Charge accumulation on a capacitor

In summary, capacitors work because of the electric field that is created between the plates and the dielectric. More charge accumulates on the plates when a dielectric is inserted because the electric field is weakened in the dielectric.
  • #1
nbsmith
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When a capacitor is hooked up to a battery, why does more charge accumulate on the plates of a capacitor when a dielectric is inserted?

I've read a lot of mathematical answers to this question (based on C = Q/V), but I'd like a more causal explanation of what's really happening. Here's my understanding so far:

1) I begin by imagining a parallel plate capacitor hooked up to a potential difference (battery) without a dielectric. Equal but opposite charge accumulates on each plate. This happens because the negative terminal of the battery has excess electrons that repel some of themselves to the plate they are connected to. And the positive terminal attracts electrons creating a positive charge on the plate it is connected to.

Charge is attracted by opposite charge and repulsed by like charge. Charge stops accumulating when the attractive and repulsive forces are equal. (The geometry of the capacitor of course also affects how much will accumulate.)

2) As a result of this, an electric field will be created across the plates of the capacitor.

3) Now imagine a dielectric is placed in between the plates of a capacitor. The molecules of the dielectric will slightly polarize against the electric field of the plates. This will in tun create a weaker electric field. This field will act in the opposite direction of the first (the one caused by the plates).

Now I'm not completely sure why more charge would accumulate on the plates, but here's what I think:

4) Back to number 1, charges stopped accumulating on the plates because they were in balance between the repulsive and attractive forces acting on them. But now because of the polarized molecules of the dielectric, there is addtional "pull" on each of the plates. (There is a slight pull from the negative side of the molecules on the positive plate and vice-versa.) This causes additional charge to be attracted to each of the plates.

Here's a common picture visualizing this: http://www.chemistrydaily.com/chemistry/upload/c/c3/Dielectric.png"

Now everything I've said seems to makes sense to me, but here's my concern: In the picture above (and I've seen a number just like it), the electric field does not seem to change in the regions between the plates and the dielectric (assuming there is a gap). It is only weakened within the dielectric. If this is true, and the opposite field does not extend outside of the dielectric, then there shouldn't be any additional pull on the plates, and I don't see why more charge would accumulate.

So I see one of two possibilities:
a) The diagram is correct and there is another explanation for the extra accumulation of charge.
b) The diagram is incorrect and my explanation for the extra accumulation of charge is correct. (In other words, the field is also weakened outside the dielectric in the region between the dielectric and the plates.)

It does seem to me that the electric field should change outside the dielectric as well, but I may be wrong.
 
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  • #2
There are two basic concepts to consider here
A. Electrons in a conductors will move and keep on moving until the conductor is an equipotential.
B. The job of a battery is to keep whatever conductors are connected to its terminals at a constant potential difference.
As you mentioned,
nbsmith said:
Now imagine a dielectric is placed in between the plates of a capacitor. The molecules of the dielectric will slightly polarize against the electric field of the plates. This will in tun create a weaker electric field. This field will act in the opposite direction of the first (the one caused by the plates).
The weaker electric field means that there is less of a potential difference between the plates, which are connected by (equipotential) wires, all the way to the battery terminals. The battery does its job and pumps more charge onto the plates until the potential difference between the plates is back to what it was before the inserion of the dielectric.
 
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Likes berkeman

1. What is a capacitor and how does it work?

A capacitor is an electronic component that stores electrical energy in an electric field. It is made up of two conductive plates separated by an insulating material, also known as a dielectric. When a voltage is applied across the plates, one plate becomes positively charged and the other becomes negatively charged, creating an electric field between them. This electric field stores the energy until it is discharged.

2. How does charge accumulate on a capacitor?

Charge accumulation on a capacitor occurs when a voltage is applied across the plates. Electrons from the negative plate are repelled and move to the positive plate, creating a buildup of negative charge on one side and positive charge on the other. The amount of charge that accumulates is directly proportional to the applied voltage and the capacitance of the capacitor.

3. What factors affect charge accumulation on a capacitor?

The amount of charge that accumulates on a capacitor is affected by the voltage applied, the capacitance of the capacitor, and the dielectric material between the plates. A higher voltage or larger capacitance will result in a greater charge accumulation, while a thicker or more insulating dielectric material will decrease the amount of charge that can accumulate.

4. How does charge accumulation affect the behavior of a capacitor?

The amount of charge accumulated on a capacitor determines its ability to store and release electrical energy. A higher charge accumulation means that the capacitor can store more energy, while a lower charge accumulation means that it can store less energy. This affects the behavior of a capacitor in circuits, as it can be used for tasks such as filtering, timing, and energy storage.

5. Can charge accumulation on a capacitor be reversed?

Yes, charge accumulation on a capacitor can be reversed by discharging the capacitor. This can be done by connecting the two plates together, allowing the charges to equalize and return to their original state. Alternatively, the capacitor can be connected to a circuit that allows the stored energy to be used, which will also result in a discharge of the capacitor.

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