Dielectric polarization and capacitance

In summary: Can someone elucidate? If you can explain it in terms of the movement of electrons, that would be best! I already understand how the increase in capacitance is derived from the equations. Thanks!
  • #1
Tjornan
1
0
Hello all! My friend and I are having a bit of a discussion. We did an experiment on liquid crystals as dielectrics with changing voltage, and we came across some weird conclusions (capacitance decreased with increasing voltage? odd. Our theory is that liquid crystals don't act as normal dielectrics).

But that's not the purpose of this post! We are discussing why the capacitance of a capacitor increases if the dielectric becomes more polarized. He says he understands it, but his explanation doesn't make any sense to me.

I understand that an internal electric field inside the dielectric decreases the net electric field across the capacitor, but I don't understand how this increases the amount of charge the capacitor can store. Shouldn't a decrease in the net electric field across the capacitor decrease the number of electrons that can be stored on the capacitor, since it is the electric field that attracts them to the other side in the first place? Since the field is less, the attractive force is less. But common physics tells me my layman attempt at a solution is wrong!

Can someone elucidate? If you can explain it in terms of the movement of electrons, that would be best! I already understand how the increase in capacitance is derived from the equations.

Thanks!
 
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  • #2
A linear capacitor obeys a simple rule, C = Q / V.
If the slope of the Q / V relationship, (= capacitance), changes then the capacitor is non-linear.
Reverse biassed semiconductor junctions can have non-linear capacitance. An example is the varactor diode.
http://en.wikipedia.org/wiki/Varicap
 
  • #3
Welcome to PF;
Tjornan said:
My friend and I are having a bit of a discussion. We did an experiment on liquid crystals as dielectrics with changing voltage, and we came across some weird conclusions (capacitance decreased with increasing voltage? odd. Our theory is that liquid crystals don't act as normal dielectrics).
This is correct - for a given value of "normal" - see post #2. But this is not your question.

We are discussing why the capacitance of a capacitor increases if the dielectric becomes more polarized. He says he understands it, but his explanation doesn't make any sense to me.

I understand that an internal electric field inside the dielectric decreases the net electric field across the capacitor, but I don't understand how this increases the amount of charge the capacitor can store. Shouldn't a decrease in the net electric field across the capacitor decrease the number of electrons that can be stored on the capacitor, since it is the electric field that attracts them to the other side in the first place?
You appear to have this backwards.
Charge is moved from one plate to the other against the electric field between the plates.
Charges are repelled from their plates, not attracted.
 

1. What is dielectric polarization?

Dielectric polarization is the process in which an external electric field causes the alignment of polar molecules or atoms within a material, resulting in the separation of positive and negative charges. This leads to the formation of an induced dipole moment and the material becomes polarized.

2. How does dielectric polarization affect capacitance?

Dielectric polarization increases the capacitance of a material by reducing the effective electric field within the capacitor. This is because the polarized molecules or atoms act as additional charges, increasing the overall charge stored in the capacitor.

3. What factors affect the dielectric constant of a material?

The dielectric constant of a material is affected by the strength and orientation of the polar molecules or atoms within the material, as well as the density and temperature. It is also influenced by the type of dielectric material used, as different materials have different dielectric constants.

4. Can dielectric polarization be reversed?

Yes, dielectric polarization can be reversed by removing the external electric field. This causes the polar molecules or atoms to return to their original state, resulting in the material becoming non-polarized again.

5. How does the thickness of a dielectric material affect capacitance?

The thickness of a dielectric material affects capacitance by increasing the distance between the plates of a capacitor, thus reducing the electric field strength and increasing the capacitance. Thicker dielectric materials also have a higher capacitance compared to thinner ones.

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