By placing dielectric between plates why capacitance increases
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When a piece of insulator is inserted into a capacitor, we call the insulator a dielectric. The side of the insulator closest to the positive plate will be negative, while the side closest to the negative plate will be positive.
The electric field from the polarized dielectric will partially cancel the electric field from the charge on the capacitor plates. This decreases the net field inside the capacitor, and decreases the potential difference across the capacitor. To return the capacitor to its original potential difference, more charge is needed.
The net effect of the dielectric is to increase the amount of charge a capacitor can store for a given potential difference. The whole point of using a capacitor is to store charge, so coming up with a way to store more charge for the same amount of effort is a good thing.
Every material has a dielectric constant κ. This is the ratio of the field without the dielectric (Eo) to the net field (E) with the dielectric:
κ = Eo/E
E is always less than or equal to Eo
, so the dielectric constant is greater than or equal to 1. The larger the dielectric constant, the more charge can be stored.
Completely filling the space between capacitor plates with a dielectric increases the capacitance by a factor of the dielectric constant:
C = κ Co
, where Co
is the capacitance with no dielectric between the plates.
For a parallel-plate capacitor containing a dielectric that completely fills the space between the plates, the capacitance is given by:
C = κ εo * effective Area / distance between plates
The capacitance is maximized if the dielectric constant is maximized, and the capacitor plates have large area and are placed as close together as possible.
If a metal was used for the dielectric instead of an insulator the field inside the metal would be zero, corresponding to an infinite dielectric constant. The dielectric usually fills the entire space between the capacitor plates, however, and if a metal did that it would short out the capacitor - that's why insulators are used instead
The electric field from the polarized dielectric will partially cancel the electric field from the charge on the capacitor plates. This decreases the net field inside the capacitor, and decreases the potential difference across the capacitor. To return the capacitor to its original potential difference, more charge is needed.
The net effect of the dielectric is to increase the amount of charge a capacitor can store for a given potential difference. The whole point of using a capacitor is to store charge, so coming up with a way to store more charge for the same amount of effort is a good thing.
Every material has a dielectric constant κ. This is the ratio of the field without the dielectric (Eo) to the net field (E) with the dielectric:
κ = Eo/E
E is always less than or equal to Eo
, so the dielectric constant is greater than or equal to 1. The larger the dielectric constant, the more charge can be stored.
Completely filling the space between capacitor plates with a dielectric increases the capacitance by a factor of the dielectric constant:
C = κ Co
, where Co
is the capacitance with no dielectric between the plates.
For a parallel-plate capacitor containing a dielectric that completely fills the space between the plates, the capacitance is given by:
C = κ εo * effective Area / distance between plates
The capacitance is maximized if the dielectric constant is maximized, and the capacitor plates have large area and are placed as close together as possible.
If a metal was used for the dielectric instead of an insulator the field inside the metal would be zero, corresponding to an infinite dielectric constant. The dielectric usually fills the entire space between the capacitor plates, however, and if a metal did that it would short out the capacitor - that's why insulators are used instead
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