Lets say I have a pipe and then another pipe inside a larger pipe.

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Discussion Overview

The discussion revolves around the electromagnetic behavior of a coaxial configuration consisting of an inner pipe carrying an AC current and an outer pipe carrying an equal and opposite AC current. Participants explore whether this setup radiates electromagnetic waves, considering the implications of the enclosed currents and the resulting electric and magnetic fields.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant posits that the configuration will not radiate due to the nature of coaxial cables, suggesting that signals are transmitted without radiation.
  • Another participant questions this assertion, asking if a single wire with AC current would radiate, implying that the inner pipe should also radiate.
  • A further response clarifies that while a single wire would radiate, the coaxial arrangement confines the fields between the pipes, preventing radiation from escaping.
  • Another participant adds that the two opposing currents create field amplitudes that cancel out at large distances, contributing to the argument that radiation does not occur.

Areas of Agreement / Disagreement

Participants express differing views on whether the inner pipe radiates. While some argue that the coaxial configuration does not radiate, others contend that the inner wire would radiate if it were alone, indicating a lack of consensus on the overall behavior of the system.

Contextual Notes

The discussion does not resolve the underlying assumptions about the nature of radiation in coaxial configurations or the specific conditions under which radiation might occur. The implications of field confinement and cancellation at large distances remain open to interpretation.

cragar
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Lets say I have a pipe and then another pipe inside a larger pipe. On the inside pipe I have an AC current I(t) and on the outer pipe I have to opposite AC current -I(t). So that when I am outside both pipes and I make my Amperian loop, the current enclosed is always zero. Now my
question is will this radiate. Between the inner and outer pipe there should be a B field because I have enclosed current and it is changing with time so there should be an induced E field. So does the inner pipe radiate. Or what radiates if anything. Any input will be much appreciated. And the pipes are infinitely long.
 
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This doesn't radiate. It's called a coaxial cable and if you watch TV you're using it to get the signals.
 


I don't understand why the inside one would not radiate. If I just had one wire with AC current would that radiate?
 
cragar said:
I don't understand why the inside one would not radiate. If I just had one wire with AC current would that radiate?

If you had just one wire it would radiate.

With the coaxial cable, the fields are confined between the two pipes. So there is radiation of a sort, but it's being channeled to run down the space between the pipes and it can't get out.
 


Another way to see why it doesn't radiate is that the two currents create field amplitudes at large distances that cancel each other out.
 

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