Calculating Self-Inductance in Coaxial Cables

[bon]

Homework Statement

A coaxial cable is made from concentric cylindrical conductors. The radius of the inner conductor is a and the outer comductor has inner radius b and outer radius d. Calculate the self inductance per unit length of the cable. You may assume that (b-a) >> a and (b-a) >> (d-b). Why is this assumption necessary.

Homework Equations

The Attempt at a Solution

Ok so i know that if we take the conducting plates to be think i can work out the inductance to be u0/2pi ln(b/a)

But what difference does it make that the outer one has an inner and outer radius? How do i work it out now?

Also why is the assumption necessary?

 

[bon]

Can Ampere's law always be applied without regard for what is outside the amperian loop?

So in this case, in the space between the inner cable and outer cable can i just draw an amperian loop and apply ampere's law? If so - why do i need to assume (b-a) >> a and (b-a) >> (d-b)?

Thanks

 

[bon]

anyone?

 

[bon]

should i post this in advanced phys?