Fsc 2nd Year chapter No :1 | ELECTROSTATICSFsc 2nd Year chapter No :1 | ELECTROSTATICS
CAPACITANCE OF A PARALLEL PLATE CAPACITOR
Parallel Plate Capacitor
The capacitance of flat, parallel metallic plates of area A and separation d is given by the expression above where:
= permittivity of space and k = relative permittivity of the dielectric material between the plates.
k=1 for free space, k>1 for all media, approximately =1 for air.
The Farad, F, is the SI unit for capacitance, and from the definition of capacitance is seen to be equal to a Coulomb/Volt.
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Capacitance of Parallel Plates
The electric field between two large parallel plates is given by
The voltage difference between the two plates can be expressed in terms of the work done on a positive test charge q when it moves from the positive to the negative plate.
It then follows from the definition of capacitance that
Electric Field: Parallel Plates
If oppositely charges parallel conducting plates are treated like infinite planes (neglecting fringing), then Gauss' law can be used to calculate the electric field between the plates. Presuming the plates to be at equilibrium with zero electric field inside the conductors, then the result from a charged conducting surface can be used:
This is also consistent with treating the charge layers as two charge sheets with electric field
in both directions.
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