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Old Apr 4th 2009, 03:01 PM   #1
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electronics

Hi.........
plz share with me that....."does insulator can conduct current & when? "
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Old Apr 5th 2009, 12:37 AM   #2
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Alternating current of sufficiently high frequency can be passed through any insulator.

High intensity direct current that can produce electric field greater than the dielctric strength of the insulator makes the insulator conducting.
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Old Apr 5th 2009, 05:35 AM   #3
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It may be better to look at it this way

If a sufficiently high voltage/ field were to be applied across the insulator such that it is more than the dilectric stength, the insulator breaks down and a current flows. It is the field/voltage which causes the current and not vice versa.
Whenever a current flows thru' a resistor for example, a voltage is found across it. From Ohm's law eqn we could say the current caused the voltage, but the fact is unless there is a voltage difference in the first place, there would be no current flow.Like when water flows thru a pipe, a pressure difference is always found across it, but it is this P.D . which caused the water to flow
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Old Apr 5th 2009, 07:12 AM   #4
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Originally Posted by Parvez View Post
Alternating current of sufficiently high frequency can be passed through any insulator.

High intensity direct current that can produce electric field greater than the dielctric strength of the insulator makes the insulator conducting.
Thanks.......... but i want know hows is this possible?what is the reason behind this?
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Old Apr 5th 2009, 08:18 AM   #5
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When an electric field is applied across a dielectric, it exert a force on the electrons. If the field is high enough, it snatches the electrons bound to atom and makes them free. This makes the dielectric a conductor. The phenomenon is called field emission.

To know the effect of alternating field (caused by alternating current) you have to study the behaviour of the electric dipole subjected to an electric field.
The dielectrics are of two types: polar and non-polar. When a polar dielectric is kept in the electric field, the dipole moments of the dipoles allign them them selves in the direction of electric field creating a surface charge density that produces a field opposite to the applied one.

Suppose the dielectric is kept in the field whose direction is from left to right. The negative charge will appear on the left face and positive on right. Now if the direction of the external field field is reversed ( as in the case of AC), the direction of the dipoles will be reversed and now left face will be positively charged and the right negatively. Thus as the direction of the field changes, the polarity of the dielectric also changes. This is equivalent to a current.

In case of non polar dielectrics, the field makes them polar and vibrate them as in the case of polar molecules.

Note that AC current is conducted by bound charges and not the free ones.
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Old Apr 5th 2009, 08:22 AM   #6
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For that you will have to go to the band theory of solids. In an insulating material, the electrons are tightly bound to the atoms in the crstal structure and are not available for conduction. When a high voltage is applied, they are torn away and thus conduction happens. In a good conductor like a metal, the electrons are not tightly bound to any single atom and roam all over; they are pictured as a cloud or sea of electrons.Due to this they are easily available for conduction.
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