1. The problem statement, all variables and given/known data An alternating current I cos wt flows down a long wire and returns along a coaxial conducting tube of radius a. Find E assuming E→0 at ∞. 2. Relevant equations 3. The attempt at a solution I did the problem from Griffiths but have some problem to understand the physical situation. I did using ∫E∙dl = (-dΦ/dt) and proceeded straight away as B=μn I cos wt. How much it is justified as I am simply neglecting the current returning through the conducting tube?