|MadSci Network: Physics|
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 now. 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 conduct. Your not-so-mad scientist, Karl KarlKolbus@ameritech.net http://www.woodsbas. demon.co.uk/calcs/calcs.htm
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