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**1. Homework Statement**

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. Homework 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?