Pleeeease Help Why a dielectric absorbs only a certain amt of field

[jtbell]

Perhaps the attached diagram will help. It shows the electric field before and after inserting a dielectric into a capacitor, and how the potential difference across the capacitor is calculated in both cases.

The arrows are electric field lines, not vectors. The strength of the electric field is proportional to the number of field lines, not to their length. The original electric field is $E_0$. When the dielectric is inserted, the polarization induces an effective electric charge on the surfaces of the dielectric, which produces an additional induced electric field $E_i$ inside the dielectric. For this example I assume that $E_i = E_0 / 2$. The net electric field inside the dielectric is $E_0 - E_i$.

The + and - represent the charge on the capacitor and the induced charge on the surfaces of the dielectric.

The final result is that the potential difference $\Delta V$ across the capacitor is reduced (in this example) from $E_0 d$ to $E_0 d - 0.6 E_i d$. The 0.6 is a purely geometrical factor. It appears because I chose the dielectric to fill 0.6 of the space between the capacitor plates. If you expand the dielectric to fill more and more of the space, this factor increases accordingly, until it reaches 1.0 when the dielectric fills the space completely.

 

[Mr Virtual]

I really appreciate and thank you that you took out time to draw the diagram.
But my question is still unanswered. A field Eo is applied across the two plates of a capacitor. When we introduce a dielectric, an induced field Ei builds up which opposes Eo, thus reducing it to E. Here, E<Eo. Now suppose that we introduce an insulator inside the capacitor. The molecules of the insulator barely stretch, so Ei is negligible. Thus, when we do E=Eo-Ei, then E~Eo (i.e. E should approx equal Eo). Is this right or wrong?

Mr V

 

[jtbell]

A dielectric is an insulator! If it were not an insulator, that is, a conductor, there would be an electric current through the capacitor and it wouldn't be a capacitor any more!

What you're describing is a dielectric with a vanishing dielectric constant. Such a material indeed would not be very useful as a dielectric. But good dielectrics with high dielectric constants are nevertheless still insulators.

The fact that a dielectric gets an induced polarization when immersed in an electric field does not make it a conductor. What makes a material a conductor is that some of its electrons are not bound to any particular atom but instead can roam throughout the material. In an insulator, all the electrons are bound to particular atoms, but in general they can still shift their average position very slightly with respect to their nucleus, so the atoms are polarizable. Some kinds of atoms or molecules are more polarizable than others, so they make better dielectrics.

 

[Mr Virtual]

I am very much clear about what you have told me. I am stating the words 'dielectric' and 'insulator' just to indicate (yes! you guessed it right) the diminishing dielectric constant. Perhaps I was using wrong words which was giving you such an impression.
But thanks a LOT! I have finally got the answer I have been trying to get for the last one month ( I started this thread on 6 June)!

Such a material indeed would not be very useful as a dielectric.

This was what I needed. So an insulator (please forgive me for my choice of words) stops flow of electrons through it, but does a bad job in blocking the field. Most of the applied field just passes through it at the other end. On the other hand, a dielectric (or an insulator with a high dielectric constant) both stops flow of electrons and blocks a considerable amount of field (due to polarization).

Thanks, thanks, thanks, once again!

 

[jtbell]

OK, I see where our confusion lies now. We're using the word "insulator" to mean different things. To me, "insulator" simply means something that blocks the flow of electric current, without regard for its properties as a dielectric. You were apparently using "insulator" to mean something that also has a high dielectric constant.

 

[Mr Virtual]

You were apparently using "insulator" to mean something that also has a high dielectric constant.

A low dielectric constant (not high) is what you mean, isn't it?

Mr V