What's the explanation of 'paper pieces and comb experiment' about static electricity on atomic level? I know that the charged comb induces static electricity in the paper pieces.how does that happen? Do the atoms in the paper get ionized,the same what happens when the comb was rubbed on  hair?please correct me if I'm wrong in any way.Thanks.
 A: Induction of Dipole Polarization by Static Electricity

A comb is charged with static electricity by combing hair.  In turn, the charged comb attracts pieces of paper. Are the paper atoms ionized in the same way the comb ionizes hair atoms?

Short Answer No, the paper's atoms are not ionized.  Ionization implies sufficient work done to the electrons in the paper to completely free them from the paper molecules.
Rather than completely freeing (ionizing) electrons from the paper's atoms and molecules, the $E$ field from the excess electrons on the charged comb displaces the positive and negative charges away from their neutral position.  
The $E$ field from the comb polarizes the atoms in the paper's molecules, 1) causing the electrons in the paper's molecules to move farther from the comb, and 2) causing positive nuclei to move closer.  Due to the inverse square law, the $E$ field from the comb's electrons is weaker farther from the comb.  Thus, the attractive force on the paper molecules' nuclei (which are closer) is stronger than the repulsive force on the paper molecule's electrons (which are farther).  As a result, the paper moves toward the comb.
Production of Static Electricity 


*

*The plastic molecules of a comb have a greater affinity for electrons than hair molecules.  The fact that we must expend more work removing an electron from plastic molecules than hair molecules is a reflection of the greater force exerted by the plastic atoms/molecules. As a result, when in close surface contact, the plastic attracts electrons from the hair and keeps some of the hair's electrons after they separate.  

*Excess electrons accumulate on the comb (called static electricity) when the comb and hair are brought close enough for the plastic molecules to capture and retain electrons from the hair molecules. 

*Note: Rubbing is not required to produce static electricity, only contact and withdrawal are necessary.  Rubbing (parallel motion with pressure) increases the chances for new surface-surface contact and capturing electrons, thus rubbing usually produces more static electricity than a single contact and withdrawal.  Due to the microscopic surface irregularities of materials (called asperities), all the surface molecules do not come close enough to capture an electron with a single contact.  But, by applying pressure and rubbing, the surfaces have more opportunity to make intimate contact, thereby allowing the more electronegative material to capture electrons.

*The effect of rubbing molecules of dissimilar electron affinity, and the resultant capture of electrons by the more electronegative material, results in static charge accumulation and is called the triboelectric effect (note: "tribo" is a Greek word referring to "rubbing").  


Induced Charge - Paper-Comb Attraction


*

*Before contact, the comb and hair molecules are neutral, having equal numbers of positive nucleons and negative electrons.  After contact with hair, the plastic comb is negatively charged with excess electrons, and the hair is left positively charged with an electron deficit.  

*The negative charge from the excess electrons in the comb produces a negative electric field, which decreases in strength with distance. (The field from a point charge diminishes at $\frac {1}{r^2}$. The comb is not a point-charge, but the $E$ field from the comb still diminishes with distance.)

*Paper is a solid and an insulator, so very few electrons move between molecules in the paper matrix below the breakdown voltage.  Nevertheless, the $E$ field from the comb causes a slight polarization of the electrons within the paper molecules throughout the paper (i.e., attraction of each nucleus and repulsion of each electron cloud). 

*The comb's negative $E$ field causes charge movement/polarization within each of the paper molecules.  The polarization is produced as follows:
1) The comb's negative $E$ field attracts each of the positive nuclei in the paper, causing them to move slightly closer to the comb (in relationship to their neutral/unbiased electron clouds).
2) The comb's $E$ field repels each of the paper molecules' negative electron clouds, causing them to move slightly away from the comb (in relationship to their nuclei).

*The paper experiences a net force in the direction of the comb because of the greater attractive force acting on the nuclei and lesser repulsive forces acting on the electron clouds of each atom in the paper:
1) The nuclei of the paper molecules are closer to the comb than the paper's electron cloud.  The negative $E$ field from the comb results in a stronger attractive force on the nuclei because they are closer than the electron cloud.
2) And conversely, the negative $E$ field and force repelling the electron cloud in the paper molecules is weaker because they are farther away from the comb. 
3) Therefore, the attractive force from the comb $E$ field acting on the closer positive nuclei of the paper molecules is stronger than the force repelling the more distant electron clouds of paper molecules.  
4) The net effect is an attractive force between comb and paper.

*When the comb and paper are still separated, the paper molecules are polarized, not ionized.  That is, the net charge of the paper is unchanged, each molecule still has a net neutral charge. 


In Summary: This comb-paper attraction effect is due to the electrons on the comb being slightly closer and therefore having a slightly stronger attraction to the nuclei of the paper atoms, and the electrons on the comb being slightly farther away and therefore slightly less repelled.  In short, the stronger attraction to the nearer nuclei than the weaker repulsion from the farther electrons, results in a net attractive force.
Charge Movement upon Touching the Comb and Paper


*

*When the charged comb and paper touch, some electrons will flow from the comb onto the surface of the paper, because the paper is relatively deficient of electrons compared to the charged comb. 

*But, since the paper is an insulator, only a few electrons will flow onto the surface of the paper and neutralize the electron-deficient paper molecules.  

*Electrons will flow from the comb on to the paper, but the paper will quickly accumulate sufficient negative charges to create a repulsive $E$ field that will neutralize the attraction and prevent further flow at the surface interface.  

*Even though the comb will lose a few electrons to the surface of the paper, the comb will still retain its charge/excess negative charges and the bulk of the paper molecules will remain polarized by the $E$ field of the comb.  As a result, the polarization of the paper molecules by the comb's $E$ field is retained, and the paper will remain stuck to the comb.

*If the charged comb and neutral paper are separated after having touched, the molecules on the surface of the paper will take away a small number of the excess electrons from the comb.  But, in the bulk of the paper, its molecules will remain neutral. 

A: First of all let's do a small experiment as follows:
Rub your comb on your hair on a dry day and bring the comb near some small pieces of paper. You would find that the comb attracts the paper pieces. We can guess that this is due to electric field, so why not check this with a stronger electric field? Now repeat the same experiment with an electric field which is 10 times or 100 times stronger, which you can easily produce in a capacitor in a laboratory. You would be surprised to find that the paper pieces are not attracted by the field.
So, electric field is important but there should be some additional conditions imposed on the electric field.
Explanation:
When you rub the comb on your hair electric charges are accumulated on it, but the charge distribution is localized. This means that the electric field is non-uniform. This electric field polarizes the paper pieces and there are both positive and negative polarization charges. The non-uniform field exerts forces on these polarization charges and we have a net force due to all these forces, whereas in the uniform field case (like the capacitor) all these forces cancel each other and there is no net force.
Q. Why are the paper pieces only attracted and not repelled?
A. This is because the net force is always directed along the direction of positive gradient, i.e., the direction along which the electric field lines gets denser.

Conclusion:
The attraction of the paper pieces is due to generation of non-uniform electric field that polarizes the paper pieces and attracts them non-uniformly so that there is a resultant attractive force due to the force gradient.
A: It is true that the pieces of paper are attracted by the comb. So, we can say that the comb are also attracted by them.
But according to the Gauss Law, if we enclose one of that pieces by an imaginary sphere, the flow of the electric field will be zero around its surface, because the paper is neutral, so there are no net charges inside the sphere. 
But for the flow, integrated along the surface be zero, it is not necessary that the E-field be zero at all of its points. It would only be the case if the positive and negative charges had a spherical symmetry inside the paper. That is the normal situation.
But after the electrified comb induces electric dipoles in the pieces, the situation changes. We can now imagine 2 spheres, both touching the comb. One centred at the geometrical centre of the positive charges, and the other at the centre of the negative charges. 
The flow of the E-field is the same for both, only having opposite sign, because there are the same number of positive and negative charges. 
But they have different radius, and the bigger sphere (related to the slighted more distant charge centre) has a bigger surface area, and as a consequence smaller E-field at each point. 
At the comb, where both spheres touch, the E-field is different from zero, and the product of that field by the amount of net charges in the comb generates the force of attraction.   
