I am using David K. Chengs's Fundamentals of Engineering Electromagnetics book. In chapter 5/question 5.2, there is a given closely wound toroidal coil, and it asks what is the magnetic flux density at the inside.
This question is okay but at the end it says;
"It is apparent B=0 for r<(b-a) and r>(b+a) since the net total current enclosed by a contour constructed in these two regions is zero."
I understand, at r<(b-a) region, there is no current therefore B=0, but I did not understand the other part which is r>(b+a) how total net current is zero here?
Please see the picture of a question to understand better.