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'[EE] Resistance between 2 points of a uniform mate'
2005\10\21@100914 by

I have a theoretical question to ask.

Consider an infinite plane of a uniform resistivity material (e.g.,
conductive foam).

Take two points A and B a fixed distance apart in this plane:

<^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
<                                                >
<        A                        B                >
<                                                >
<vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv>

How, using techniques of electrical engineering, can the resistance in ohms
be calculated between points A and B?

Obviously it can be measured easily in        practice by simply placing one
ohmmeter probe at A and the other at B. But how could it be calculated from
theory?

It would seem this problem should be easy, similar to EMF field
calculations for potential difference, etc., however it's not obvious to me
as to how this could be done.

Any suggestions or experience?

The importance of the theoretical analysis is obvious: It allows answers to
questions about parametric dependence on length and width of a finite
rectangular piece, extends to non-uniform resistivity, etc.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

{Quote hidden}

http://rec-puzzles.org/new/sol.pl/physics/resistors

Number 5

Regards

Mike

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At 10:17 AM 10/21/2005, Michael wrote:
> >How, using techniques of electrical engineering, can the
> >resistance in ohms
> >be calculated between points A and B?
>
>http://rec-puzzles.org/new/sol.pl/physics/resistors
>
>Number 5

Thanks! This seems directly in point.

I found you can a copy of this book ("Random Walks and Electrical
Networks") under a GNU license at:

http://arxiv.org/abs/math/0001057

However, I was unable to find the solution formula given on the website you
reference anywhere in the book.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

Robert A. LaBudde wrote:
> Consider an infinite plane of a uniform resistivity material (e.g.,
> conductive foam).
>
> Take two points A and B a fixed distance apart in this plane:
>
> <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
> < >
> < A B >
> < >
> <vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv>
>
> How, using techniques of electrical engineering, can the resistance in
> ohms be calculated between points A and B?

I remember working out exactly this case many years ago back in school.  The
resistance is infinite unless "points" A and B have a non-zero radius.

> Obviously it can be measured easily in practice by simply placing one
> ohmmeter probe at A and the other at B.

Not so obvious.  Try it and you'll see it has a lot to do with the contact
area at each probe tip.  If the material deforms easily, you will get very
different answers depending on how hard you press the probes into the
material.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--- Olin Lathrop <olin_piclistembedinc.com> wrote:
Yeah, I remember that one. The no-zero radius requirement comes from the
resistivity equation :- R = rho x length/CSA. If CSA --> 0, R goes infinite. As
Olin says, it applies to a probe pressed against the surface (point contact) as
well as an imaginary cylinder of radius "r" drilled thru the block.
Debbie

{Quote hidden}

> --
The black antic static foam that protects ic's is a good example of such a
resistive material.   Would be interesting to measure resistance across it
and then cut off pieces to narrow path while logging resistance.

Also, this stuff will change resistance when compressed.  So it could be
used as a push thingee and maybe even give a degree of how hard it was
pushed.  Would it be possible with more than one resistance probe connected,
to determine the xy position of the push?

Bill

{Original Message removed}
> The black anti-static foam that protects

> Also, this stuff will change resistance when compressed.  So it
> could be used as a push thingee and maybe even give a degree
> of how hard it was pushed

Tried it a long time ago when experimenting trying to make a
linear joystick. Trouble is, the stuff is loaded with so much
carbon that it doesn't have a very long life-time. It loses its
bounce quite quickly. For occassional use or slight compression
it might do as an on/off sensor. Into a comparator for example

I also tried the black conductive (anti-static) plastic bags. That
might work as a sensor, but the lineal resistance is very high

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