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

Speed of Electricity in Wiring - Why near light speed?

  1. May 14, 2008 #1
    Electricity (the flow of current) is said to travel at near the speed of light (75-90%), but I read that electrons have a drift velocity of only 2-3 mm/hr or something close to that. So, if the electrons are so slow, should I think it is the holes that travel fast? Suggestions.
  2. jcsd
  3. May 14, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Whilst the electrons travel slowly, the signal, i.e. the electromagnetic field, propagates close to the speed of light in a vacuum.
  4. May 14, 2008 #3
    Have you ever been stopped at a traffic light ? Are the speeds of individual cars and of the information on green light the same ? Does this surprise you ?
  5. May 14, 2008 #4
    If it is the EM field that is moving down the wires, then would a 1 Telsa rare earth magnet cause the current in the wire to change?
  6. May 14, 2008 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes external magentic fields will affect the current flowing in a conductor.
  7. May 14, 2008 #6
    Guess I'll get out a DC circuit with a light bulb and see if it changes.
  8. May 14, 2008 #7
    Does the EM field move in the same direction that the electrons move? I read somewhere that accelerating charges emit EM waves perpindicular to the direction of movement of the charges (not parallel). Where am I going wrong?
  9. May 14, 2008 #8
    Based on my reading, the electrons only move very slightly and moved forward but with random motion. This suggests the electrons themselves might generate EM waves in random directions.

    You are correct, that free electrons and protons that travel around synchrotrons do emit EM waves when they are accelerated by the magnetic wigglers and that the EM is perpendicular.

    If Hootenanny's picture is correct that an EM field is moving down the wire, then it seems that electricity (flow of current) could be viewed as a form of light, aka electromagnetic radiation.

    Does that make sense?
  10. May 14, 2008 #9
    That is certainly not Hootenanny's picture, and is wrong. Think about the traffic light. For once, you do have a mechnical model.
  11. May 14, 2008 #10


    User Avatar
    Homework Helper

    Free electrons travel at high speed in random directions. The "drift" velocity is just the net component of this high speed travel in a specific direction.
  12. May 14, 2008 #11
    Sorry Hootenanny for the mis-quote. I mis-read. You simply wrote that EM fields travel at the speed of light in vacuum which is an accepted concept.
  13. May 15, 2008 #12


    User Avatar
    Homework Helper

    The speed of propagation of a change in current in a wire is near light speed.

    As a weak analogy, imagine billiard balls spaced 1 mm apart, and then colliding the first ball with another ball at slow speed. Each ball moves slowly, but the rate of propagtion of energy from one end of the chain of balls to the other moves much faster than the balls themselves.

    There is some interaction between free electrons and the molecules of a wire. When a free electron is captured by a molecule, the molecule frees up another electron quite rapidly. I don't know how fast this "information" regarding the excess molecule is propagated. From what I understand, most of the electron movement involves free electrons, that pass between molecules as opposed to colliding with them.
  14. May 15, 2008 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Thanks for clearing this up in my absence humanino.
    Let me just clarify something here. As I'm sure you know, current is a measure of the flow of charge through a particular surface. Now, the current is dependent on the applied external fields, but by no means are the Electric or Magnetic fields considered to be the current. Furthermore, the EM fields do not continuously propagate down the conductor, once a steady current has been established, the EM fields of the wire remain constant.
  15. May 15, 2008 #14


    User Avatar
    Homework Helper

  16. May 15, 2008 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I like to explain this in terms of the water-flow analogy that is popular with electrical engineers.

    Imagine a garden hose full of water, where the faucet is closed but the end of the hose is open. The water is just sitting there in the hose.

    Now turn on the faucet. Water starts coming out the open end almost instantly, even though water actually takes several seconds to travel the length of hose from the faucet to the opening. Actually, the time it takes for water to start flowing out the end is related to the speed of sound in water. It's the time it took for the pressure increase at the faucet end to propagate to the open end.

    Like water molecules, electrons exert a force or push on each other. Electron motion at one location in a circuit propagates at the speed of light in that medium, so that electrical currents are initiated quite quickly.
  17. May 15, 2008 #16
    About 7" per nanosecond as opposed to 12 in real life.
    Last edited: May 15, 2008
  18. May 16, 2008 #17


    User Avatar

    If the electrons in the wire accelerate on random directions, do the electrons emit EM radiation ? (DC current on steady state).
  19. May 16, 2008 #18


    User Avatar
    Homework Helper

    Signal progration velocity depends on the dielectric constant of the wire and the surrounding insulator. The equation is:

    [tex]V = \sqrt{\frac{1}{\varepsilon _r}}[/tex]

    A vacuum is defined to have a constant of 1, so it doesn't slow down the propagation rate. Air is just a bit more at 1.0059 (at 20C), while water is 80.4. So if a wire is submerted in water, the propagation rate could be slowed by a factor of almost 9. A link to some dielectric constants:


    This gets more complicated for AC signals. A couple of links about coax cables, signal speeds, and dielectric constants.


  20. May 16, 2008 #19
    There is little or no comparison between the movement of drifting electrons and those subjected to a Voltage (EMF). Plus in Physics, we are more more concerned with the holes left by the electrons as they move down the wire. So the direction of the current is reversed. But I digress, This is an apples vs oranges discussion and the EMF is the difference!
  21. May 16, 2008 #20
    Sorry guys, but I'm not sure yet.
    What is it that is traveling at near light speed down our copper wires?
    While your at it, why do you we need two wires for AC?
    And has anyone heard of the Catt anomaly about electric current that is heavily debated in the UK?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Speed of Electricity in Wiring - Why near light speed?
  1. Speed of light (Replies: 5)

  2. Speed of electricity (Replies: 17)

  3. Speed of light (Replies: 10)

  4. Speed of light (Replies: 7)

  5. Speed of Light (Replies: 1)