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Speed of elecrons/signal

  1. Apr 2, 2012 #1
    I know that when a battery is connected to a lamp and the switch is closed the lamp comes on 'immediately'. I also know that the current consists of electrons moving very very slowly.... a few mm per sec.
    My problem is : what is the speed of the signal/current from the switch to the lamp. Is it immediate, can it be measured and explained.
    To explain this we compared it to opening a tap to let water flow, it is immediate even though the watr moves slowly.
    I know that the wire is full of electrons and the pipe is full of water but I cannot see how to get a speed for what the switch or tap does.
     
  2. jcsd
  3. Apr 2, 2012 #2

    jedishrfu

    Staff: Mentor

    another comparison would be a pipe filled with marbles. When you push a marble in one end then another marble pops out at the same speed from the other end.
     
  4. Apr 2, 2012 #3
    that is the same thing that I mean. For marbles read electrons or water molecules. Is there any way to find the actual speed taken for the marble to drop out after the first marble is pushed?
     
  5. Apr 2, 2012 #4

    AlephZero

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    Science Advisor
    Homework Helper

    In the water analogy, when you open the tap and apply pressure to the water at one end of the pipe, the pressure wave takes a finite (but small) amount of time to travel the length of the pipe. The water at the end doesn't move until a force acts on it to make it move, and that force is the pressure wave. The speed of the pressure wave is the same as the speed of sound in water (about 1.5 km/sec) which is much quicker than the velocity of the water itself.

    For the electricity example, the equivalent of the water pressure wave is an electromagnetic wave travelling along the conductor. Its speed is not exactly equal to the speed of light in a vacuum, but it is the same order of magnitude.
     
  6. Apr 2, 2012 #5
    that sound sensible, I never thought of it as a wave thing but we have seen a slinky pushed and seen the time it takes for the disturbance to get to the other end.
    I wondered whether you could say speed of light for the electric current.
    Thankyou....gives me something to think about
     
  7. Apr 2, 2012 #6
  8. Apr 3, 2012 #7
    Unlike the electric wave in a conductor, the electrons move very slowly (at about mm/s for currents in order of 1A).
    In spite the wave description is the best for explaining the propagation of a signal in a conductor, a simpler image of corpuscular description in layman's terms can also be given: when an electron is entering one end of a wire, its repulsive Coulomb force acts onto the other electrons around, and so on, the influence of electrons on each other propagates very quickly to the other end where another electron goes out.
     
  9. Apr 3, 2012 #8
    A simpler image is not always correct.
    The electrons move under the influence of the electric field. An electron at the other end of the wire starts acquiring a drift velocity as soon as it "feels" the electric field, which propagate as described in previous posts.
    On the other hand, if the electron-electron repulsion plays a role, with what speed will this propagate along the wire? When you say "the influence of electrons on each other propagates very quickly", aren't you describing a wave-like phenomenon?
     
  10. Apr 4, 2012 #9
    I agree that such an image is very simplifying and so, it can't be "correct" stricto sensu.
    Its purpose was not to modelize a physical process, but just to show why the "speed of electrons" is not a relevant data when it is question of a signal transmission.
    Your underlying image is that of a wire with a potential difference between the two ends so that the electrons moves as one and alone under the electric field along the conductor, and so the Coulomb force between electrons would be out of interest.
    But in order to deal with a transmission time, we must tag the flow of electrons, otherwise we would be unable to say at which time something enters the conductor and then goes out. For example we could use a voltage pulse at one end of a bifil
     
  11. Apr 4, 2012 #10
    Sure, but I don't use not correct images :-)
    I agree that my image is very simplifying and so, it can't be "correct" stricto sensu.
    Its purpose was not to perfectly modelize a physical process, but just to show why the "speed of electrons" is not a relevant data when it is question of a signal transmission.

    The underlying image that your reply suggest me is that of a wire with a potential difference between the two ends so that the electrons move as one and alone under the electric field along the conductor, and so the Coulomb force between electrons would be out of interest.
    This is true only for constant current or in the quasi-stationary state approximation, and in any case, in order to create the field, you must present negative charges at one end and/or positive charges at the other end. When you say "the electrons move under the influence of the electric field", not false but it is the story of the chicken and the egg, because an electric field is caused by charges.

    Now in order to deal with a transmission time, we must tag the flow of electrons, otherwise we wouldn't be able to say at which time something enters the conductor and then goes out. For example we could use a voltage pulse at one end of a bifilar transmission line and see at what time we receive it at the other end on the load. This implies a not constant gradient of electric field at the position of the pulse front in the wires, which is time dependent. So the electrons have to rearrange their position where and at the moment of the passage of the pulse. While rearranging, they influence other electrons and progate the field disturbance further. So my image is not too bad. You asked about the speed. Of course it is the same speed as that of the signal, in the order of the speed of light. And you are right when you say that finally I'm describing a wave-like phenomenon, but now it is causal and local.
     
  12. Apr 4, 2012 #11
    2 questions to expand on this a little...

    First, does voltage play a roll in signal velocity?
    Intuitively it seems like the harder I push on the first electron in the wire the faster it should accelerate and the sooner it should start pushing on the next electron, and so on. Is this intuition borne out by experiment?

    Second, suppose I have a series circuit consisting of the following components in the following order...
    Battery -> switch A ->bulb A ->100,000 miles of wire ->bulb B -> 100,000 miles of wire -> bulb C -> switch B ->Opposite side of battery
    Intuition would tell me that if switch B was already closed when I closed switch A then the bulbs should light in the order ABC. If switch A was already closed when I closed switch B they should light in order CBA. If I close both switches at the same time A and B should light simultaneously, followed by C.
    Is my intuition right on this? What happens If I insert a switch at bulb C and use that to close the circuit?
     
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