dipole in dielectrics

[UIUCstudent]

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A dipole in an external electric field has potential energy depending upon its orientation.

A dielectric contains dipoles as well and can be oriented in random fashion. So one explanation by EK is that "When the electric field begins to build up between the plates of a capacitor, the dipoles are rotated to point in the direction of the electric field. This rotation requires energy in the form of work done on the dielectric. The work is conserved in the field, thus the capacitor is able to store more energy."

I'm a bit confused about this. Dielectrics are insulators so their electrons can't be moving. What's going on here?

 

[Rabolisk]

I can give you an example. Water is an insulator, but a dielectric as it is polar.

 

[DougTPR]

I can give you an example. Water is an insulator, but a dielectric as it is polar.

To piggy back on Rabolisk's answer...

Electrons are not moving, but the water molecules can rotate.

 

[Bernoull]

I should be hangin out here more, i definitely miss physics, math too!

Let me see help u a little..

First off electrons are always moving extremely fast (random motion), the presence of an electric field simply results in net displacement of electrons (drift velocity). But this is somewhat beside the point for dielectrics (unless say you have dielectric breakdown).

Think about capacitance, C=Q/V = Q/Ed so it inversely related to V & E. Now to increase capacitance u want as much charge density while keeping voltage low (sufficiently high voltage will short the capacitor causing current flow = discharge of capacitor). To increase capacitance, one can reduce d (plate separation distance) or increase source voltage (increases Q) WITHOUT having current flow b/t the two plates. A dielectric allows you to do accomplish this. U can apply higher voltages (more charge buildup) and bring the plates closer (reduced d) without shorting the capacitor.

In the presence of an electric field, dipoles (of the dielectric) will reorient, negate some of the charge on the plates (think about equal and opposite coulomb forces) and this reduces the effective electric field (and voltage) across the parallel plates. Reduction in the effective electric field mean u can bring the plates even closer or build up more charge (increase capacitance essentially) while maintaining a low voltage = => more effective capacitor.

Coming back to insulators and the movement of their electrons (drift velocity); it's better to think of insulators as "poor conductors" rather than "insulators", because ALL insulators will conduct electricity given a sufficient electric field. They all have electrons, given enough force, the electron will move. In fact this is how u can induce charge separation in an "insulator" to create dipoles to begin with. An external electric field will push electrons away from the positive nucleus hence a dipole results.

Hope this helps.