It is NOT a homework question. I am doing my revision and get stuck at this question. I am confused with the angle θ shown in this link: http://www.physicspages.com/2013/10/06/mutual-inductance/ Professor who wrote this solution stated that θ is the angle between unit vector z and unit vector r, but as what i understand, the angle between two unit vectors (in this question) is cos(180-θ) = -cos(θ) but it is different with the solution shown in the link. (Help me, I am so confused!) And also one more question. Why is the integration of area taken from 0 to b? If it is, aren't we taking a integration of disk, then? but in the question, it says it is a loop. Can someone please explain it briefly?