# I How can insulators have charge density inside?

1. Sep 21, 2016

### edgarpokemon

I thought that insulators cannot be charged in the inside or the outside, so how can they have any charge density inside? I know that electric fields pass through a insulator, so is that why they can have charge density? I am currently reading about electric flux.

2. Sep 22, 2016

### Orodruin

Staff Emeritus
If you have a charged insulator the excess charges cannot move away. Now, a conductor on the other hand cannot carry charge inside (in the stationary case). It would quickly dissipate.

3. Sep 22, 2016

### sophiecentaur

An insulator becomes Polarised when a field is applied. The charges around individual molecules get displaced a bit (negative one way and positive the other) but they are not released to move to another molecule. The amount of this effect is the Dielectric Constant of the material. An insulator, inserted between the plates of an Air Capacitor will increase its capacitance. Current will flow whilst the capacitor is charging up but it is a transient effect because the charges will not flow through the insulator. I guess that, whilst the capacitor is charging up, you could consider that a charge density exists. But, even in a metal, the net charge density is zero (each mobile electron will have a corresponding static proton).
PS Fields do not "flow through"; they are set up by the surrounding arrangement of charges.

4. Sep 22, 2016

### Aaron Crowl

You can have a static charges on an insulator. It's just charges that got trapped there by some means (like friction). I suppose that in theory you could have static charges distributed throughout an insulator but I'm not familiar with any way to do that.

Edit: Ahh, a cloud would be a good example of this. Positive and negative charges will be distributed unevenly by mechanisms of weather. The cloud is normally non-conductive until the electric field gets high enough then *boom* you get lightning.

5. Sep 22, 2016

### Olioli

When it comes to micro-fabrication, some insulator can be "charged" because the coating/deposition method will not make a stoichiometric material. Especially with alumina (aluminium oxide Al2O3), when this material is deposited on silicon and is annealed, alumina becomes negatively charged. Those charges are considered as fixed charges but you can also have some mobile charges due to impurities in the thin film. This charge density in the alumina is used to passivate a p-type silicon surface, in order to decrease the surface recombinaison velocity (SRV) at the interface. In fact, the field created by this charge density will repel minority carriers front the surface and will prevent them to recombine at the interface (where their are a lot of defects).

That is one example where we can find charge density in insulator.

Last edited: Sep 22, 2016