What is happening in those wires?

Discussion in 'General Electronics Chat' started by Gerry Rzeppa, Sep 9, 2019.

  1. crutschow

    Expert

    Mar 14, 2008
    23,063
    6,843
    The mobile (free) electrons slowly* move in the wire to provide the current going from a higher to a lower potential.
    If you want to get into the quantum physics of what's happening, that's beyond my pay-grade. :rolleyes:

    *For example, the electrons move about 1/4 mm/s for 10A of current flowing in a 12AWG wire.
     
    Last edited: Sep 13, 2019
  2. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
    3,535
    664
    The TS would not like the world I lived in. Wires vibrating in a constant temperature environmental chamber generated measurable currents
    Only vibration counts.
     
  3. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
    2
    I understand about current; that definition is quite clear and simple: one ampere of current is one coulomb of electrical charge (6.24 x 10^18 charge carriers (ie, electrons)) moving past a specific point in one second.

    But to what differing physical states do we attribute potential differences? I'm told that a charged capacitor has a potential difference across the plates because there are more free electrons on the one plate versus the other. And that makes sense: more electrons, more negative charge, with the like-charged electrons repelling each other and thus producing the electronic "pressure" we call voltage.

    But when I surmise that a potential difference across a resistor must similarly be the result of more free electrons congregating in the wire at the one end versus the wire at the other end, I'm frequently (and often rudely) told that I'm wrong -- which would be fine, if the rebuke included a description of the cause of the potential difference across the resistor, say, in terms similar to the description of the charged capacitor.
     
  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    8,077
    2,285
    No analogy I know of comes close to explaining everything. Some do well with some aspects; that's as good as it gets. All analogies fall apart as you push the details.

    An analogy is a crutch for both the teacher and the student, and should be used sparingly and carefully. By definition, all analogies have faults, problems, and whatever else you call it when it is wrong. The problem with giving a student an analogy is that (also) by definition, a student doesn't know when the analogy is helping and when it is hurting.

    I understand what you are trying to do; I have explained many aspects of physics in general and electricity in particular to all strata of people. Usually it went well, sometimes not so much. But there is a reason an EE degree takes years of study and requires an understanding of complex matrix algebra and n-dimensional tensors. Electronics is hard, and the true nature of how energy moves down a wire (or a waveguide) is very hard (Feynman got the Nobel Prize for it). There is no good analogy for the Poynting vector, and that's ok.

    ak
     
    Sinus23 and nsaspook like this.
  5. crutschow

    Expert

    Mar 14, 2008
    23,063
    6,843
    Marbles in a tube is the best analogy I can think of.
    If that doesn't work for you, then good luck with finding something better. :p
     
  6. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
    2
    I suppose that's possible, under certain circumstances. But please consider this small portion of a guitar amplifier circuit:

    pile up 3.png

    We know that the heating element (not shown) at the bottom of the tube causes a cloud of free electrons to form on and a round the tube's cathode. We also know that a varying potential applied to the tube's grid will allow more or fewer of these electrons to cross the tube's vacuum and collect on the tube's positively-charged plate. We know, as well, that the smaller input signal on the grid is thus inverted and amplified as shown by the two waves in the diagram. Now three deductions:

    1. Since the fundamental frequencies of these waves vary, in actual practice, from about 80 to 1300 cycles per second, it stands to reason that these electron "bursts" across the tube's vacuum must proceed at commensurate rates. Yes?

    2. Since the electrons cannot travel backward inside the tube, and since they cannot pile up on the tube's plate indefinitely, it stands to reason that these electrons must be dispersed throughout the rest of the amplifier's wires and components at similar (varying) rates. Yes?

    3. Since there may be as much as 12 inches of wire between the tube's plate and the electron "sink" capacitor in the amplifier's power supply, it stands to reason that electrons in the amplifier circuit may be travelling as fast as 12 inches/cycle * 1300 cycles/second = 15,600 inches/second. Yes?
     
  7. MrChips

    Moderator

    Oct 2, 2009
    19,111
    6,142
    No.
    The physical velocity of electrons and the velocity of the wave are two different things.

    The wave velocity (and hence the flow of information) approaches the speed of light (300,000 km/s or 186,000 miles/s). In a metallic conductor, the wave propagates at about 60% to 75% the speed of light.

    The speed of electrons in a vacuum tube depends on the acceleration voltage, and is less than 1% the speed of light.

    Ref: http://www.vias.org/basicradio/basic_radio_02_03.html

    Meanwhile, the drift speed of electrons in a metal is very slow, lower than 1 mm/s.
     
  8. crutschow

    Expert

    Mar 14, 2008
    23,063
    6,843
    Not under certain circumstances.
    It happens under all circumstances for an electron in a wire.
    Yes.
    Yes.
    No. Your reasoning just went off base.
    You are confusing signal velocity with the velocity of an electron in a wire.
    The signal in the wire travels at a high percentage of the speed of light (think one marble bumping into another in the tube).
    Each electron, however, only moves a very small fraction of an inch back and forth (you might say vibrates) with each cycle of AC signal.

    You need to understand that there is a huge number of free electrons in a conductor.
    For example if the electrons only move 1/4 mm in one second when moving 10 coulomb of charge (10 amperes) in a 12AWG wire, then each cm of that wire contains 10*10*4 = 400 coulombs of free charge.
    Even a large lightning bolt only transfers about 350 coulombs of charge.

    On the other hand, the number of electrons at any instant in the vacuum between the plate and cathode of a vacuum tube is very small.
    Those electrons thus move a significant distance in a very short time (about 10E6 meters/sec).
     
    Last edited: Sep 14, 2019
  9. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
    2
    No, to which of my three questions? If would be more helpful if I knew whether you disagreed with my premises or my conclusions.

    I'm sorry, but the rest of your response seems to miss the point altogether. Or I'm too stupid to see the correlation. Interesting link, though.

    Let me simplify the example by replacing the musical, analog "wave" with a 1hz digital signal -- instead of playing a note on the guitar in question, I'll just tap on the pickup with a screwdriver at a rate of one tap per second.

    1. This tapping will cause a high-speed cascade of electrons across the vacuum in the tube once each second. Yes?

    2. These "bursts" or "batches" or "packets" of electrons will be dispersed through the rest of the circuit in less than a second, resulting in an audible and amplified "tap, tap, tap" coming out of the attached speaker. Yes?

    3. The electron concentration at various places in the circuit (eg, on tube's plate, in the wires, before and after the various resistors, on the plates of the various capacitors, etc) must therefore vary, considerably, during each one-second period. Yes?

    4. Measurable changes in electrical potential at various points within the circuit during each one-second period will correlate with these varying electron concentrations. Yes?


     
  10. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
    2
    Thanks for your help, but I find myself regretting that I mentioned velocity in this context at all. I was only trying to picture what happens to the electrons after they leave the tube's plate.

    I understand how a wave can travel along a rope that is fixed at both ends, so that the wave moves further and faster than the up-down movements of the rope itself; but I've never seen anyone compare the crossing of electrons in a vacuum tube to the shaking of a rope. Applying the analogy in this case, it seems to me that the varying concentrations of electrons on the tube's plate would be the up-down movements of the "rope". Which would be end-of-story if those same electrons simply bounced back-and-forth inside the tube -- faster for high notes, and slower for low ones. But they don't. They exit the tube and make their way through the rest of the circuit. Which makes me think the whole circuit is the rope, and the varying electron concentrations (as they make their way through the circuit) are the up-down movements of the rope. But that just takes me back to the picture I originally presented, with the electron conglomerations making their way through the circuit at the frequency of the note currently being played.
     
  11. MrChips

    Moderator

    Oct 2, 2009
    19,111
    6,142
    If your goal is to teach other people how electrons and electricity behave, I am afraid you are taking the wrong approach.

    We teach the behaviour of fundamental particles in physics. When it comes to electricity and electronics we move on to the aggregate behaviour using electrical current. While the two are not incompatible, it is easier to understand from the two different perspectives.
     
  12. bertus

    Administrator

    Apr 5, 2008
    19,895
    4,137
  13. nsaspook

    Expert

    Aug 27, 2009
    6,166
    6,954
    Exactly: https://www.physicsforums.com/threa...ansfer-in-actual-circuits.826868/post-5195394
     
  14. crutschow

    Expert

    Mar 14, 2008
    23,063
    6,843
    What happens is that they enter the wire and then the signal is transmitted down the wire from one electron to another, the same as if the tube wasn't there.
    Only the signal moves down the wire, not the electrons.

    Suppose you bang the end of a rod with a hammer.
    The sound wave travels down the rod from atom-to-atom, with each atom only moving a very small amount.
    It don't see why that is so hard for you to visualize. :confused:
    Or are you really just trolling us?
     
    Sinus23, shortbus and SamR like this.
  15. SLK001

    Senior Member

    Nov 29, 2011
    1,433
    590
    Probably the direction of current? Charging it goes one way, discharging it goes the other.
     
  16. SamR

    Well-Known Member

    Mar 19, 2019
    1,015
    399
    Apparently, he has been doing this on multiple forums around the net.
     
    nsaspook likes this.
  17. KeepItSimpleStupid

    AAC Fanatic!

    Mar 4, 2014
    3,535
    664
    In class, I argued that my answer was correct and won.

    The teacher's comment was "Your not supposed to know that yet"

    Two examples of stuff I had to re-learn:

    Conventional current flow because Ben Franklin was wrong with the charge on an electron sign. Most of the time, it doesn't matter if you use electrons or conventional current. In certain aspects of Chemistry, it does.

    The "water analogy" breaks down, but it does work, when your in 3rd grade.

    Electron orbits I learned as circular. Nope, probability theory.

    Darn, I wish I could have been able to differentiate when learning about the slope of a line.

    You need age appropriate behavior.
     
    SamR and nsaspook like this.
  18. SamR

    Well-Known Member

    Mar 19, 2019
    1,015
    399
    Took a long while to unlearn and reorient my thinking as to conventional current flow. I do understand the concept of using simple models to help form a basis for understanding principles especially when I taught HS Chemistry. At the same time I also explained that there was much more to it than the simple model used to learn by and that if they ever made it to College there was far more to learn in-depth than just the basic simple model. I taught the orbital model of the atom but also tried to get them to understand Heisenberg's Uncertainty Principle and that what we were learning as a nice circular planar orbit was actually a hollow sphere of possibilities as to exactly where the electrons was as it orbited the nucleus.
     
  19. Wolframore

    Well-Known Member

    Jan 21, 2019
    1,160
    356
    I like the quantum atom model it explains crystalline structure much more than the Bohrs model but the Bohrs model is still useful and easier understand covalent bonds.
     
  20. Gerry Rzeppa

    Thread Starter Member

    Jun 17, 2015
    170
    2
    Yes, thanks. I especially empathize with the author when he says: "I've done this numerous times and I am still amazed at the emotional response this issue generates."

    This is true. Four years and one life-threatening heart attack ago, I raised similar questions on this and other forums. These questions have bothered me throughout the whole interim, and are still unresolved in my mind. So now, fully recovered, I thought I would "poke the bear" again in the hope that different people would be frequenting the forums (four years later) and that we might make progress, together, in settling these unresolved issues.

    MOD: links deleted.
     
    Last edited by a moderator: Sep 14, 2019
Loading...