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why don't charged insulators conduct?

Posted on 2010-08-12
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Hi

I was just reading a webpage about how things can become charged and it lists the mechanisms as charging by friction, induction and conduction. With charging my conduction there is an example that if a neutral aluminium plate is put into contact with a negatively charged foam plate, the charge will not move from the foam plate to the aluminium plate.

I was wondering why this is the case. I thought the obvious answer was because insulators do not allow the movement of electrons, but this foam plate is negatively charged so electrons have migrated to it. If they migrated TO it, why can't they migrate away to the aluminium plate

thanks
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Question by:andieje
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top_rung earned 125 total points
ID: 33423832
The insulator holds a charge and that charge can influence surrounding objects (polarization indicated in the graphic).  Insulators limit electron mobility (no conduction) but again, can hold a charge.

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by:top_rung
ID: 33423872
Generic yet simple explanations.....
 
http://www.exo.net/~pauld/summer_institute/summer_day14electrostatic/Electrophorus.html
 
"The Styrofoam is an electrical insulator which means that charge cannot move through it. The negative charge stays on the Styrofoam plate until it slowly leaks away through the air.
The aluminum pan is a conductor, charge can move through the pan, as well as onto it and off it when it touches another conductor."
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by:aburr
ID: 33425264
"
I was wondering why this is the case"
because the charges on the insulating object do not move (or at least VERY slowly)
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by:BigRat
BigRat earned 125 total points
ID: 33427872
As your posted diagram shows, the charge difference is on the insulator not on the aluminium, and since the insulator does not conduct the charge, cannot be neutrialized. If on the other hand it was placed in an exactly fitting aluminium box....

The difference between insulators and conductors is that conductions have many free roaming valence electrons, often called outer band electrons. Metals are good conductors, the best conductor is gold which has the electronic configuration of [Xe] 4f14 5d10 6s1 The unpaired outer 6s electron is shielded from the nucleus by no less than 24 electrons plus the Xenon shell. Platinum is also another good condutctor and it has a lone 6s electron shielded by 23 other and the Xenon shell. (http://www.science.co.il/ptelements.asp)

In the metallic crystal lattice these outer unpaired electrons wander around with different energies forming a sort of layer above the atoms. These electrons can therefore conduct energy and also can interact with photons making the metals shiny.

The lattice of atoms in an insulator however has no free electrons, they are all involved in bonding, so the material cannot conduct readily nor can it reflect light so good.

The materials which display properties of both are semi-conductors. In a silicon lattice all the Si atoms are bonded covalently with each other. The material is of a dull appearance and a poor conductor of energy. If however one infuses just the odd atom of valency 3 or 5 (Si has valency 4) then an electron will be "missing" or an electron will be superfluous. the resultant material is of a bright appearence and conducts reasonably well. Of course the number of infused atoms must be small and the atoms must be roughly the same size as the Silicon atom, otherwise the lattice structure will not be maintained. That is the basis of semiconductor science.
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by:andieje
ID: 33428136
But if the charge cannot move through the insultator (which is what i thought as you will see in my question) how did the insultator get charged in the first place? There must have been some movement of electrons in the insulator to charge it surely? Perhaps the charging of the foam plate occured by some form of intervention that I don't know about.

So really the question should be, how do you charge an insulator?
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by:NovaDenizen
NovaDenizen earned 125 total points
ID: 33430568
It's easy to charge a surface of an insulator.  It will take a charge just like anything else.

But an insulator ion does not pass the charge along to its neighbors.  It is perfectly happy to have one side of itself at a very different potential from the other side of itself.

In contrast, if you apply a charge to one surface of a conductor then that charge will stay on the surface for only a very short time before it spreads out evenly throughout the conductor.

It's very similar to thermal conductors and insulators.  Thermal insulators can be hot on one side and cool on the other with very little transfer of heat between the sides, the same way electrical insulators can be at a high voltage on one side and a low voltage on the other with very little current between the sides.
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by:BigRat
ID: 33430658
>>There must have been some movement of electrons in the insulator to charge it surely?

By rubbing the insulator with a cloth or similar you can "rub off the electrons" and thus charge it up. Quantum mechanically the energy done in rubbing raises the covalent electrons into higher orbits (by gaining energy) which then allows then to leave the surface of the material making it charged. If you heant an insulator the electrons gain energy and so the resistance decreases.
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by:aburr
aburr earned 125 total points
ID: 33432575
"Perhaps the charging of the foam plate occured by some form of intervention that I don't know about."
That depends on what you know.

Conductors can be charged by placing one end (+) of a battery onto the conductor. Because charges are mobile all the conductor will take the potential of the battery and have a charge on its surface.
(note which, if confusing, can be disregarded. In the usual case the electrons are mobile so the battery sucks up some of the surface electrons from all over the surface of the conductor.). If you replace the conductor with an insulator the battery will charge up a small area of the insulator but almost all of the insulator's surface will remain as before because the charges will not move. The result will be a small patch on the surface of the insulator which will be positively charged.
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Author Comment

by:andieje
ID: 33433647
That makes sense then. I didn't understand how an insulator could gain charge and for this charge to spread out but yet this charge can't move to a conductor. But if you can charge an insulator but the charge is localised then everything is A-ok with me
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Author Closing Comment

by:andieje
ID: 33433655
thanks
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