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

B Problem of electricity and electrons

  1. Aug 5, 2017 #1
    Hi, the today definition of electric current says it is done by moving electrons. But this wikipedia article https://en.wikipedia.org/wiki/Speed_of_electricity says:
    "the signals or energy travel as electromagnetic waves typically on the order of 50%–99% of the speed of light, while the electrons themselves move (drift) much more slowly."
    "AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field"

    This comment about AC voltages remember us that the inventor of the AC induction motor, Tesla, didn't believe in the existence of the electron https://en.wikipedia.org/wiki/Nikola_Tesla#On_experimental_and_theoretical_physics

    In order to prove or disprove the existence of electrons in electric current I searched the internet if the discharged plate of a capacitor lose mass and if a charged plate of a capacitor gain mass. The experiment was never made. The mass is considered negligible but we have to measure it to show that electricity is an actual movement of electron (it should be feasible with today instruments and a big capacitor to have lot of electron involved)

    Additional info :
    _how the notion of electron was accepted https://www.nobelprize.org/educational/physics/vacuum/experiment-1.html
    _formula to evaluate electron speed https://en.wikipedia.org/wiki/Drift_velocity "Therefore in this wire the electrons are flowing at the rate of 23 µm/s. At 60 Hz alternating current, this means that within half a cycle the electrons drift less than 0.2 μm. In other words, electrons flowing across the contact point in a switch will never actually leave the switch."
     
    Last edited: Aug 5, 2017
  2. jcsd
  3. Aug 5, 2017 #2

    Nugatory

    User Avatar

    Staff: Mentor

    Don't guess when you can calculate.
    What is the difference in the weight of a plate of a 1F capacitor when discharged and when charged to a few tens of kV, and how much is that as a fraction of the total weight of the capacitor? You can make reasonable assumptions about the size and weight of the capacitor.

    However, we don't need this experiment to satisfy ourselves that electrons are the charge carriers in ordinary electrical circuits. There are plenty of other experiments that support the existence of electrons and their role as charge carriers, and plausible alternative theories that match the results of these experiments.
     
  4. Aug 5, 2017 #3

    Dale

    Staff: Mentor

  5. Aug 5, 2017 #4

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    And compare the change in gravitational force to the change in force between the capacitor plates.
    In most but not all materials electrons are the moving charges.
    You can calculate the conductivity of materials based on the number of free electrons in the material.

    Tesla had a lot of weird ideas, and we learned more in the last 100 years. How would CRT monitors work without electrons, for example?
    That is (sort of) correct. What is your point?
     
  6. Aug 5, 2017 #5
    we have F=C/V . We need to know the value of coulomb to have the quantity of electrons. For 10 000V we would have 10 000C for 1F. For -1C meaning 6.242×10^18 electrons we would have 6.242×10^22 electrons in -10 000C. With the accepted mass of an electron ( 9.10938356 × 10-31 kg) we would have 5.68607722x10^-8 kg of electrons. The Copper has one free electron per atom we would have 6.242×10^22 atoms (0.1036508469mol) of copper in each plate thus, with a atomic weight of 63.546 g/mol, 0.00658659671kg. The ratio difference of weight would be 0.0000086328

    I am not a physicist. I could have mistaken in some points.

    Hall effect was discovered 18 years before the electron discovery. It has been reinterpreted after electron discovery.

    So for you electricity is at the same time a electromagnetic wave (a photon ?) traveling at the speed of light and moving electrons traveling at µm/s ? The calculation here https://en.wikipedia.org/wiki/Drift_velocity#Numerical_example was made with an AC of 60hz but AC voltage can alternate at tens to thousands of megahertz https://en.wikipedia.org/wiki/Alternating_current#Information_transmission making electron nearly immobile. I guess you will ask me to calculate how much the electron would move (or oscillate) in such a case and if the distance is inferior to the size of an orbital (making AC a quantum phenomenon, if electron exist lol. And I wonder if the "electric conductivity of the medium at the temperature considered" of the drift velocity formula apply)
     
    Last edited: Aug 5, 2017
  7. Aug 5, 2017 #6

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    You cannot make a 1 F, tens of kV capacitor with 6 gram of copper.
    The results of the experiment were better understood later. What exactly is your point?
    See the "sort of". Changes in the electric potential propagate at speeds of the order of the speed of light, while the net motion of electrons is very slow, yes.

    If you push a metal bar, the end will start moving quickly, even if the bar with all its atoms is moving very slowly, and the two speeds are unrelated. This is similar to motion of electrons in a conductor.
     
  8. Aug 5, 2017 #7
    Last edited: Aug 5, 2017
  9. Aug 5, 2017 #8

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    A rough estimate: We want to put the plates on independent scales, so we cannot combine them with a dielectric in between. Let's make a 0.5 mm gap filled with SF6 at high pressure to avoid an electric arc. Then we need 50 km2 for our plate capacitor.

    A 1 nm thick layer of copper with this area has a mass of 500 kg.

    It gets worse. The two plates attract each other with 2*1011 N. The 1 nm foil doesn't have any relevant structural strength. You need a huge support structure to keep the foils apart, increasing the mass by orders of magnitude.
    You can save a factor 2 by interleaving the plates, but it should be obvious that the project is completely infeasible.


    What you can do is measure field emission of negatively charged spiky things: If you push too many electrons in, they get ejected into the environment.
     
  10. Aug 5, 2017 #9

    Nugatory

    User Avatar

    Staff: Mentor

    Yes, but it doesn't matter. These additional considerations just mean that the ratio is even smaller than the 1 part in 100,000 that you calculated above, and that ratio is already far below what is practical to measure (Remembering that we have to separate this effect from the electrostatic force between the two plates, and considering the physical characteristics of a 1F 10Kv capacitor).

    We do the calculation to check whether this might be a practical experiment, and we have our answer without investing any more work in it.
     
  11. Aug 5, 2017 #10

    Dale

    Staff: Mentor

    And what is your point? The Hall effect is one of the reasons that we know about electrons. It was one of the main effects that electrons explain. Sometimes an effect is discovered first and then a model is developed to explain it and sometimes the model is developed first and then effects predicted by the model are discovered later. The order doesn't matter much.

    In any case, the Hall effect demonstrates conclusively that the charge carriers in metals are negative. What else do you want to show?
     
  12. Aug 5, 2017 #11
    Ok,
    If what I said about AC current (which is mostly quotes of wiki) is true and that the Hall effect is truely measuring charges carriers movements. Then hall effect would be unable to measure AC current above a certain frequency since electrons would oscillate in a very small distance. Likely considered immobile
     
  13. Aug 5, 2017 #12

    Dale

    Staff: Mentor

    Please post a valid reference for this claim.
     
  14. Aug 5, 2017 #13

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    The distance doesn't really matter, the force on the electrons is independent of the distance they move during a cycle, it only depends on the current and the material. You cannot measure the Hall effect properly if the frequency is too high, because the voltage changes its polarity too often and voltage measurements need some time. So what? This is mainly an experimental limitation.
     
  15. Aug 5, 2017 #14
    Say there OP, thanks so much for the link to electron drift. I have played around with calculations involving a 1 mm square wire and movement of "free electrons" through it for some time. Somehow never ran across that formula. Duh.

    When you think about electrons "bumping" from one valence shell to another in even a very small wire and do calculations on how many valence electrons are present in a single cross section of the wire, it is mind boggling. I am speaking of a copper wire with one electron in the outer shell.

    My crude calculations indicate that if one Ampere was fed into the wire one electron at a time in line through the length of the wire it would take seconds for a given electron to move through a 1 meter long wire.

    If you did the same by "bumping" one cross-section worth of electrons at a time to move one cross-section out the other end of the wire, it would take years for one electron in a given cross-section to get to the other end of the wire (at one Ampere current flow).

    Likely the interval is somewhere in between. I think I will be amusing myself with that formula for a quite some time.
     
  16. Aug 5, 2017 #15
    http://en-us.fluke.com/training/tra...p-meters/inside-hall-effect-clamp-meters.html
    It seems to concern a commercial tool.

    https://en.wikipedia.org/wiki/Hall_effect_sensor
    spurious signals systematic errors:
    http://aip.scitation.org/doi/abs/10.1063/1.1657320

    May be.

    This thread was manly motivated on my side by the difference of electicity as a electromagnetic wave speed-of-light and electricity as µ movement of electrons. Before recently I always believed electricity was purely electron movement done at the speed of light. I was wrong and came in that forum when I noticed that.

    Please if you can, mesure the movement of electrons in a cycle of AC voltages alternating tens to thousands of megahertz. And confirm or not that it is smaller than an orbital.
     
    Last edited: Aug 5, 2017
  17. Aug 5, 2017 #16

    Dale

    Staff: Mentor

    Looks like a good article. I can't read more than the abstract.

    That is a pretty common misconception. The fields travel at nearly the speed of light, but not the charge carriers. The energy is also transported in the fields, not as little packets of energy attached to the charge carriers.

    The Hall effect can be directly used to determine the charge carrier charge density. With that you can calculate the velocity of the charge carriers, even without a notion of an electron (i.e. as a classical continuum of charge carriers)
     
  18. Aug 5, 2017 #17
    I think you mean calculate, not measure? Would be interesting to calculate the minimum frequency movement that would equate to the orbital dimension of the valence electron for a given conductor. Does that make any sense?
     
  19. Aug 5, 2017 #18

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That doesn't matter.
    (a) it is a motion of unbound electrons, they are not localized at an atom
    (b) it is an average motion. It doesn't even make sense to assign positions to individual electrons
     
  20. Aug 5, 2017 #19

    davenn

    User Avatar
    Science Advisor
    Gold Member

    that makes no sense at all
    you misunderstand what Amps and current is .... you Don't feed 1 amp into a length of wire ( or other circuit) and you cannot do it 1 electron at a time
    the current through a circuit is determined by the resistive load of the circuit

    google the definitions of ampere


    Dave
     
  21. Aug 5, 2017 #20
    Of course one electron at a time doesn't make sense, but when one is thinking about what actually is going on in the wire, not just a formula, you think about all the possibilities. As in max and minimum. I do anyway. As to "feeding" one amp into a wire perhaps you could explain to me whether it is push or pull. Chuckle.

    After looking at the formula for the drift rate through a wire due to a voltage I notice that there is no temperature value there. Interesting considering that for "normal" conditions that the thermal movement of valence electrons is much larger that that due to current flow. Seems that would mean that while the electrons might move "forward" the distance given in one second, they might be moving in some other direction multiple times that value due to thermal excitement.

    Think I have to check on thermal excitation again. Actually I think where I got that from was a comment in a research paper on electrons and current flow.

    On the google definition lookup, I probably should mention that I calibrated the electronics equipment that was used in designing and building the antennas that went to the moon. Both of them. chuckle.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Problem of electricity and electrons
Loading...