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I Conservation of momentum in antennas

  1. Mar 14, 2017 #1
    When an RF antenna absorbs a signal, the momentum is transferred in quantized photon-electron interactions. The electrons in the metal will be given both perpendicular momentum (the actual signal information) and momentum in the direction of propagation (shown below):


    The momentum in the direction of propagation is a conservation of the photon's linear momentum, but what about the perpendicular momentum? Where does that come from? The photon's angular momentum/spin?
  2. jcsd
  3. Mar 14, 2017 #2


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    Unfortunately, that movie of a cheap and cheerful radiometer is not demonstrating radiation pressure. It is showing the difference in momentum imparted to air molecules by a hot and cooler surface. The dark surface gets a slightly higher equilibrium temperature than the shiny surface. But there are parallels where momentum is concerned. The normal force on the surface produces the turning effect whilst the lateral forces cancel out - whether it's photons or bouncing air molecules. To eliminate the air molecule effect, you need to drop the ambient pressure well below what you get in a 'Science Shop' version of the radiometer.​
  4. Mar 14, 2017 #3


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    Are you visualizing a cloud of free electrons? It sounds like scattering is a good mental model. Things scatter at many angles, not just linear and perpendicular. I say go look up the statistics of scattering.

    If the electrons are bound to atoms, the whole picture is more complex.

    If you want to analyze just the quantum states of a photon electron system, it is more of a QM question.
  5. Mar 14, 2017 #4


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    Wow, that's an interesting take on things. I'm sure the OP doesn't include the idea of photoemission of electrons. It's surely to do with the idea of internal motion of electrons within the metal. There's no way that incident radiation (except perhaps UV and higher) can do more than warm up the surface or be reflected. (the vanes are not made of potassium or any other group 1 metal.)
    Remember, you don't need QM to explain Radiation Pressure; it can be explained with classical EM theory. Yet again, photons can cloud the issue when they're introduced where not needed.
  6. Mar 14, 2017 #5
    Wow I'm a moron.

    I will look into scattering a little more. Thanks!
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