What happens if plates of charged capacitor gradually separated?

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Discussion Overview

The discussion revolves around the behavior of a charged parallel plate capacitor when the plates are gradually separated. Participants explore the implications for charge, capacitance, voltage, and energy within the system, considering both theoretical and practical aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions what happens to the charge when the plates are separated, suggesting that excess charge might radiate or cause arcing.
  • Another participant challenges the idea of "excess" charge, arguing that in a theoretical vacuum, charge does not vanish simply due to separation.
  • It is proposed that separating the plates increases the charge strength as energy stored in the electric field is returned to the plates, while also noting that work is done against the attractive force during separation.
  • A later reply states that the charge remains constant, while capacitance decreases and voltage increases inversely, leading to an increase in stored energy due to the work done in separating the plates.
  • One participant expresses surprise at the idea that energy is added to the system and indicates a desire for more information about capacitance.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of charge and energy in the system, with no consensus reached on the implications of separating the plates of the capacitor.

Contextual Notes

Some assumptions about ideal conditions, such as the presence of a perfect vacuum, are mentioned but not fully explored. The discussion also touches on the relationship between capacitance, voltage, and energy without resolving the mathematical details.

Who May Find This Useful

This discussion may be of interest to those studying capacitor behavior, energy storage in electric fields, or related concepts in physics and electrical engineering.

Rolliet
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Say you have a charged, parallel plate capacitor, isolated from any external circuit. If you were then to gradually separate the two plates (or thicken the dielectric) what would happen to the charge on the two plates ?
 
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What do you think would happen? Where could the charge go?
 
My guess is that excess charge must radiate. perhaps arcing or coronal discharge
 
What "excess" charge? Why should it go anywhere?
 
If you think of a capacitor like that in a perfect vacuum (theoretical conditions) there is no where for the charge to go and just by separating the plates doesn't make the charge to vanish.
Indeed what you would get is a increase in charge strength because now the energy that was stored in the electric field goes back to the plates.A capacitor works by that as it stores energy in electric field , hence separating the plates further and further would diminish the field between the plates.
+ you are doing work on the plates as you are separating them.
 
Crazymechanic said:
If you think of a capacitor like that in a perfect vacuum (theoretical conditions) there is no where for the charge to go and just by separating the plates doesn't make the charge to vanish.
Indeed what you would get is a increase in charge strength because now the energy that was stored in the electric field goes back to the plates.A capacitor works by that as it stores energy in electric field , hence separating the plates further and further would diminish the field between the plates.
+ you are doing work on the plates as you are separating them.

Thanks crazymechanic. Yes, an increase in charge strength makes sense. Is there a quantum physics description of what would be going on here ?
 
The correct answer is that the charge remains constant. The capacitance decreases as the plates are separated, and the voltage increases in inverse proportion. I.e., if you halve the capacitance, the voltage doubles.
Since the energy stored is proportional to capacitance and to voltage squared, the energy is increased! Where does the extra energy come from? It comes from the work you do in separating the plates because you are working against the attractive force. This is the principle of the Electrophorus and of the Wimshurst Machine (q.v.).
 
Thanks. I never thought it'd add energy. I have many questions about capactance.
 

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