Discussion in 'Feedback and Suggestions' started by David Lewis, Jun 29, 2009.

1. ### David Lewis Thread Starter Member

Jun 29, 2009
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The web page says:

"Physical dimension also impacts conductivity."

Physical dimension should have no effect on conductivity (an intensive property). Physical dimensions would only affect conductance (an extensive property).

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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This is correct. Conductance is a dimensionless figure, while conductivity is per/unit volume.

eric

3. ### David Lewis Thread Starter Member

Jun 29, 2009
12
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I think the unit of conductance is siemen (formerly mho), which is ohm to the -1 power, and the unit of conductivity is siemen/metre.

Last edited: Jun 30, 2009
4. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Well spotted David, and welcome to AAC, you are quite correct if that is what the E book says, but for the life of me I can't find the reference.

Can you be more specific please?

We will look into it.

5. ### Unregistered Guest

Right below the bullet list of conductors and insulators, it's the first sentence of the third paragraph.

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Might also mention that there is both "volume conductivity" and "linear conductivity". Conductivity of a wire, would be in siemens/ft, while conductivity of soil, for instance, would be siemens per cubic foot.

7. ### Dcrunkilton E-book Co-ordinator

Jul 31, 2004
416
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"Physical dimension also impacts conductivity. For instance, if we take two strips of the same conductive material -- one thin and the other thick -- the thick strip will prove to be a better conductor than the thin for the same length. If we take another pair of strips -- this time both with the same thickness but one shorter than the other -- the shorter one will offer easier passage to electrons than the long one. This is analogous to water flow in a pipe: a fat pipe offers easier passage than a skinny pipe, and a short pipe is easier for water to move through than a long pipe, all other dimensions being equal."

If we were to replace conductivity with conductance the paragraph would be technically correct. However, I do not see that conductance is discussed at all in this section. Therefore, I would be inclined to delete the whole paragraph, unless there is a strong opinion to add the discussion of conductance to this section.

8. ### studiot AAC Fanatic!

Nov 9, 2007
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Thank you for highlighting the offender Dennis.

I aggree the paragraph adds nothing useful at this point in the explanation, but the concept it contains is important and should appear somewhere in the Ebook, with the appropriate wording.

9. ### Dcrunkilton E-book Co-ordinator

Jul 31, 2004
416
11
I can effectively remove the paragraph from the html and pdf by commenting it out of the .sml source. It will still be there in the source. It just won't show.

At this time I have no idea where to move it to in the books.

10. ### David Lewis Thread Starter Member

Jun 29, 2009
12
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KL7AJ wrote: "...conductivity of soil, for instance, would be siemens per cubic foot."

If the conductivity of a soil sample doesn't vary with respect to sample volume
then the conductance of two cubic feet of soil needs to be twice the conductance
of one cubic foot of the same soil.

Last edited: Jul 1, 2009
11. ### David Lewis Thread Starter Member

Jun 29, 2009
12
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Subsequent research seems to suggest
when conductivity is specified in siemens per cubic foot,
"cubic foot" doesn't refer to a unit of volume but rather
describes the shape of the test sample (a cube that measures
one foot on each side), so conductivity under this scenario would
be numerically equal to the conductance of a 1' x 1' x 1' sample cube,
and the unit should more correctly be called seimen-foot/square foot
(or siemen/foot).

No matter what you're measuring, or how you're measuring it,
or what units you use, conductivity has nothing to do with
the size, shape or dimensions of the test sample,
assuming the sample is isotropic and homogenous.

12. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
Was going to post but you seem to be working this through OK David, keep it up.

conductivity can be thought of as a limit in calculus and I agree that it is an intensive property.

Howevr conductance is not an extensive one as it also depends upon the shape of the matter present as well as the quantity. You can get a range of conductance by different arrangements of the same matter particles. You cannot do this with mass no matter how you arrange the particles.

Last edited: Jul 2, 2009
13. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

This is correct. However the reason volume resistivity (or volume conductivity) is an important parameter, is because in many cases, (such as soil measurement) the INTERFACE to the sample is of a different dimension than the sample itself.

For example, of you drive a couple of ground rods into the earth, you have MULTIPLE conductive paths between the rods...in fact, an infinite number. The current density through each of these paths is not uniform, but rather, follows an elliptic function as the rods are separated.

These same issues arise when trying to characterize electrolytic solutions (as in batteries) or other chemical processes.

Adding to the "fun" of this whole process, we find that ground conductivity (admittance) is actually a COMPLEX function....which becomes very significant at radio frequencies.

So...nothing is ever as simple as it seems.

eric

14. ### David Lewis Thread Starter Member

Jun 29, 2009
12
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Kind thanks to KL7AJ and studiot for correcting my errors.

15. ### Dave Retired Moderator

Nov 17, 2003
6,960
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Thanks for your suggestions, David Lewis.

Would it be an idea to modify the text to introduce the allied concepts of conductivity and conductance at this point?

Something to the effect of, a statement that conductivity is the specific conductance where conductance is a function of the physical dimension (size and shape). This would allow us to move on to embellish this point with examples from this thread. The idea would provide a rudimentary introduction without omitting fundamental concepts.

What do you think?

Dave

16. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
515
I agree, indeed I started on just such a paragraph.

I think the author was talking about 'the ability to conduct' as a concept, but unfortunately used words which have specifically defined meanings.

So replacing the instances with my phrase would preserve the valuable idea presented in the paragraph.

A second paragraph or a link to it in another part of the Ebook could then provide the formal definitions.

17. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Ok. I'll try and flesh something out and we can work on a solution.

Like a glossary? That would be real desirable for the e-book.

Dave

Jun 29, 2009
12
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19. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
As a curiosity, what are the conditions of use on your glossary page? Would someone be able to use that glossary on their own site with appropriate attribution to yourself?

Dave

20. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Have you guys noticed the pdf version has a complete index at the end? It would be nice to have something similar for web version, but I think I understand why this would be a major hassle. Still.