Capacitors and Capacitance

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By Tim Raney...bald engineer guy with glasses

Apparatus for determining capacitance with different dielectrics. One places a sample between two parallel conductive plates connected to a multimeter as shown. Important - use same thickness and diameter for all samples for comparative purposes.

Capacitance (C) is the ability to store an electric charge. Measured in farads (F), capacitance equals the amount of charge (Q) in coulombs stored, divided by the potential (E) in volts applied across the capacitor plates when the charge was stored1. The farad is the SI unit for capacitance - one farad = 1 coulomb/volt  =  1C/E. This very brief fact sheet only shows selected equations, dielectric examples and an experimental set-up for determining the relative capacitance for different dielectrics.

The equation below is the mathematical representation for capacitance.

Energy Stored in a Capacitor. This is the fundamental equation for determining the electrical energy stored in a capacitor:

E = Capacitor energy (joules)
C = Capacitance (in mF)
V = Potential (kV)

Dielectric Constant (K) of a material equals the ratio of capacitance (C) of the dielectric between the capacitor's plates compared to a vacuum (C0).  Thus, we can show this ratio as K = C/C0 where C0 is unity. When conducting the experiment in air, C0 suffices for practical purposes.

Three factors determine capacitance of a parallel-plate capacitor expressed by C = e (A/d), where A is the area of either plate (m2) and d is the distance between the plates (m); e is the separating medium’s constant – its permittivity, found by e = eo K. eo = 8.85 x 10-12 C2/(N)(m2), the permittivity of free space2.



Notes:

[1] DoE Fundamentals Handbook Electrical Science (vol. I – III), June 1992, pg. 17.

[2] N.C. Harris & E. H. Hemmerling, Introductory Applied Physics (4th Ed.), Gregg Division/McGraw-Hill Book Company, Inc., New York, 1980, pg. 506.

 

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One Response to Capacitors and Capacitance

  1. Tim,
    Most of the literature seems to use one form or the other of your definition for capacitance — “Capacitance (C) is the ability to store an electric charge”
    I tend to favour the one that I learned in school, a million years ago.
    Capacitance is the characteristic of a circuit component to oppose any change of voltage across it.
    And likewise:
    Inductance is the characteristic of a circuit component to oppose any change of current through it.

    Ralph

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