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I read in book recently that when a glass rod is rubbed with a silk cloth, some sort of 'electrostatic interaction' take place that both of them acquires a net electric charge. It also said that the negative charge (electrons) are more mobile than the positive charge so what was going on is simply 'electron transfer' between silk cloth and glass rod. But I had read in chemistry that electrons are held tightly within the atom and breaking that bond (covalent or ionic) releases large amount of energy. Now, how come simple mechanical rubbing provide that sort of energy that electrons escape the atom? I don't know whether my understanding is correct. Please enlighten me!

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The interaction is the triboelectric effect. If two materials are in close contact (such that they adhere), then overlap between the orbitals of the atoms in both materials enables electron transfer because the reduction in distance decreases the potential energy barrier. Once the materials are separated, they retain their charge because the potential barrier is once again too great for charge movement.

The transfer of electrons during this interaction has the macroscopic effect of (at least partially) equalising the electric potentials of the two materials.

You might also consider the triboelectric series, which ranks materials according to their tendency to gain or lose electrons. See http://soft-matter.seas.harvard.edu/index.php/Triboelectric_series.

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