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Shouldn't a DC current emit EM waves?

  1. Nov 26, 2009 #1


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    So in a DC current we have a relative uniform movement of electrons in one direction. If we would have a very narrow coil around the dc current carrying wire, (like an atom wide), every time an electron pass trough the plan of the coil, should induce a voltage in the coil, so theoretically we should get a high frequency current in the coil circuit. Is this true in reality or the magnetic field of the moving electrons will blend (physically) together, forming a much uniform field?
  2. jcsd
  3. Nov 26, 2009 #2
    You are correct in that a "DC" current is actually a series of pulses as individual charges pass by. In currents in wires, the non-uniform statistical time interval between charges produces an AC signal, called the shot noise or Schottky noise. In radio receivers, this noise sounds like a hiss, called white noise.
    en.wikipedia.org/wiki/Shot_noise (I can't open this link)
    It is actually easier to visualize beams of electrons or protons in a vacuum beam tube. These currents of particles are not confined to wires, and pass by a signal pickup at nearly the speed of light. These currents produce a toroidal magnetic field pulse as they pass by. If a coil of wire is put around a magnetic toroid surrounding the beam, the beam signals can be picked up. For extremely relativistic particles, the pulses from individual particles are picoseconds wide.
    Bob S
  4. Nov 26, 2009 #3


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    Thanks for your clear answer! Perfect!
  5. Nov 28, 2009 #4
    Last edited: Nov 28, 2009
  6. Nov 28, 2009 #5
    I don't understand. How does this explain Schottky noise from individual electrons? See
    Bob S
  7. Nov 28, 2009 #6
    The general question says, Shouldn't a DC current emit EM waves? The simple answer is yes, a static B field is generated by a DC current per Ampere's Law. There is a coupling of the electric current and magnetic field whenever charge flows.

    If the specific question of detecting each electron individually is about Schottky noise I didn't pick up on that implication.
  8. Nov 28, 2009 #7
    Read the OP carefully:
    "If we would have a very narrow coil around the dc current carrying wire, (like an atom wide), every time an electron pass through the plane of the coil, should induce a voltage in the coil, so theoretically we should get a high frequency current in the coil circuit."
    This describes shot noise, not a steady dc field.
    Bob S
  9. Nov 28, 2009 #8
    According to your wiki link, shot noise only applies when there are a relatively small number of electrons in the flow of current, so that noise becomes a statistically significant factor in the average flow, but this factor diminishes in significance as the magnitude of current flow increases.

    I am not sure the OP was making such a subtle distinction, but I see the value of your interpretation of the problem. The hall effect sensors on the page link show a similar principle of current sensing for typical values of current flow.
  10. Nov 28, 2009 #9


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    now or the sensitivity of the sensor is not enough...orrrr the magnetic field of each of the electrons close enough is melted together, they overlap, in one single field ...which would be very weird ...
  11. Nov 28, 2009 #10
    In a good conductor (such as copper) there are many free electrons in the conduction band. The discrete energy levels of each electron in a single atom merge into one continuous band of energy in the crystal lattice structure of many atoms, so a band with no discrete energy levels occurs.

    The electric current in a good conductor is more like a continuous flow (a micro-fluid) than a flow of discrete particles. Fluids have wavelike properties and so do EM fields.

    The particle-wave duality appears to be what you're trying to conceptualize (if I'm not mistaken). This Wiki does an OK job of discussing the history of the idea but maybe not the core principles:


    Let me know if I've mistaken your comment ...
  12. Nov 28, 2009 #11
    At FermiLab, physicists measure the position of up to 50 milliamps of circulating dc beams of antiprotons in a circular strong-focusing magnetic ring using multi-GHz electronics. The shot noise (also called Schottky signal) is the only measurable non-dc signal available. The shot noise is due to individual discrete antiprotons passing the pickup detectors in sub-nanosecond intervals.
    Bob S
  13. Nov 28, 2009 #12
    This quote is from the Wiki article link cited by Bob S above.

    In p-n junctions (diodes, transistors, photodetectors) the charge carriers, which are holes and electrons, are generated via some random process that introduces shot noise.

    In a good conductor there are always many free electrons in the conduction band which do not have to be generated. They just exist. Send a signal into the conductor and the free carriers bump into each other at nearly the speed of light to conduct direct or alternating current. Thus the charge carrier medium has an impact on the relative significance of shot noise.
  14. Nov 29, 2009 #13


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    ...kinda sorta maybe... The truth is that not so long time go i really believed in particle-wave duality, photon, relativity, but i didn't get anywhere, so now i try to look at things in other "non mainstream", non conventional ways even taking the risk that i will end up to be burned at the stake...just kidding ...So you say that i should look at the electrons and their individual magnetic field as having waves proprieties rather than individual particles. I can rather than imagine electrons behaving like bosons, or like a fluid something that floes between the atoms, electrons losing their individual proprieties.....
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