This result is interesting when applied to the potential energy PEe = ½C(ǻV)^2 stored by a capacitor. After you insert a dielectric, the potential
energy becomes
In other words, the potential energy decreases when you insert a dielectric between the plates of a capacitor. Why? The capacitor is doing
work on the dielectric as you insert it, rotating many of its small dipoles into alignment.
26.10 - Interactive checkpoint: dielectric in a capacitor
A parallel plate capacitor has a
capacitance of 3.50 ȝF when a
vacuum separates its plates. You
insert an insulator with a dielectric
constant of ț = 5.60 and apply a
potential difference of 145 V across
the plates.
How much charge builds up on a
plate?
Answer:q = C
26.11 - Physics at work: commercial capacitors
Capacitors in electronic equipment, like a stereo
system, that you might have at home are often made
from two layers of metallic foil backed by thin sheets
of a dielectric material. When these layers are rolled
up, the result is a multilayer cylindrical capacitor with
alternating positive and negative cylindrical
conductors, separated by the dielectric material.
Several capacitors of this type are shown in Concept
Higher voltage capacitors can consist of sets of thin
parallel metallic plates immersed in insulating silicone
oil. Other capacitors, capable of storing large
amounts of charge, are made with insulators
consisting of ceramic materials that can have very high dielectric constants.
Avariable capacitor, like the one in the picture above, features interleaved metal plates.
You change the capacitance by rotating the knob, increasing or decreasing how much
of the area of each plate is close to the neighboring plates on either side. Devices like
this can be used to change the capacitance of a circuit in a radio tuner, altering what is
called the resonant frequency of the circuit and enabling it to “tune in” a radio station
broadcasting at a particular frequency.
Electrolytic capacitors can operate with very high potential differences across their two
conducting surfaces. An electrolyte is a nonmetallic conductor, or a substance that
when dissolved in a suitable solvent becomes a conductor. In an electrolyte, current is
composed of ions, not electrons.
In an electrolytic capacitor, one of the conductors is a solid or porous metal electrode,
the dielectric is an insulating metal oxide coating on the surface of the electrode, and
the other conductor is an electrolytic liquid or solid. Since the distance between the
“plates” is the thickness of the insulating metal oxide, which is quite small, tiny electrolytic capacitors can have extremely high capacitance. The
capacitors used in certain fuel-cell automobiles are a form of electrolytic capacitor.
A variable capacitor used for tuning a radio.Commercial capacitors
Rolled foil capacitors