dielectric/capacitors

[monkeyvokes]

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Wikipedia has this today about dielectrics:
"A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field


How does this reduce the overall field within the dielectric itself? I guess I'm not sure exactly what that means. Any help is appreciated, thanks

 

[phys2med]

The electric field in the dielectric opposes the direction of the external field. Since E fields are vector fields, you can think of "subtracting out" the internal field from the external so the overall effect of the field is less than the original external.

 

[monkeyvokes]

The electric field in the dielectric opposes the direction of the external field. Since E fields are vector fields, you can think of "subtracting out" the internal field from the external so the overall effect of the field is less than the original external.

the external would be the field created by change separation between two plates of a capacitor?

 

[phys2med]

No its the applied field

 

[monkeyvokes]

No its the applied field

So an applied field between the plates of a capacitor is created by the circuit. An internal field can be created by a dielectric between the plates if the particles inside shift their orientation. What is the external field? I am a bit confused

 

[phys2med]

Forget any circuit. A dielectric is just there. Someone creates an electric field (who cares how or how strong). This field is the one I'm calling applied or external. These terms are used quite often to mean the same thing

 

[sps27]

Wikipedia has this today about dielectrics:
"A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field


How does this reduce the overall field within the dielectric itself? I guess I'm not sure exactly what that means. Any help is appreciated, thanks

So I have a capacitor correct. And I charge it with an external source, say a battery. So what happens. Positive charge builds up on the plate connected to the positive end of the battery. Negative charge builds up on the plate connected to negative end of the battery. So essentially, an electrical field has now been created between the two charged plates of a capacitor. The direction of the electrical field between those charged plates, is from +ve plate to -ve plate. So ok. Now what. Well, let's put a dielectric in between the plates. So when we do that the ions, dipoles, charged particles within the dielectric near the +ve plate of capacitor will re-orient themselves so that -ve charges starts to accumulate on that edge, because of attraction. Similarly the edge of the dielectric near the -ve plate will start to accumulate +ve charges, ions, dipoles with +ve ends etc. So because of that there is an electric field within the dielectric itself which is exactly opposite in direction to the electric field created by the plates of the capacitor. So then what. Well, when that happens, the electric field between the plates of capacitor has now gone down because of opposing electric field in the dielectric, which means, charge begins to flow once again from the battery to the plates of the capacitor to charge it. Hence by introducing the dielectric we have essentially increased the capacity of the capacitor to hold more charge. And I think that is why Capacitor with a dielectric (stronger the better) holds more charge than one without.

The wording about reducing the electric field within the dielectric is a bit confusing but I think all they mean is introducing the dielectric in capacitor will reduce the electric field between the plates of capacitor, which allows it to store more charge and hence have more capacitance.

At least that is my understanding. Hope it helps....

 

[monkeyvokes]

So I have a capacitor correct. And I charge it with an external source, say a battery. So what happens. Positive charge builds up on the plate connected to the positive end of the battery. Negative charge builds up on the plate connected to negative end of the battery. So essentially, an electrical field has now been created between the two charged plates of a capacitor. The direction of the electrical field between those charged plates, is from +ve plate to -ve plate. So ok. Now what. Well, let's put a dielectric in between the plates. So when we do that the ions, dipoles, charged particles within the dielectric near the +ve plate of capacitor will re-orient themselves so that -ve charges starts to accumulate on that edge, because of attraction. Similarly the edge of the dielectric near the -ve plate will start to accumulate +ve charges, ions, dipoles with +ve ends etc. So because of that there is an electric field within the dielectric itself which is exactly opposite in direction to the electric field created by the plates of the capacitor. So then what. Well, when that happens, the electric field between the plates of capacitor has now gone down because of opposing electric field in the dielectric, which means, charge begins to flow once again from the battery to the plates of the capacitor to charge it. Hence by introducing the dielectric we have essentially increased the capacity of the capacitor to hold more charge. And I think that is why Capacitor with a dielectric (stronger the better) holds more charge than one without.

The wording about reducing the electric field within the dielectric is a bit confusing but I think all they mean is introducing the dielectric in capacitor will reduce the electric field between the plates of capacitor, which allows it to store more charge and hence have more capacitance.

At least that is my understanding. Hope it helps....

yeah that does make sense thanks

 

[sps27]

That fish made me laugh, i dunno why, but it did....good drawing....

 

[monkeyvokes]

lol 😀