How Can the Potential Due to an Infinitely Large Plate Be Justified?

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    Plate Potential
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Discussion Overview

The discussion revolves around the justification of the potential due to an infinitely large conducting plate in the context of the classical image problem. Participants explore the implications of potential at infinity and its behavior near the plate, considering both theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how the potential due to the sheet can be justified as zero at "all" infinities.
  • Another participant argues that if the potential goes to zero at infinity in one direction, it must do so in all directions due to the nature of the Laplace equation and the behavior of Legendre polynomials.
  • There is a challenge regarding the potential at a point infinitely separated from the point charge but just above the plate, with participants asserting that it is also zero.
  • One participant suggests that the symmetry of the field lines above the plate leads to a decrease in potential as an electron moves along these lines, emphasizing the significance of the term "infinitely."
  • Another participant points out that the situation is highly unsymmetrical, complicating the justification of potential values.
  • A participant notes that at a point far from the point charge, the electric potential from the charge can be neglected, leading to a potential near that of the plate, which is also zero.
  • It is mentioned that the potential due to a point charge is inversely proportional to the distance, reinforcing the idea that it approaches zero at infinity.
  • One participant summarizes that the fundamental point is the inverse proportionality of potential with distance and the continuity of potential.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of potential near the plate and its justification at infinity. While some agree on the potential being zero at certain points, others raise challenges regarding the symmetry and the implications of distance from the point charge.

Contextual Notes

The discussion highlights limitations in assumptions about symmetry and the dependence of potential on distance from charges, as well as the unresolved nature of certain mathematical steps in the justification process.

pardesi
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i was going through the proof of the classical image problem in which u have agrounded conductor and you have a charge q above it and u r asked to find potenntial at all points above the conductor the proof uses the fact that the potential due to the sheet at "all" infinities is 0.how does one justify this
 
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If the potential goes to zero at infinity as approached in one direction then it goes to zero in all directions because outside the region containing the charge (far from the plate and the pt charge) I am just solving the Laplace equation. Thus, the angular dependence is given by Legendre polynomials and the radial dependence is at most 1/r. Since the Legendre polynomials never blow up the potential falls off at least as fast as 1/r regardless of the direction.
 
what about a point infinitely separated from the point charge but just above the plate
 
pardesi said:
what about a point infinitely separated from the point charge but just above the plate

it is also zero!
 
walkinginwater said:
it is also zero!

well how is that proved?
 
pardesi said:
well how is that proved?

Because of the symmetry, the field line above the plate will be upwards. Assume you move the electron along the field lines. The electron will always accelerated along the field lines, i.e., the potential will decrease. The key issue here is the word "infinitely", it means that you can move an electron along the field lines infinitely until the potential decrease to zero!
 
yes far above the plates that is true but what i asked before and now is a point far from the point cahrge but just close to the plate
 
and one more thing adding to the problem is the situation is highly unsymmetrical
 
pardesi said:
and one more thing adding to the problem is the situation is highly unsymmetrical

hi, Pardesi:
A point far from the point charge means that the electric potential caused by the point charge can be neglected; close to the plate means that its potential is near the potential of the plate. so it is also zero
The key point is that the potential caused by the point charge is inversely proportional to the distance from the charge. So basically , at the infinitely far away from the particle charge, the potential due to the particle charge is zero.
 
  • #10
It doesn't matter. The potential of a point charge is [tex]\propto 1/r[/tex]; so it drops to zero at infinity and therefore so does the (potential of the) induced charge in the plate.
 
  • #11
so if i am not wrong the fundamental point is inverse propotionality of v with r and the continuity of v
 
  • #12
pardesi said:
so if i am not wrong the fundamental point is inverse propotionality of v with r and the continuity of v
Ja, you got the point!
 
  • #13
thank u all
:rolleyes:
 

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