Why does a glass rod when rubbed with silk cloth aquire positive charge and not negative charge? I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, and not negative charge?  
It is also said that glass rod attracts the small uncharged paper pieces when it is becomes positively charged. I understand that a positively charged glass rod attracts the uncharged pieces of paper because some of the electrons present in the paper accumulate at the end near the rod, but can't we extend the same argument on attraction of negatively charged silk rod and the pieces of paper due to accumulation of positive charge near the end?
 A: This is because glass is above silk in the triboelectric series (attracts electrons less than silk) and when rubbed, silk 'takes' its electrons.
And yes, if you had a silk rod it would also attract neutral paper, because paper pieces are turned into dipoles, as you explained.
A: Well this can be explained by the work function of materials. Due to rubbing, heat is generated which supplies energy for removal of electrons. As the work function of the glass rod is smaller than the silk cloth, it easily loses electrons to the silk cloth which then releases energy (electron gain enthalpy) and thus ensures conservation of energy.
A: You might know that all matter is made up out of atoms. Now, atoms themselves have a core, or nucleus, and electrons orbiting around the nucleus. The core has positive charge, the electrons have negative charge. 
When you are rubbing the glass rod with the silk cloth, electrons are stripped away from the atoms in the glass and transferred to the silk cloth. This leaves the glass rod with more positive than negative charge, so you get a net positive charge. 
Why do the electrons go from glass to silk and not from silk to glass? That depends a lot on the minute details of the material. Ultimately, for every two materials there is one of them where electrons are happier. It just turns out that for glass and silk, electrons are happier at the silk cloth.
Now to your second question. Here, the important thing to note is that in your typical solid material, the positive charges, which are the cores of the atoms, cannot move around much. They are locked into a rigid structure. The tiny electrons, however, can move around much better. That's why the glass rod can induce a net negative charge at one end of the paper clips. 
EDIT: Let me add that there should also be some attraction between the silk and a bunch of paper: The electrons in the paper will be pushed away by the electrons in the silk, leaving the end of the paper that is closer to the silk with a net positive charge that then gets attracted. However, it might very well be that in your silk cloth the electrons are overall too spread out to have a strong enough attractive effect. 
A: As we all know that matter in our environment is made up of basic element "atoms" well silk is obtained from cocoons that are living thing thus made from "amino acid" that is the fundamental compound of living being and the components of amino acid are H2NCHRCOOH  thus we see that R letter then requires to gain electron thus then making it rubbed by a glass rod gives +ve charge to the rod. 
A: Silk has lots of scraggly fibers that provide lots of surface area and lots of pointy parts. 
Glass has flat and smooth surfaces with relatively small surface area for a given volume.  
Silk is also not a great insulator so any accumulated charges can spread out and dissipate.
Clean glass is an excellent insulator so any charges do not move around much and can accumulate. 
My guess is that the point of rubbing contact on the silk very quickly looses its excess charge to the surrounding material and air, so by the time you bring it near the paper there is no noticeable attractive force left.
There are also differences in the mechanics of positive and negative coronal discharges, but my guess is that the texture and conductivity of the silk is the dominant effect at work here rather than the polarity of the objects. The fact that you can rub acrylic with wool and the acrylic attracts paper seems to confirm this since acrylic/wool has the reverse charging direction as glass/silk. (Acrylic is smooth and is a good insulator)
