A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the plates. The effect is greatest between wide, flat, parallel, narrowly separated conductors.An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current.
An electric field E exerts a torque on an electric dipole of diploe
moment p. The torque is p*E*sin(theta), where theta is the angle between the E-field and the dipole moment
vector. The torque tends to align the dipole with the field.
The potential energy of an electric dipole placed in an electric
U = p.E = -p*E*cos(theta).
When a parallel plate capacitor is filled with a dielectric
(insulator) of dielectric constant K, the dielectric becomes polarized (a dipole moment is induced in the
dielectric). This, in turn, causes a reduction in the value of E by K, and a corresponding increase in the value
of the capacitance by K.
Examples that show how to calculate the capacitance of a parallel
plate capacitor if it is partially filled with a metal or a dielectric are given in the lecture.
More on Filled Capacitors
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