I simulated your problem about inductance of a wire,and actually the surface current density on the boundary flows from the ground to the terminal to form a loop circuit.

My question is if you plot the surface current density in the tutorial model “integrated square-shaped spiral inductor”,there will be also the so called loop, because the spiral inductor model is almost the same to your wire model,except a little geometry difference. So, does it also mean the spiral inductor’s inductance will also change if the dimensions of the outer air domain is changed?

What’s more, I didn’t try to test the relationship between the radii and convergence. ]]>

L = mu_r*mu_0*length/(2*pi)*ln(Rext/Rint) (H)

since both Rint and Rext are known when we design our cylinder.

and then compare this also to the known inductance of an “INF” straight conductor

L = mu_r*mu_0*length/(2*pi)*(ln(2*length/R)-3/4) (H) (approximation, ignoring cap effect)

And by normalising Wm, by using Rext, one can easily see the effect of the two cylinder caps, or even attempt the exercice by surrounding the straight inductor by a sphere instead of a cylnder

Having fun Comsoling

Ivar