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Speed of electricity

  1. Mar 3, 2007 #1
    when a light switch is flicked, does the light have to wait for electrons to get from the switch to the bulb before it lights up, or would electrons already lined up along the length of the wire simply move towards the bulb and so light up the bulb in a much shorter time?
  2. jcsd
  3. Mar 3, 2007 #2

    Meir Achuz

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    Yes, an individual electron does not travel very far.
  4. Mar 3, 2007 #3


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    That's yes to the second part....
  5. Mar 3, 2007 #4
    If I remember correctly, I think electric flow can be modelled as a diffusion of electrons, similar to heat flow.
  6. Mar 3, 2007 #5
    Because of resistance it would take a percievable period of time to turn a light on with a distant switch (say 10 ft away) if the electrons were not already "lined up" as you say.
    (someone correct me if I'm wrong)
    The drift speed of electrons in a copper wire is something around 0.0043m/s.
    See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html for more details.
  7. Mar 3, 2007 #6
    Yes, that is more or less how it works. There are a lot more theoretical details that go into this, but what you've said reflects the basic idea. If you want numbers, I remember that my junior year experimental physics book mentioned that electric signals travel down a wire at about 2/3 the speed of light (we were using wires of different length to get two signals to reach an apparatus at different times). But if you want an explanation as to why this is the case, I'll need to think a bit before I respond.
    Last edited: Mar 3, 2007
  8. Mar 3, 2007 #7
    An electric wire is a bit like a pipe crammed full of peas. Push a new pea in one end and one falls, almost instantly, out the other.
  9. Mar 4, 2007 #8
    No. It takes time for the process to reach the other end. I'm fairly sure it's a diffusion process.
  10. Mar 4, 2007 #9
    Yes, it takes time, otherwise it would exceed the speed of light, which is a gigantic no no. But that's why I said "almost".

    I would call it more of a propagation.
  11. Mar 4, 2007 #10
    Like DaveFaz says, a pipe crammed full of peas...

    and another one said

    which one is right.. must be the one with the 2/3 right?
  12. Mar 4, 2007 #11


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    They are all correct. This is because in the semi-classical treatment of conduction electron in metals, these electrons are considered to be an "electron gas" within the material. And if you have learned anything about the ideal gas law, you will know that these particles continue to make "random" motion with collisions with each other and the lattice ions. The average speed (not velocity) of each of these electrons is LARGE, while the whole group speed, which is the drift velocity, can be rather low. In fact, without any external field, the drift velocity is zero.

  13. Mar 4, 2007 #12
    velocity is speed in a straight line (or same direction) right ?

    all right so the speed the electrons bang into each other with is great but the "forward going" speed is quite low..
  14. Mar 4, 2007 #13
    The drift velocity figure given was absolutely correct. The (2/3)c value is the approximate speed of the signal itself. In practical terms: if you connect a wire to an apparatus that can acquire the signal (a DAQ card, for instance), and then send a signal to it, the computer will not receive the signal instantaneously. The information is transmitted at about (2/3)c, but any given individual electron still travels at a very slow speed. Make sense?
  15. Mar 5, 2007 #14
    yeah it does.. Could you define what a signal is?
  16. Mar 5, 2007 #15
    Here I'm using the word to describe the information transmitted by the electric current. When an electric current flows, it's because an electric field is created in the wire, and this field causes the migration of charge. Though the speed of the charge carriers (drift speed) is very low, the net flow rate of current is high.
  17. Mar 5, 2007 #16


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    The "signal" is essentially the electromagnetic version of a pressure wave. If you were to open a water valve quickly, the water near the valve would start flowing right away, but water at some distance away would not. This creates a shock wave that travels through the water (starting at the valve) at the speed of sound. Once the shock wave passes any point in the pipe, water at that point will flow. Effect isn't noticeable when opening a valve, but it can be very noticeable when closing one: it is the cause of the "water hammer" effect.
    Last edited: Mar 5, 2007
  18. Mar 5, 2007 #17
    Very good Russ.. I like these very essential explanations.. "Water hammer" effect is where the valve kind a shocks or how to say it...right? (I'm not from an English speaking country)
  19. Mar 5, 2007 #18


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    In the water hammer effect, the shockwave will travel back and forth until it dampens itself out, banging against valves, elbows, everything that is a little bit of an obstruction.
  20. Mar 5, 2007 #19
    if you stop the water at the valve by closing it then all the water some distance away will come smacking in to the valve and then going back and forth ?
  21. Mar 6, 2007 #20


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    I might point out that your wires probably are AC (alternating current) rather than DC (direct current) so NO electrons ever travel from the lightswitch to the lamp- they just move back and forth a very short distance.
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