Electrical Engineering ⇒ Topic : Uniform Dielectric Medium
Uniform Dielectric Medium
The two parallel plates C and D are separated by a distance d as shown in Figure (a). The relativem permittivity of dielectric between C and D be εr. Let a charge +Q coulomb be given to plate C,the flux passing through the medium is:
ψ = Q coulomb C
Flux density in the medium is:
The electric intensity E is given by
Capacitance of Parallel-Plate Capacitor with Uniform Medium
We have seen that the capacitance of a capacitor can be determined from its electrical properties using the relation C = Q/V. However, it is often desirable to determine the capacitance of a capacitor in terms of its dimensions and relative permittivity of the dielectric. Although there are many forms of capacitors, the most important an-angement is the parallel-plate capacitor.
Consider a parallel plate capacitor consisting of two plates, each of area A square metres and separated by a uniform dielectric of thickness d metres and relative permittivity εr, as shown in Figure. Let a p.d. of V 'volts applied between the plates place a charge of +Q and Q on the plates. With reasonable accuracy, it can be assumed that electric field between the plates is uniform
Electric flux density between plates is
D = Q/A coulomb/m2
Electric intensity between plates is
The ratio Q/V is the capacitance C of the capacitor.
...in a medium
The following points may be noted carefully
(1) Capacitance is directly proportional to εr and A and inversely proportional to d
(2) Relative permittivity of medium
(3) Re-an-anging the relation for C in air
Obviously, permittivity can also be measured in F/m.
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