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Dielectric polarization and capacitance

by Tjornan
Tags: capacitance, dielectric, polarization
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Tjornan
#1
Jan25-14, 10:39 PM
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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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Baluncore
#2
Jan26-14, 02:02 AM
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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
Simon Bridge
#3
Jan26-14, 06:56 PM
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Welcome to PF;
Quote Quote by Tjornan View Post
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.


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