Equipotential of transmission or telegraph line

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Discussion Overview

The discussion revolves around the concept of equipotential in the context of transmission or telegraph lines, particularly focusing on the presence of different voltages along a single conductor during wave propagation. The scope includes theoretical considerations and practical implications related to electrical circuits.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions how different voltages can exist in a single copper wire, asserting that equipotential implies uniform voltage throughout the conductor.
  • Another participant clarifies that different voltages can occur in a conductor at any time, citing examples such as dipole antennas where current and voltage distributions vary along the length of the wire.
  • A participant notes that voltage changes take time to propagate through a conductor, leading to temporary differences in potential until equilibrium is reached.
  • It is suggested that equipotential can be a useful approximation in circuits with small resistance, capacitance, inductance, and slow voltage changes.
  • Another participant adds that differences in voltage along a wire can occur only if the wire has zero resistance or no current is flowing, particularly under DC conditions, and highlights the role of inductance in AC scenarios.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the equipotential concept to conductors, with some asserting that different voltages can exist simultaneously while others emphasize the conditions under which equipotential can be considered valid. The discussion remains unresolved regarding the implications of these differing perspectives.

Contextual Notes

Participants mention assumptions such as zero resistance, the nature of current flow (DC vs. AC), and the time required for voltage changes to propagate, which may affect the applicability of equipotential concepts.

iVenky
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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.
 
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iVenky said:
Equipontential means that the voltage should be same everywhere in a single copper.

At equilibrium.
 
iVenky

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:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/equipot.html

Cheers,
Bobbywhy
 
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.
 
iVenky said:
T Now the question is how can we have two different voltages in a single copper wire?

Adding to the other excellent responses:

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

*and*

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.

*and*

We assume zero flight time.
 

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