Suggested correction

Discussion in 'Feedback and Suggestions' started by ksquare, Oct 11, 2009.

  1. ksquare

    Thread Starter New Member

    Oct 11, 2009
    3
    0
    First - what a great site! I've been assigned the task of teaching electronics to a group of guys with no real background in the field (other than working on large 3 phase UPS systems with high voltage). Although I'm an engineer, teaching this material to this kind of audience is an interesting challenge. Your site has given me ideas on how to explain things.

    On the Ohm's Law (again!) page in Vol. 1, the text states "the more resistance a body offers to current, the slower electrons will flow for any given amount of voltage." Is this accurate? I would suggest that increased resistance reduces the number of electrons passing a given point, not their speed. One coulomb per second is one amp. True, if we could slow the electrons down, we would reduce the number of coulombs per second and therefore reduce the current. But I don't think a resistor slows down electrons. I think it reduces the quantity by reducing the number of free electrons. Yes, the result would be the same - reduced current - but the mechanics are different. It seems to me that the sentence should state "the more resistance a body offers to current, fewer electrons will flow for any given amount of voltage." Am I wrong (it has happened)?

    Then again, if we examine things from a quantum viewpoint, perhaps those electrons are being diverted to a galaxy in a different universe <g>
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The definition of current is a quantity of electrons passing a point per unit of time. So the statement
    says the same thing as
    The net result is that fewer electrons pass the measuring point in either case. It's a difference that makes no difference.
     
  3. ksquare

    Thread Starter New Member

    Oct 11, 2009
    3
    0
    Hi beenthere,
    Yes, I agree that the result would be the same as far as current is concerned. That's why I said "the result would be the same - reduced current". However, the atomic mechanics are what I'm questioning. Carbon has more resistance than copper because it has fewer free electrons. Yes, there is also resistance due to electron/ion collisions. Therefore fewer electrons are available to flow past a point at any instant. If the electrons were actually made to flow more slowly through a resistance, we should be able to measure the change in velocity. I realize that the electron flow is a bit below C but the statement in question suggests that the resistance reduces it further. If so, what is the mathematical relationship? Does a 100K resistor slow electrons 100 times more than a 1K resistor? If this were true, we would have to factor in propagation delay in a circuit. Actually we sometimes do, but the most important factors there are distance a signal has to travel and reactive components. So I ask which is correct:

    1. resistance slows down electron flow even though the number flowing doesn't change

    2. resistance reduces the number of electrons flowing even though their velocity doesn't change

    Yes, in one sense "It's a difference that makes no difference" because the result would be less current. I don't mean to nit-pick. I'm just curious and want to be corrected if I'm wrong.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    As I understand it, the number of electrons flowing stays the same, the velocity (which is low) is reduced. You measure the number of electrons passing a given point, which will be reduce due to the slower flow.

    A common analogy is voltage is pressure, amperage is volume/minute.
     
  5. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    ksquare,

    You're wondering that if you add a resistor to a circuit, will the quantity of electrons decrease and the velocity remain the same (near the speed of light)? Yes I believe that's true. When I turn on a circuit with a bunch of resistors, they only limit the current not the duration it takes to flow. The higher quantity of electrons I have, means I have a higher current.

    Maybe I'm wrong, maybe resistors only shorten the duration by a slight fraction of a second, but I don't think they do.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Actually that is incorrect. The actual speed of an individual electron is inches per hour or so, and their motion is pretty brownian, but we are talking a really huge number of electrons. It is the electric field that travels at the speed of light.
     
  7. notxjack

    New Member

    Sep 7, 2009
    20
    0
    What BM said is mostly correct. Also, ksquares is giving a QM correction to the classical definitions of currents and voltages with respect to ohms law which are of minimal use for the purposes of this site's audience.

    For Ksquares, just don't think of the circuit as bound. Electrons may pop in and out of the wiring's insulation and from there in and out of the atmosphere, and as such any particular electron may have any particular speed.

    Also, out of curiosity, by no background in the field do you mean these guys are more hard science or more layman? Good luck teaching electrical engineering to string theorists.
     
  8. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Interesting...you're saying that the speed of electrons determines the current? Whatever speed you may have, I'd think the quantity of electrons would remain the same.
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Not just speed, quantity and speed. Wider wire has more electrons per inch of length, which is why larger wire can carry more current. Amps is a number of electrons passing a given point.
     
  10. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Oh, ok. Thanks for clearing that up for me!

    So coulombs describe the quantity of eletron flow pass a givin point. Do they have a name for the speed?
     
  11. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Sorry-correction. Coulombs describe the quantity of electron flow after 1 second.
     
  12. notxjack

    New Member

    Sep 7, 2009
    20
    0
    Coulomb is a quantity of charge. Current, or Amperes, is a measure of charge per unit time. In a wire, more than one electron moves at a time. The individual speed of an electron does impact the current, but not nearly as much as one would initially believe.
     
Loading...