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Conductivity Speed

  1. Apr 18, 2010 #1
    Can anyone tell me at what speed an electrical signal travels through copper wire?

    How about fibre optic cable? (I'm REALLY new here).

  2. jcsd
  3. Apr 18, 2010 #2
    In fiber optics the signal has the speed of light in glass. That is about 2/3 of the light speed in air. For electrical wires it's comparable.
  4. Apr 19, 2010 #3
    I have always had an issue with the speed of electricity in a wire.
    As an electron has mass (although extremely small) it can not travel at the speed of light (einsteins relativity).
    But, a lot of physics teachings have said that conventional electricity is the movement of holes in the atoms not the movement of the electrons.
    This means that, altough an electron can jump from one atom to the next slowly, the hole will travel at a very fast rate in the opposite direction.
    For example, if a wire has ten atoms in it's length. The electron that enters the wire will take ten hops to exit the wire. But the hole that allows the movement of the electrons will travels from one end to the other ten times in the same period. As the holes have no mass, they can travel at the speed of light.
  5. Apr 19, 2010 #4
    Electricity is a conga line. Let's say that the dancers in a conga line begin by standing still. The individual dancers in the line can't move very fast, but, if the dancer at the back pushes the person in front of him, and that person pushes the person in front of him, and so on, the person in the front of the line will begin moving in next to no time. The signal to begin moving can move much faster than the individual carriers.
  6. Apr 19, 2010 #5
    You need to differentiate between the speed of electrons and the speed of signals in wires. If you have two parallel wires with a signal traveling in (actually between) them, the signal velocity is

    v = 1/sqrt(μ0ε0)

    where μ0 and ε0 are the permeability and permittivity of free space. This is exactly the definition of the speed of light.

    Bob S
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