Capacitors, Electric Fields, and Dielectrics

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Discussion Overview

The discussion centers around the behavior of electric fields in capacitors when a dielectric is introduced while a constant potential difference is maintained. Participants explore the implications of dielectric materials on capacitance, charge, and electric field strength, considering both parallel and non-parallel plate capacitors.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the electric field in a capacitor decreases when a dielectric is inserted while maintaining a constant potential difference, suggesting that capacitance and charge would increase but the electric field should remain the same.
  • Another participant agrees with the initial claim that the electric field does not decrease under these conditions.
  • A request for credible sources to support the claim is made, indicating a disagreement with a physics teacher who believes the electric field decreases.
  • A formula for the electric field in a parallel plate capacitor is provided, emphasizing its dependence on potential difference and plate separation.
  • One participant speculates that the teacher's misunderstanding may stem from the effect of the dielectric's induced electric field, which could reduce the electric field if the voltage source is removed before inserting the dielectric.
  • Another participant confirms that the electric field is determined by the applied voltage and geometry, not by the dielectric's presence, and clarifies the sequence of events that could lead to a misunderstanding.

Areas of Agreement / Disagreement

Participants generally agree that the electric field does not decrease when a constant potential difference is applied with a dielectric present. However, there is a recognition of differing views, particularly from the physics teacher, regarding the effects of dielectrics when the voltage source is removed prior to insertion.

Contextual Notes

Participants note the importance of the sequence of actions (applying voltage, removing the source, inserting dielectric) in determining the electric field's behavior, indicating that assumptions about these actions may lead to different conclusions.

gokugreene
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I have a question that is confusing me perhaps one of you can help me.
If I hook up a constant potential difference to a capacitor and place a dielectric inside of it, will the electric field decrease even if the plate separation remains constant?
I think that the capacitance will increase as well as the charge on the plates, meaning that the electric field would have to remain the same, since the potential is constant? or am I wrong? if so why?
Thanks for the help
 
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You are correct.
 
Do you know of anyplace that offers credible proof like in a book or an article?

This was a test question and I argued with my physics teacher about it. He thinks the electric field will decrease.. where I do not and he said I am wrong.

I would like to show him that the electric field doesn't decrease when a contstant potential difference is applied.
 
The electric field in a parallel plate capacitor, as you apparently realize, depends only on the potential difference and the distance between the plates:
E = \frac{\Delta V}{d}[/itex]<br /> <br /> This kind of thing should be in any textbook; here&#039;s a website that discusses it: <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html#c2" target="_blank" class="link link--external" rel="noopener">http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html#c2</a>
 
That is the formula I explained to him. I don't see why he doesn't get it
 
Is it because he thought that the electric field is decreased due to the presence of the opposite electric field set up by the dielectric? But he forget that the voltage is still applied after the insertion of dielectric, which will bring the electric field back.

Pls correct me if I'm wrong.
 
That must be what he is thinking.

DocAl, would this also apply to non parallel plate capacitors?
 
gokugreene said:
DocAl, would this also apply to non parallel plate capacitors?
Sure. The electric field is determined by the applied voltage and the geometry of the conductors, not by the presence of a dielectric.

As Alpha2005 notes, your instructor is probably thinking of this sequence:
(1) Apply a voltage to the (empty) capacitor
(2) Remove voltage source
(3) Insert dielectric​
In this case, she would be correct. The induced polarization charge within the dielectric will reduce the effective electric field.
 

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