Question about capacitors with dielectrics

  • Context: Undergrad 
  • Thread starter Thread starter anon6912
  • Start date Start date
  • Tags Tags
    Capacitors Dielectrics
Click For Summary

Discussion Overview

The discussion revolves around the behavior of a capacitor when a dielectric material is introduced to one of its plates after disconnecting the voltage source. Participants explore the implications of charge density changes and energy considerations in this scenario.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the net charge density on each plate remains the same or differs after introducing the dielectric to only one plate.
  • Another participant suggests that while the charge is conserved, the polarization of the dielectric increases the capacitance, effectively reducing the voltage across the capacitor.
  • A third participant agrees that the charge distribution remains unchanged and discusses the relationship between stored energy and capacitance, indicating that energy decreases when the dielectric is inserted.
  • One participant adds that the work done in moving the dielectric is analogous to the work done if the plates were allowed to move closer together, implying a connection between physical movement and energy changes.

Areas of Agreement / Disagreement

Participants generally agree on the conservation of charge and the effects of dielectric polarization on capacitance and energy. However, the initial question regarding the charge density on each plate remains unresolved, with differing interpretations of the implications of introducing the dielectric.

Contextual Notes

The discussion does not clarify the specific assumptions regarding the nature of the dielectric or the initial conditions of the capacitor, which may affect the interpretations of charge density changes.

anon6912
Messages
21
Reaction score
3
Say i have a capacitor and i supply it with a voltage V and after that it has charge density sigmainitial on each plate.

Now i remove the voltage source and then introduce a dilectric with area A that it equal to the area of one of the plates in the capacitor to the capacitor.

But this dielectric is only touching one plate.

My question is: is the net charge density on each plate(sigmainitial - sigmainduced) the same in both plates or different?

please explain...
 
Physics news on Phys.org
I think the answer is that the same charge is there but the molecules in the dielectric have all polarised a bit. This has increased the capacity ov the capacitor. Equivalent to moving the plates closer together. Now
Q=CV
so the voltage will have dropped. As for the energy difference, some work has been done in moving the dielectric into place- nothing violated either way, I think.
 
Hi Anon-
Sophie is correct. Charge is conserved, and the charge distribution is unchanged. The stored energy is

E = Q2/2C

So when the dielectric is pulled (not pushed) in, the stored energy drops, and work is done.

Bob S
 
Yes- the same work that the plates would do if they were allowed to come together and have the equivalent capacitance.
 

Similar threads

High School Rotating Capacitor: Concentrating energy using accessible technology
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Is capacitor energy the electrostatic energy of the whole system?
  • · Replies 9 ·
Replies
9
Views
2K
Undergrad Why is a Dielectric slab ejected from the capacitor when energized?
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Parallel plate capacitor, charge imbalance VS charge redistribution
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Charging Capacitor with one terminal grounded
  • · Replies 16 ·
Replies
16
Views
3K
Undergrad Magnetic field when the area of the plates of a capacitor increases
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Capacitor surface charge movement current, relativistic effects
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad A capacitor with two dielectrics in series
  • · Replies 6 ·
Replies
6
Views
7K
Graduate An electrolytic capacitor keeps charging by itself
  • · Replies 15 ·
Replies
15
Views
3K
Graduate A Capacitor With A Dielectric But Also Free Space in Between
  • · Replies 5 ·
Replies
5
Views
2K