one for the theorist

[Martin Czech]

I've recently read a book where the inductance of a single piece of wire
was given. This is strange, I thought, because you can only speak of the
inductance of a loop. And, after all what I know, the magnetic energy
of a single wire without "back" current path goes towards infinity and
so does L. 

Contrary a normal twisted pair arrangement makes sense.


Example:

If you have a cylindric shaped wire then H=i/(2*PI*r).  The magnetic
flux will be phi=u*l* int{H(r)dr}

= i*u*l/(2*pi)*ln(r/r0)

(u=my, r0 radius, r distance)

The total flux will need r->infinity and this is clearly also infinite,
thus the inductance is also infinite.

This makes sense, you simply need the back current path to have finite flux.

So, is this a stupid book, or is it me who is the stupid?

m.c.