See figure attached.
The Attempt at a Solution
I have some concerns about his solution for part a).
I agree with how he's evaluated Ampere's law to find the field due to one of the strips at any point between the plates, but if you want to know the net field inbetween the strips, I think you must consider what the opposing plate is doing as well.
He seems to have neglected this, am I correct?
For example, let's say we want to find the field a distance z from the bottom plate, or in other words a distance (a-z) from the top plate. (Assuming the Y-axis starts at the top edge of the bottom plate)
The net field should have two contributing fields, one from each plate respectively.
Does it just so happen that due to the symmetry of the problem that the summation of these two contributing fields always works out to be the answer he's given? (Anywhere inbetween the two plates of course)
How can you prove that this is indeed the case?