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

  • Thread starter cragar
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  • #1
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Main Question or Discussion Point

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.
 

Answers and Replies

  • #2
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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.
 
  • #3
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I don't understand why the inside one would not radiate. If I just had one wire with AC current would that radiate?
 
  • #4
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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.
 
  • #5
Ken G
Gold Member
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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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