Factors affecting capacitance
There are three basic factors of capacitor
construction determining the amount of capacitance created.
These factors all dictate capacitance by affecting how much
electric field flux (relative difference of electrons
between plates) will develop for a given amount of electric
field force (voltage between the two plates):
PLATE AREA: All other factors being
equal, greater plate gives greater capacitance; less plate
gives less capacitance.
Explanation: Larger plate area
results in more field flux (charge collected on the plates)
for a given field force (voltage across the plates).
PLATE SPACING: All other factors
being equal, further plate spacing gives less capacitance;
closer plate spacing gives greater capacitance.
Explanation: Closer spacing results
in a greater field force (voltage across the capacitor
divided by the distance between the plates), which results
in a greater field flux (charge collected on the plates) for
any given voltage applied across the plates.
DIELECTRIC MATERIAL: All other
factors being equal, greater permittivity of the dielectric
gives greater capacitance; less permittivity of the
dielectric gives less capacitance.
Explanation: Although it's
complicated to explain, some materials offer less opposition
to field flux for a given amount of field force. Materials
with a greater permittivity allow for more field flux (offer
less opposition), and thus a greater collected charge, for
any given amount of field force (applied voltage).
"Relative" permittivity means the
permittivity of a material, relative to that of a pure
vacuum. The greater the number, the greater the permittivity
of the material. Glass, for instance, with a relative
permittivity of 7, has seven times the permittivity of a
pure vacuum, and consequently will allow for the
establishment of an electric field flux seven times stronger
than that of a vacuum, all other factors being equal.
The following is a table listing the
relative permittivities (also known as the "dielectric
constant") of various common substances:
Material Relative permittivity (dielectric constant)
============================================================
Vacuum ------------------------- 1.0000
Air ---------------------------- 1.0006
PTFE, FEP ("Teflon") ----------- 2.0
Polypropylene ------------------ 2.20 to 2.28
ABS resin ---------------------- 2.4 to 3.2
Polystyrene -------------------- 2.45 to 4.0
Waxed paper -------------------- 2.5
Transformer oil ---------------- 2.5 to 4
Hard Rubber -------------------- 2.5 to 4.80
Wood (Oak) --------------------- 3.3
Silicones ---------------------- 3.4 to 4.3
Bakelite ----------------------- 3.5 to 6.0
Quartz, fused ------------------ 3.8
Wood (Maple) ------------------- 4.4
Glass -------------------------- 4.9 to 7.5
Castor oil --------------------- 5.0
Wood (Birch) ------------------- 5.2
Mica, muscovite ---------------- 5.0 to 8.7
Glass-bonded mica -------------- 6.3 to 9.3
Porcelain, Steatite ------------ 6.5
Alumina ------------------------ 8.0 to 10.0
Distilled water ---------------- 80.0
Barium-strontium-titanite ------ 7500
An approximation of capacitance for any pair
of separated conductors can be found with this formula:
A capacitor can be made variable rather than
fixed in value by varying any of the physical factors
determining capacitance. One relatively easy factor to vary
in capacitor construction is that of plate area, or more
properly, the amount of plate overlap.
The following photograph shows an example of
a variable capacitor using a set of interleaved metal plates
and an air gap as the dielectric material:
As the shaft is rotated, the degree to which
the sets of plates overlap each other will vary, changing
the effective area of the plates between which a
concentrated electric field can be established. This
particular capacitor has a capacitance in the picofarad
range, and finds use in radio circuitry. |
