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Equipotential of transmission or telegraph line

  1. Feb 1, 2013 #1
    Take a telegraph line or transmission line which is long. Now slowly the wave propagates through the wire in the forward direction. In the beginning one end of the wire will have a non-zero voltage while the other end will have zero voltage and it takes some more time to get settled after some reflections. Now the question is how can we have two different voltages in a single copper wire? Equipontential means that the voltage should be same everywhere in a single copper.
  2. jcsd
  3. Feb 1, 2013 #2


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    At equilibrium.
  4. Feb 1, 2013 #3


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    You ask “Now the question is how can we have two different voltages in a single copper wire?” I answer that we do have different voltages in a single conductor all the time. If not, radio and TV would not function, among lots of other things.

    In a simple dipole antenna, like a TV rabbit-ears the current distribution is roughly sinusoidal. It falls to zero at the end and is at a maximum in the middle. Conversely the voltage is low at the middle and rises to a maximum at the ends. It is generally fed at the centre, at the point where the current is at a maximum and the voltage a minimum.

    Equipotential lines are like contour lines on a map which trace lines of equal electric potential or voltage. Please note that it does not apply to a conductor like an antenna or copper wire. See:

  5. Feb 1, 2013 #4
    voltage changes require a finite amount of time to propagate through a conductor. If I apply a voltage to one end of a wire the other end will be at a different potential until the charges have had time to move around and equalize the potential. The principal of equipontential can be used as a good approximation for circuits where resistance, capacitance, inductance, and the physical size of the circuit are small and/or the rate of change of voltage is slow.
  6. Feb 1, 2013 #5
    Adding to the other excellent responses:

    Only if the resistance of the wire is zero or there is no current flowing.


    We are at DC. If we have AC, then the wire has inductance and the two ends of the wire are not the same node in an equivalent circuit.


    We assume zero flight time.
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