Understanding Conductance and Resistance in Coaxial Cables

[brad sue]

Hi ,
I have this problem but I cannot figure out the last question.

A coaxial cable has an inner radius of a and an outer radius of b. between the two conductors is a material with conductivity σ.
1-Determine the resistance between the two conductors for a one meter length of cable.
R=1/(σ*pi*(b^2-a^2))


2- What would be the conductance G be?
G=σ*pi*(b^2-a^2)

3- What would be the conductance be for a 100 [m] length cable?
G=σ*pi*(b^2-a^2))/100

4-Explain why the cable is characterized by a conductance per unit of length (not a resistance per unit of length)

I have no sure answer for it.
I would say to avoid to have infinite resistance if a tends to b!??
Please can someone help me with this question?

 

[berkeman]

I have never, ever heard of a coaxial cable characterized by its conductance per unit length. What is the source of this question? Transmission line media (especially coax cables) are characterized by their characteristic impedance Zo. Can you show the general equation for Zo = ?

There are only a couple places in EE where it is more convenient to use the conductance, and transmission line impedance is not one of them IMO.

 

[brad sue]

I reall cannot answer you. the problem was asked as is.
I was as shoked as you are when I read the problem.
B

 

[berkeman]

Have you been able to ask the prof or TA about this problem? I've been working in the industry for a fairly long time, including lots of time with transmissions lines of many different bandwidths, and haven't heard of conductance per unit length used for anything except for the extended terms in the Zo equation.

Again, can you write the general equation for Zo, and show where the conductance per unit length term shows up? That's the only related thing I can think of.