Something about parallel capacitors

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Discussion Overview

The discussion revolves around the behavior of parallel capacitors when the distance between the plates is altered, specifically comparing scenarios where the switch is opened versus when it remains closed. The scope includes conceptual understanding and technical reasoning related to capacitance, voltage, charge, and energy transfer.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants note that moving the plates before opening the switch keeps voltage (V) constant, while charge (Q) and electric field (E) are not constant.
  • Others argue that moving the plates after opening the switch keeps charge (Q) and electric field (E) constant.
  • One participant mentions that moving the plates involves doing work, which can be converted to potential or kinetic energy depending on whether there is a conducting path.
  • Another participant states that increasing the distance between the plates decreases capacitance and leads to an increase in voltage, resulting in some charges moving back to the power supply.
  • There is a claim that the dielectric constant would not change unless the dielectric material itself is altered.
  • A later reply humorously challenges the attention of participants regarding the dielectric constant statement.

Areas of Agreement / Disagreement

Participants express differing views on the effects of moving the plates with respect to voltage, charge, and dielectric properties, indicating that multiple competing views remain without consensus.

Contextual Notes

Some participants reference textbook explanations, which may introduce confusion regarding the relationships between voltage, charge, and electric field when the switch is opened or closed. There are also assumptions about the nature of the dielectric constant and its relevance to the discussion.

illuminati23
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If two parallel plates are facing one another and one of them are moved without opening the switch, what's the difference with moving the plates after opening the switch?
 
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Sorry, You'll have to be more detailed than that.
 
If they are moved before opening, V is constant. If after, Q is constant.
 
willem2 said:
Sorry, You'll have to be more detailed than that.

So my textbook says that if the distance between the parallel plates are increased from d to d' without opening the switch first(so that means that electric current is flowing), then only V is constant and Q, E is not constant. But if d is increased to d' after opening the switch(stopping the current), then Q and E is constant.
 
Moving the plates involves doing work. That work will be transferred to potential energy or Kinetic Energy (a flow of charge), depending on whether there is a conducting path or not.
 
illuminati23 said:
So my textbook says that if the distance between the parallel plates are increased from d to d' without opening the switch first(so that means that electric current is flowing), then only V is constant and Q, E is not constant. But if d is increased to d' after opening the switch(stopping the current), then Q and E is constant.
When the switch is closed then V is the value of the power supply. So if you then increase d you put some energy back in the power supply. Increasing d means lowering the value of the capacitor. Since Q=C*V then Q will decrease. Also since E=V/d it means that E decreases. So your text reads: only V is constant, and Q and E are not constant. I think your book is a bit confusing (but correct).
The second part is OK.
 
illuminati23 said:
If two parallel plates are facing one another and one of them are moved without opening the switch, what's the difference with moving the plates after opening the switch?

-dielectric constant will decrease, capacitance will decrease, voltage between the plates will increase, and some of the charges will move back to the power supply. The energy to move the electrons back to the source will come from the work of the force that is used to depart the plates.
 
Last edited:
crx said:
-dielectric constant will decrease, capacitance will decrease, voltage between the plates will increase, and some of the charges will move back to the power supply. The energy to move the electrons back to the source will come from the work of the force that is used to depart the plates.

Dielectric constant is a material property, it would not change here unless one were to change the dielectric between the plates.
 
Born2bwire said:
Dielectric constant is a material property, it would not change here unless one were to change the dielectric between the plates.

Of course it doesn't! I just wanted to see if you pay attention...:redface:
 
Last edited:

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