Abstract :
Capacitor Construction
The
parallel plate capacitor is the simplest form of capacitor. It can be
constructed using two metal or metalized foil plates at a distance parallel to
each other, with its capacitance value in Farads, being fixed by the surface
area of the conductive plates and the distance of separation between them.
Altering any two of these values alters the value of its capacitance and this
forms the basis of operation of the variable capacitors.
Also,
because capacitors store the energy of the electrons in the form of an
electrical charge on the plates the larger the plates and/or smaller their
separation the greater will be the charge that the capacitor holds for any
given voltage across its plates. In other words, larger plates, smaller
distance, more capacitance.
By applying
a voltage to a capacitor and measuring the charge on the plates, the ratio of
the charge Qto the voltage V will give the capacitance value of
the capacitor and is therefore given as: C = Q/V this equation can
also be re-arranged to give the more familiar formula for the quantity of
charge on the plates as: Q = C x V
The
property of a capacitor to store charge on its plates in the form of an
electrostatic field is called the Capacitance of the capacitor. Not
only that, but capacitance is also the property of a capacitor which resists
the change of voltage across it.
Just like
the Resistor,
the Capacitor, sometimes referred to as a Condenser, is a simple
passive device that is used to “store electricity”. The capacitor is a
component which has the ability or “capacity” to store energy in the form of an
electrical charge producing a potential difference (Static Voltage) across its
plates, much like a small rechargeable battery.
There are
many different kinds of capacitors available from very small capacitor beads
used in resonance circuits to large power factor correction capacitors, but
they all do the same thing, they store charge.
In its
basic form, a capacitor consists of two or more parallel conductive (metal)
plates which are not connected or touching each other, but is electrically
separated either by air or by some form of a good insulating material such as
waxed paper, mica, ceramic, plastic or some form of a liquid gel as used in
electrolytic capacitors. The insulating layer between capacitors plates is
commonly called the Dielectric.
Due to this
insulating layer, DC current cannot flow through the capacitor as it blocks it
allowing instead a voltage to be present across the plates in the form of an
electrical charge. The conductive metal plates of a capacitor can either be square,
circular or rectangular, or they can be of a cylindrical or spherical shape
with the general shape, size and construction of a parallel plate capacitor
depending on its application and voltage rating. When used in a direct current
or DC circuit, a capacitor charges up to its supply voltage but blocks the flow
of current through it because the dielectric of a capacitor is non-conductive
and basicallsy an insulator. However, when a capacitor is connected to an
alternating current or AC circuit, the flow of the current appears to pass
straight through the capacitor with little or no resistance.
The
Dielectric of a Capacitor
As well as
the overall size of the conductive plates and their distance or spacing apart
from each other, another factor which affects the overall capacitance of the
device is the type of dielectric material being used. In other words the
“Permittivity” (ε) of the dielectric. The conductive plates of a capacitor are
generally made of a metal foil or a metal film allowing for the flow of
electrons and charge, but the dielectric material used is always an insulator.
The various insulating materials used as the dielectric in a capacitor differ
in their ability to block or pass an electrical charge.
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