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

  1. Jan 8, 2012 #1
    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.
  2. jcsd
  3. Jan 8, 2012 #2
    Re: Radiation?

    This doesn't radiate. It's called a coaxial cable and if you watch TV you're using it to get the signals.
  4. Jan 8, 2012 #3
    Re: Radiation?

    I don't understand why the inside one would not radiate. If I just had one wire with AC current would that radiate?
  5. Jan 8, 2012 #4
    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.
  6. Jan 8, 2012 #5

    Ken G

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    Gold Member

    Re: Radiation?

    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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