# Dialectric inserted into capacitor => creation of a double-capacitor

## Main Question or Discussion Point

A dialectic of width d is inserted into a capacitor made up of two plates, with the plates being a distance D apart. D>d. Afterwards, the capacitor acts like it is two capacitor connected in series. Why? I mean how the heck is the dialectic supposed to get induced charges equivalent to the ones on the capacitor-plates?

the situation is illustrated in the solution manual. Look for assignment 4b ("Oppgave 4 b", page 4) http://folk.ntnu.no/sveinoll/ov/TFY/4155%20Elektrisitet%20og%20Elektromagnetisme/V11/LF/Ov06los.pdf [Broken]. The situation as described is like A, and the capacitor acts like in B.

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mfb
Mentor

I mean how the heck is the dialectic supposed to get induced charges equivalent to the ones on the capacitor-plates?
It is not, why do you expect this?

You can imagine adding an additional conducting plate of "0" thickness between the dielectric and the air (or whatever) without changing the capacity. But then you just have two different capacitors in series.
This is possible without dielectric, too, by the way.

A dialectric in series has equal charges on all the plates. Why doesnt this dialectric have that?

BruceW
Homework Helper

I'm not 100% certain what your question is. But putting a dielectric in-between the plates causes the permittivity in-between the plates to change. And this means that the capacity of the capacitor will change. So by selecting a material of a certain permittivity, you can choose the capacitance of the capacitor. pretty neat.

sophiecentaur
Gold Member

With or without a layer of metal between the dielectric and the air, I think the same situation exists. There will be a potential gradient across the air gap and a different (shallower) potential gradient across the dielectric because the molecules of the dielectric can polarise. It's just like two capacitors in series, sharing the total PD. The equivalent capacitance will be greater than for an air gap and less than when full of dielectric.

davenn
Gold Member
2019 Award

A dialectric in series has equal charges on all the plates. Why doesnt this dialectric have that?
its spelt .... Dielectric :)

It seems you dont realise or understand what a dielectric is
its the non-conductive layer between 2 plates of a capacitor
it may be composed of air, paper or ceramic amongst other things

The thickness and composition of which, as others have said, will have an effect on the value of capacitance

Dave