1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

EM waves basic question

  1. Feb 2, 2014 #1
    EM waves do not need a medium for propagation.
    But isn't high frequency signal (like USB) flowing in a wire also electromagnetic.
    How come it needs copper as a medium.
  2. jcsd
  3. Feb 3, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    EM does not need a medium to propagate, but it may use a medium if one is available.
    If we cunningly contrive for it to use a carefully selected medium then we don't need so much power to get a useful signal. It's also much easier to get that signal to go exactly where we want it to go.

    Consider - radio waves propagate without a medium, but to hear a radio station we need to channel that energy into a speaker ... probably adding more energy to the signal first. We could just hold the speaker in the air and, if carefully designed, it could, in principle, vibrate with the radio waves. But it is much easier just to hold an antenna in the air, converting some of the EM waves in the air into EM waves in the wire ...
  4. Feb 3, 2014 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What Simon said.

    High frequency signal like USB can "leak" out of the wire they are propagating in (especially if not screened) and can travel through air or space. People who design systems such as computers and USB devices have to go to some lengths to ensure such "leaks" don't interfere with, for example, the users TV, his pace maker or the local police radio system :-)
  5. Feb 3, 2014 #4
    Thanks, Simon and CWatters.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook