Conductivitiy vs Resistance

[bphgravity]

Basic Ohm's Law tells us there are directly and inversely proportional relationships between electrical fundementals. Of those relationships, I assumed conductivity related to resistance in that as resistance goes up, conductiveity goes down and vice-versa.

However, I was reading a report on electrical burns and injuries the other night and it stated that people have varying resistances and conductivities and that one's particluar resistance is independent of one's conductivity. The paper indicated that your body's conductivity changes constantly with different variables and resistance will change as well, but not in response to your conductivity.

Is this acurrate? Can something have a fixed conductivity that does not change as the reistanc eof the same material varies?

 

[steve66]

Re: Conductivitiy vs Resistance

You are right in that in the normal use for "conductance" and "resistance" , they are directly inversely proportional. They are really two different ways to refer to the same thing.

It might be possible that the author wanted to refer to two different resistances, and he used conductance and resistance to differentiate between the two. For example, there is skin resistance, and there is the resistance of internal organs. There is resistance between your hands, and there is resistance from one hand to ground. The resistances could even vary at different voltage levels. But the author should have made that clear.

 

[physis]

Re: Conductivitiy vs Resistance

I knew I'd see you here Steve.

Resistance and conductivity are not closely related.

They are both mathematical representations of a single property.

They're reciprocals of each other. You can't change only one of them and not be wrong.

Edit: Error A again.

[ June 08, 2005, 06:25 PM: Message edited by: physis ]

 

[rattus]

Re: Conductivitiy vs Resistance

G = 1/R and vice-versa. That is all there is to it.

Like Yogi Berra once said, "Good pitching beats good batting anyday and vice-versa"!

 

[bphgravity]

Re: Conductivitiy vs Resistance

Let me ask the question in antoher way.

Doesn't copper have a fixed conductance, yet I can change the resistance of that copper by making it longer or smaller in circular mills? Or does that indeed also change the conductance of the copper as well?

 

[charlie]

Re: Conductivitiy vs Resistance

As Sam and Rattus has already said, they're reciprocals of each other. By the way, the units for resistance is ohms; the units for conductivity is mhos. I wonder how they came up with that?

 

[physis]

Re: Conductivitiy vs Resistance

Doesn't copper have a fixed conductance, yet I can change the resistance of that copper by making it longer or smaller in circular mills? Or does that indeed also change the conductance of the copper as well?

I think you're talking about resistivity. That's also a measure of resistance, but with defined parameters. You know what K is. And if you divide 1 by that quantity you'll also get conductance.

The only thing fixed about it is that you can expect to measure the same quantity for two identical samples if all of the relevent parameters are also identical.

 

[rattus]

Re: Conductivitiy vs Resistance

Originally posted by charlie:
As Sam and Rattus has already said, they're reciprocals of each other. By the way, the units for resistance is ohms; the units for conductivity is mhos. I wonder how they came up with that?

Charlie, you are showing your age. Mhos are obsolete, the unit of conductance is now "Siemens".

But, that is OK, I am older than you and prefer Mhos.

Sam, resistivity is a property of the material, say copper.

R = rho x length/area.

where R is in Ohms, rho is in Ohm-cm, length is in cm, and area is in square cm.

"cm" means centimeters, not circular mils.

 

[physis]

Re: Conductivitiy vs Resistance

the units for conductivity is mhos

Mho should have his own law. something like:

1/V=(1/I)(1/R)
1/R=(1/V)/(1/I)
1/I=(1/V)/(1/R)

It's easier to read like this if the exponents -1 turn out right.

V????-? = I??-?R??-?
R??-? = V??-?/I??-?
I??-? = V??-?/R??-?

Edit: I wonder what the symbol would have been for a mho?

[ June 09, 2005, 01:48 AM: Message edited by: physis ]

 

[physis]

Re: Conductivitiy vs Resistance

By Rattus:

Sam, resistivity is a property of the material, say copper.

Thanks Rattus. If I only knew what T represented.

Edit: Left out a word.

[ June 09, 2005, 02:07 AM: Message edited by: physis ]

 

[steve66]

Re: Conductivitiy vs Resistance

Physis:

I wonder what the symbol would have been for a mho?

I don't know if you are kidding or not, but that is indeed the symbol for conductance.

STeve

 

[charlie b]

Re: Conductivitiy vs Resistance

We are using four separate terms here. Let me try and put them into their respective places.
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Conductivity is the reciprocal of resistivity. These two terms have a relationship to length.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Resistivity is measured in units of ohm-meters.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Conductivity is measured in units of siemens per meter. This unit is the same as the reciprocal of ohm-meters</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Conductance is the reciprocal of resistance. These two terms have had the ?length term? removed from consideration.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Resistance is measured in units of ohms.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Conductance is measured in units of siemens, which is equivalent to the reciprocal of ohms.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Now for the conversion process:
</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">A chunk of Copper, as an example, will have the property of resistivity (ohm-meters).</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Multiply resistivity times the length of the copper, and divide by the cross-sectional area, and you get resistance (ohms).</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">The units of measure work out as follows: (ohm-meters) times (meters) divided by (meters squared) gives you (ohms).</font>

<font size="2" face="Verdana, Helvetica, sans-serif">

 

[rattus]

Re: Conductivitiy vs Resistance

We have been talking about bulk resistivity, but there is also sheet resistivity in Ohms per square used in thin film work for example.

Chew on that for a while Sam.

 

[physis]

Re: Conductivitiy vs Resistance

You are indeed correct Charlie B. When I read Bryans post I saw conductivity as conductance. And I would venture to say the rest of us made the same mistake. When I went and read the post again, sure enough, he never did say conductance. Not a defense, but I guess not using these terms a lot and almost never conductivity.

That's why grounded, grounding and ungrounded bothers me. If you don't have a lot of practice with the terms, they're easily mixed up. And the people who write and use the NEC don't even see that it's a problem because, for them, it isn't.

________________________________________________________________________

Steve, my sarcasm and humor can be "too" subtle sometimes. Check out the dude sticking his tongue out.

________________________________________________________________________

Rattus,

By Rattus:

We have been talking about bulk resistivity, but there is also sheet resistivity in Ohms per square used in thin film work for example.

Chew on that for a while Sam.

Charlie B. and Bryan have, I don't know about the rest of us.

Adding dimensions to resistance is very interesting to me. I've done area resistance calculations a pretty long time ago for sheilding, (using this spray on conductive stuff) and I don't remember it being very tricky. But I don't remember how to do it and I'd have to look it up or think about it.

Three dimensions would be even chewier.

 

[physis]

Re: Conductivitiy vs Resistance

And 1 typed in the 1etters that make the word conductivity too.

This is a great illustration of how important 1anguage is.

it's not good that two very similar ideas are represented by two very similar symbos.

[ June 10, 2005, 03:52 AM: Message edited by: physis ]

 

[rattus]

Re: Conductivitiy vs Resistance

Sam, think of laying out a power bus on a circuit board with a maximum resistance of "R". The width is easily calculated as follows:

W = L*rho/R

where L is the length, and rho is the resistivity in Ohms/square.