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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?

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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.

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