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The principal factors determining capacitance are the areas of the metallic plates themselves and any dielectric (non conducting material) between them.
Capacitance is directly proportional to plate area.
The formula for a two plate capacitor is C = 0.225KA / S where C is in pF, K is the dielectric constant (air = 1), A = area of the plates in square inches and S is the spacing between plates in inches. One inch = 25.4 mm or 2.54 cm.
If we had two metal plates each measuring 2" X 2" we have a net area of 4 inches square. Should the plates be separated by 1/100" then the capacitance exhibited would be:
C = ( 0.225 X 1 X 4 ) / 0.01 = 90 pF
For more than two plates the formula becomes C = 0.225KA (N - 1) / S where N = number of plates. This is the standard formula for air variable capacitors.
The dielectric constant K varies with the dielectric used. Some examples are:
Glass 4 - 7, Mica 5 - 7, Porcelin 6 - 7, Paper 2 - 3, Ceramics 3 - 7. Note the dielectric constant of solid dielectrics may vary with increasing frequency - an important property to remember.
Capacitors were represented above as two or more parallel plates and some, such as mica or the old paper types, were made this way.
If we have a flexible dielectric then we can have two long sheets of thin metal foil separated by the thin flexible dielectric. In this way we can then roll up the foil / dielectric combination with a wire connected to one foil and another wire connected to the other foil. This is the basis of the modern electrolytic capacitor.
A modern electrolytic capacitor consists of an aluminium foil positive plate immersed in a solution called an "electrolyte" which is an ionised solution capable of carrying current. The aluminium foil is the positive plate and the electrolyte is the negative plate.
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Updated 15th May, 2000