What do we mean exactly by inductance of two conductors? I'll illustrate what I mean with a problem, but the question is not about this particular question. Instead I am looking for a general principle.
The problem is:
Calculate the inductance of a unit length of a double tape line as shown in figure. The tapes are separated by a distance h which is considerably less than their width b.

I have two questions:

*

*what do we mean exactly by inductance for system of two current carrying conductors? For a single current carrying loop its defined as the flux linked due to its own current divided by current. But how to extend this idea to systems like this?


*what linked flux are we interested in general to calculate inductance? I mean what area we take in account to calculate flux and thus inductance of two conductors?
 A: (a) The system in your diagram usually arises because the ribbons go on beyond the jagged ends and eventually connect to a load of some sort, so that the ribbons  are 'forward and return' conductors of the same circuit. That's what I've assumed below. I don't think we can define the self inductance of two conductors that are not part of the same circuit. All we could do in that case, if we are to include both conductors, is to define their mutual inductance. Judging by your diagram, with equal and opposite currents in the ribbons, this question is not about mutual inductance.
(b) The set-up is actually surprisingly similar to a solenoid of length $b$ and of rectangular cross-section with its axis parallel to the ribbon width $b$. The ribbons are the top and bottom sides of the solenoid; the 'side sides' of the solenoid are missing, but that doesn't matter because the ribbons are so long and so close together that the missing sides hardly affect the field in the gap between the ribbons.
(c) You should now see, by comparison with a solenoid, or by going back to Ampère's Law, that (except at the edges) the magnetic flux between the ribbons is uniform and directed right to left.
You should check that the flux, $\Phi$, linked with length $l$ of the double ribbon is
$$\Phi=BA=\frac{\mu_0 I}{b} hl$$
From this it is trivially easy to work out the inductance per unit length of the double ribbon.
