When we rub two glass rods with their respective pieces of silk cloth, the two glass rods would repel each other. What if we rub the glass rod against the other glass rod? Will they repel each other? Or they only get charged when they are rubbed by silk cloth?
[Physics] Charging Glass Rods
chargeelectricity
Related Solutions
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.
Conductivity is not just about how tightly bound electrons are, but equally about how easy it is for them to travel.
Example: a bunch of islands in a shark-infested sea. You cannot swim from one island to the next although it is close. At low tide you can walk across no problem. The first example is an insulator, the second is a conductor.
Rubbing (google triboelectricity) causes unlike atoms to stick and unstick frequently. Atoms "fight" over electrons, and the stronger one gets to take the electron home. It is like air lifting them from the island - shark infested waters or not.
There are lists of materials (the triboelectric series) that tell you which material will give up its electrons when in contact with another material. Glass is high on the list - it loses electrons easily. The can't move sideway, but they can be picked off the surface.
Best Answer
When you run two different materials together you will usually transfer electrons from one material to the other. For example if you rub glass with silk the electrons transfer from the glass to the silk and you end up with positively charged glass and negatively charged silk. That's why the two glass rods repel each other: they both carry a positive charge and two positive charges repel each other.
Which way the charge is transferred is hard to predict from first principles. However, it's been measured for many combinations to draw up the triboelectric series. The farther apart the materials are on the triboelectric series the bigger will be the charge transfer. Two glass rods are obviously in the same place on the series, so if you rub two glass rods together there will be no significant charge transfer between them, and therefore they will neither repel nor attract each other.
I used the phrase "no significant charge transfer" because I suppose it's possible that you might get random charge transfers between the rods to give a very small charge. However, if this happened it would leave one rod positive and the other rod negative, so the two rods would attract each other rather than repelling each other.