1. The problem statement, all variables and given/known data A thin hollow cylinder of radius a is surrounded co-axially by another hollow cylinder of radius b, where b>a. An electric current I flows through them (I is into the plane of paper (x) in outer cylinder and coming out of plane of paper (.) in inner cylinder). Find the: a) Self inductance per unit length. b) Magnitude of the pressure exerted on each cylinder and state whether the force on each cylinder is tending to burst apart or to collapse the cylinder. 2. Relevant equations ∫B.ds=μoIinside Φ=B.A L=Φ/I F=(j.ds)*lxB 3. The attempt at a solution The magnetic field inside the inner cylinder and outside and outer cylinder will be zero. Magnetic field will exist only in a<r<b region. So magnetic field at a distance r will be B=μoI/2πr. We then take a rectangular strip of length l and width dr. Flux through it will be: dΦ=B.l.dr Integrating this from a to b, we get total flux through that region. Self inductance/unit length will be Φ/(i*l) which can be easily calculated. In the first part, I don't understand why we are calculating flux through a rectangular strip and then integrating it. Why aren't we taking total flux through a circle of radius r and thickness dr? In the second part, all I can think is to calculate force on each of the two cylinders. For calculating force, we need net B on the cylinder. Can anyone tell me how to do it? Kindly help. Edit: I got the second part. It was similar to calculate electrostatic pressure at a point. Please tell why we have assumed a rectangular strip in first part. Thanks.