks
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                http://www.si-list.net
>
> List archives are viewable at:
>                //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
>                http://www.qsl.net/wb6tpu
>
>
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                //www.freelists.org/archives/si-list
 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: