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Basic Electronics , Part 2 
Elements of AC Electricity  
  

Unit 8: Capacitance 
Module 3: Capacitor Formulas

Copyright © 1997, David L. Heiserman 
All rights reserved
 
 ContentsFeedback 
 
Before starting this module, you should be able to: When you complete this module, you should be able to: 
  • Define a coulomb of electrical charge.
  • Define capacitance in terms of two conductors and a dielectric.
  • Describe the meaning of charging capacitor and discharging a capacitor.
  • Cite the units of measure and common ranges of values for capacitance.
  • Describe how charge, capacitance, and voltage are related in a capacitor circuit.
  • Calculate the amount of charge on a capacitor, given the values of capacitance and voltage.
  • Define the term dielectric constant.
  • Describe how the common area of the plates of a capacitor affect its capacitance value.
  • Describe how the spacing of the plates of a capacitor affect its capacitance value.
  • Calculate the value of a capacitor, given the values for plate area, plate spacing, and the dielectric constant.


Practical Definitions for Capacitors
 
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 Symbols, Units and Values 

This is the schematic symbol for a capacitor. 

The unit of capacitance is the farad, F 
The math symbol for capacitance is C 
 

 Common ranges of values for capacitors are: 
    microfarad: mF = 1 –´ 10-6 F 
    nanofarad: nF =  1 –´ 10-9 F 
    picofarad: pF =   1 –´ 10-12 F
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Capacitor Charge, Value and Voltage
 
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 Charge On a Capacitor
 

The amount of charge on a capacitor is given by:

Q = CV

where 

    Q = electrical charge in coloumbs, C
    C = value of capacitance in farads, F
    V = voltage across the plates of the capacitor
 		 About Coulombs
  • The coulomb is a unit of measure for the quantity of electrons:
1 coulomb = 6.25 x 1018 electrons
  • The abbreviation for coulombs is the letter C. Don't be botherd by the fact that the math symbol for capacitance is also the letter C. 
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E		 How much charge is stored on the plates of a 100 nF capacitor when the voltage across the capacitor is 20 V?
Ans: 2 mC
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What Determines the Value of a Capacitor?
 
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 Value Of a Capacitor
Given area and spacing of the plates

When you know the common area between the plates of a capacitor and the distance between them in meters, you can calculate the value of the capacitance this way:

C = 8.85 x 10-12 (kA/s)

where

    C = value of capacitance in farads, F
    k = dielectric constant of the material separating the plates
    A = common area between the plates in square meters, m2
    s = spacing between the plates in meters, m

When you know the common area between the plates of a capacitor and the distance between them in inches:

C = 2.25 x 10-7 (kA/s)

where

    C = value of capacitance in farads, F
    k = dielectric constant of the material separating the plates
    A = common area between the plates in square inches, in2
    s = spacing between the plates in inches, in
 Dielectric Constant

The dielectric constant, k, is an expression of how much effect dielectric materials between the plates affect the value of a capacitor. The term has no units of measure. Here are a few examples:
 
Vacuum, air k = 1
Aluminum oxide k = 8
Ceramics k = 100 to 1000 or more
Glass k = 8
Polyethylene film k = 3


These Equations Also Show:
The value of a capacitor is proportional to the area shared by the plates.
  • Increasing the area of the plates increases the value of capacitance.
  • Decreasing the area of the plates decreases the value of capacitance.
The value of a capacitor is inversely related to the spacing between the plates.
  • Increasing the spacing between the plates reduces the value of capacitance.
  • Decreasing the spacing between the plates increases the value of capacitance.
 
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 Calculate the value of a capacitor where:
    The common plate area is 0.5 square inches
    The spacing between the plates is 0.01 inch
    The dielectric material is air
     
    Ans: 11.3 mF
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