MadSci Network: Physics

Re: electrical conductivity, permittivity, dielectric constant?

Date: Tue May 1 02:28:32 2001
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Physics
ID: 988009177.Ph

Hi Manik!
First off, dielectric is not the opposite of electric. A dielectric is a 
material which does not conduct an electric current because the lines of 
force of an electrostatic field will pass through it, making it the seat 
of the strain. 

Permittivity is the same as dielectric constant (also sometimes 
called 'specific inductive capacity'), and is a measure of the ratio of 
electric displacement to electric field intensity. Permittivity( Greek 
symbol sigma) determines the potential capacitance of 2 parallel plates 
separated by a dielectric, and is affected by temperature, pressure, and 
the frequency of the applied voltage, and is subject to the inverse square 
law, more commonly called Coulomb's Law. Permittivity calculations are in 
relation to the sigma of a vacuum (8.85 x 10 to the minus 12th) being 
assigned a value of 1.00.

The dielectric constants of some common materials are: air 1.0004947, 
water (at 25 deg. C.) 78.54, water (at -22 deg. C.(ice)) 40, wood (oak) 
2.4 to 6.8, Chloroform 4.806, Zinc Carbonate 100.  As you can see, some 
liquids have a higher value than some solids and vice-versa. Most gasses 
are slightly above 1. The higher the value, the less conductance. Given 
the same area of the plates, and the same distance between them, a 
material with a higher dielectric constant (permittivity) will conduct 
less, allowing a greater buildup of opposite charges on the plates, 
resulting in a greater capacitance. 

I've included a link to a simple capacitance calculator. Play with the 
numbers and materials. I think everything will become clear, if it isn't 

I hope this puts things in perspective for you.

By the way, you're probably thinking "What's this nut thinking about -  
using water as a dielectric? Water conducts electricity".
........ not PURE water!! It's all that other junk in water that makes it 

Your not-so-mad scientist,



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