How is E Calculated in a Coaxial Cable with Alternating Current?

  • Thread starter Thread starter Kolahal Bhattacharya
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    Magnetism
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Homework Help Overview

The discussion revolves around calculating the electric field E in a coaxial cable carrying an alternating current. The original poster presents a scenario involving a long wire and a coaxial conducting tube, seeking to understand the implications of the current distribution and its effect on the electric field.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply Griffiths' approach using the integral form of Faraday's law, questioning the justification for neglecting the return current through the conducting tube. Other participants discuss the application of Ampere's law and the relevance of the current distribution in determining the magnetic field B.

Discussion Status

Participants are exploring different aspects of the problem, particularly the justification for the choice of Ampere's loop and the implications of the current distribution on the magnetic field. There is no explicit consensus, but guidance is being offered regarding the application of Ampere's law and the relevance of the fields inside and outside the coaxial setup.

Contextual Notes

There is a focus on the assumptions regarding the current distribution and the physical setup of the coaxial cable, with some participants questioning the validity of neglecting certain currents in the analysis.

Kolahal Bhattacharya
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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 ∞.

Homework Equations





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?
 
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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.
 
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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