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Details of how electricity and radio work

  1. Oct 5, 2008 #1
    I've been doing some work with amateur radio recently, and realized that I don't have an adequate detailed, low-level understanding of the basic physics behind exactly how electricity and radio work. My background is in math, chemistry, biology and the physics of "medium sized" objects (orbital dynamics, Newtonian motion, etc). I've looked for books and online references about this, and haven't had any luck.

    My first point of confusion is about "electron flow." What does that really mean? A single electron doesn't really move from one end of the wire to the other, does it? Doesn't it actually collide with a neighboring electron, causing it to move? That's the DC side.

    AC is much more confusing. What is really happening with AC at the atomic level? What does it mean for electricity to have a certain frequency? I understand that voltage goes up-and-down, but how? And how can voltage and current get out of phase?

    Then AC, which has to do with electrons, can be converted into radio waves (photons) at an antenna. I understand how it works with DC and a wire / lightbulb -- the wire heats up, which causes photons to be emitted. But how does it work for RF?

    What about ground / earth? Why do electons and radio waves need to flow back to ground? Why is earth such an active part of their flow?

    Also, how do photons (light or RF) really manifest a particular frequency? It seems to always be drawn as big sine waves in the sky. But does a single photon really swing back-and-forth in space across the width of its wavelength?
  2. jcsd
  3. Oct 5, 2008 #2
    think of current as flowing water and wires as pipes. voltage is pressure

    applied voltage and current get out of phase

    totally different from a light bulb

    at radio frequencies forget about photons. just use the classical explanation.
  4. Oct 5, 2008 #3
    I'm familiar with the water analogy for DC -- but how does it apply to AC? How can it be used to show how voltage and current can get out of phase along the pipe?

    I don't understand what you mean by forgetting about photons. RF is photons, right? And antennas are transducers that convert AC to photons and photons to AC. Or am I confused about something?
  5. Oct 5, 2008 #4
    I already answered your first question. (applied≠net)

    I mean just what I said. just use the classical explanation.
  6. Oct 5, 2008 #5
    My understanding is that with AC, applied current and voltage can be in-phase, but can become out of phase along a transmission line, particularly in the presence of an impedence mismatch. Are you saying that's not correct?

    But I'm trying to understand the very low-level physics of what's going on. First, how do electrons flow? In copper wire, does an atom of Cu temporarily become Cu- while carrying electricity (my chemistry background makes me doubt that)? Or is the electron carried some way other than by entering the electron cloud on the copper atom?

    What causes the electrons to "flow"? I understand that it's a difference in "potential," but what does that really mean? How does an electron at one end of a wire know that there's a difference in potential at the other end?

    BTW, I realize this is a big subject. I would appreciate some general answers (or pointers to answers), but if anyone can recommend a good book, paper or website on the subject, that would be great too. I've Googled around and the only things I've found are the usual high school level descriptions.
  7. Oct 5, 2008 #6
    no that isnt what I'm saying. perhaps you should look up kirchoffs law.
  8. Oct 5, 2008 #7


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    And maybe Chapter 1 of Galperin's lecture notes "Non-stationary Electrical and Optical Properties of Bulk Conductors and Low-Dimensional Structures"
  9. Oct 5, 2008 #8


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    Try Walter Lewin's Lecture 28 (talks about oscillating charges producing radiation starting around 34 minutes):

    Edit: I'm watching it now, it's good, at least for me. I hadn't seen this lecture before, but his rainbow one was fantastic, which is why I thought his other lectures might be good too.
  10. Oct 5, 2008 #9
    Thanks for the links. I'm working my way through them.

    The answer to my first question seems to be that conductors tend to have a single electron in their outer shell. Copper's electrons are [Ar] 3d10 4s1, so the single 4s electron is "loose," and the application of a field to the wire can induce a small net motion of those electrons, which is current (although I'm not sure if the copper drops to [Ar] 3d10 when an electron moves, or if it temporarily goes up to [Ar] 3d10 4s2, and then immediately loses the extra electron). Electrons from one atom to the next on the full length of the wire are always pushing against each other (like charges repel), so when they move in one end of the wire, the pressure is felt at the other end, just as with a fluid in a pipe. Then with AC, the field that causes the movement of the electrons just reverses direction. I hope that's right.... But how is the field created in the first place (not the mechanical part of how a generator works; I understand that)?

    I also see that inductors and capacitors can change the phase of current and voltage (Kirchoff's Law only applies when there is no net reactance). But I still don't understand the "how" part of my original question.

    I think the area that I'm having trouble with has something to do with fields, which I don't understand very well (not the definitions, etc; those are straightforward). I know that the flow of current induces a field (and vice-versa), but how does it do that?
  11. Oct 5, 2008 #10
    Kirchoff's Law applies perfectly well to inductors and capacitors.
  12. Oct 6, 2008 #11


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    The cutest explanation for this is relativistic length contraction
    http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html (see section 7)
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