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

What does an infinite line of charge represent?

  1. Mar 13, 2015 #1
    In my EM book, Wentorth, he used the example of a transmission line as an infinite line of charge. The book states "...a test charge placed a couple of centimeters from an elevated transmission line will see what appears to be an infinite length line (of charge)." I'm confused why this would appear to be a line of charge if there is an equal amount of positive and negative charges inside the transmission line or conductor. It would seem to me that a single conductor would appear as an infinite line of current. While a transmission line would seem like two opposite direction lines of current.
     
  2. jcsd
  3. Mar 13, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    A line of current is a line of charge - a line of moving charge.
    In a neutral conductor, the equal amounts of positive and negative charge may not be uniformly distributed. In a transmission line the free electrons are usually on the outside.
    The model would probably be for a charged transmission line.
     
  4. Mar 14, 2015 #3
    I do not have your book, but we usually consider the electrons to be light and mobile and the positive charges to be heavy and fixed. If a charge is suddenly placed near the line, the nearest electrons will move and then the subsequent ones, forming a ripple which travels along the line near the speed of light. If two wires are used, the impulse travels along both in the same direction, but with 180 degree phase difference. I have confirmed myself that a single wire transmission line does work, even without ground beneath it.
     
  5. Mar 15, 2015 #4
    Think about it , in a transmission line where we have put a voltage source at the one side, current will flow that will flow far away (miles and tens of miles) from the source. What moves the electrons that are located in the transmission line at points far away from the voltage source? Its the electric field that exists there. That electric field is created by the surface charge densities that are created through out the whole line for miles far away from the voltage source.So the line as a whole doesnt have a net electric charge , yet the charge density across its surface is not everywhere zero because the electrons and the protons are not uniformly distributed. What causes that non-uniformity is the voltage source that initiates a ripple in the charge density , in the current density, and in the electric and magnetic fields around and inside the transmission line that can travel as wave far away from the voltage source according to maxwell's equations.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook