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Another problem from magnetism

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?
 

Answers and Replies

Meir Achuz
Science Advisor
Homework Helper
Gold Member
2,169
63
B comes from Ampere's law, which includes only the current within the loop.
The current in the outer tube is outside the loop.
 
How do you know where to construct Ampere's loop?At first you are to justify that the B field must be 0 outside the tube.
 
Meir Achuz
Science Advisor
Homework Helper
Gold Member
2,169
63
Ampere's loop is a circle within the tube, where you need to know B.
The field outside the tube ils irrelevant.
 

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