Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is RF/EM?

  1. Jul 9, 2004 #1
    What is RF/EM??

    Did you notice how modern sources (textbooks, tutorials, industry experts) treat Electrical Force and Magnetic Force quite differently than propogating RF/EM waves? Why?

    When I'm learning all this, for some weird reason, I always assumed that "proagatin EM waves" were just magnetic and electrical forces interracting at large distances?

    How about this?: Say a cell phone is talking to a basestation. Sure RF is flying through the air to dive into the basestation anntenna. BUTTTT Can't we also think of it as: The vibrating electrons in the antenna of the cell phones are causing corresponding vibrations in the electrical and magnetic forces they offer the very far electrons in the basestation. I am picturing a group of electrons in my cell phones jumping around wildly in some pattern.. and another group of electrons following in the same relative pattern far away (in basestation antenna). If you move a magnet, you can move a similar magnet that is a certain distance away. If you move a charge, you can cause relative movements in a charge a certain distance away. Why doesn't anyone consider RF propagation to be "FORCES AT A DISTANCE"? Why is the study of ELECTRICAL AND MAGNETIC FORCES so disjoint from EM THEORY?

    Can someone explain?
     
  2. jcsd
  3. Jul 11, 2004 #2

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm not sure that I understand your question but I think you are asking if EM effects can be viewed as action at a distance. No. The EM wave travels along from the source and acts directly on the electrons at the reciever. No "spooky action at a distance" in EM AFAIK. The signal propogation delay is evidence of the "travel" from one point to another. Also, Maxwells equations describe and accurately predict the behavior of a self sustaining, travelling wave. Also, we can block or intercept the wave somewhere in between the source and reciever. Again this argues against any spooky action.
     
    Last edited: Jul 11, 2004
  4. Jul 11, 2004 #3
    Radiation is different than magnetic and electrostatic fields ( even moving ones) The propogation of energy ( meaning it is lossed to the generator ) is due to accelerated electrons in the antenna conductors and it represents kinks in the other wise smooth fields, These kinks travel at ~ 'c' the light velocity in all sorts of directions -- they are subject to reflection diffraction refraction etc and multipath so that what ends up at the receiving antenna is not a one to one correspondence. Also the energy picked up by the antenna is also an accelleration phenonemon.
    so for example if a steady current flowed in the transmitter then despite their being magetic and elecric fields ( even as far as the receiver) no energy would be detected.
    An example of non- correspondance is when a receiver a few meters from a transmitter receives nothing because the arriving waves cancel out ( think of radio or cell phone fading).
     
    Last edited: Jul 11, 2004
  5. Jul 13, 2004 #4
    But.

    Yes there is a delay with EM. Yes they can be blocked. But electric fields and magnetic fields can be blocked too. Thank you for the wealth of information, I haven't gotten to the maxwell equations part of my study of EM- still dealing with first year courses (basic fields).
     
  6. Jul 19, 2004 #5

    turin

    User Avatar
    Homework Helper

    Consider two charges, Qsource and qtest, 1 kilometer apart. Now, imagine that Qsource disappears. The other charge, qtest, would (according to the WELL TESTED theory) actually feel the electrostatic repulsion from Qsource for about 3 more μs, even though Qsource does not exist for those 3 μs. An EM wave is basically this same idea, except, instead of vanishing, the charge simply moves back and forth.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?