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Rubbing a glass rod with silk causes charges to be exchanged and consequently both objects get charged.

Why do the objects have to be "rubbed"? I get that one has a stronger pull on the electrons than the other, but shouldn't just allowing the objects to make contact be enough.? I would appreciate a "visualization" of whats happening.

Similar questions: Why two objects get charged by rubbing?

How does rubbing cause the transfer of electrons from one object to the other?

Neither question addresses why the objects need to be rubbed instead of just making contact.

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    $\begingroup$ I think the answers you got are sufficient at the level the question is formed. Simple contact forms new bonds that allow for mobility of electrons, breaking them once leaves a bit of charge separation, rubbing allows a cumulative effect of bond breaking and leaving charges separated. $\endgroup$ – anna v Jan 25 '14 at 5:21
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The link you posted in your question contains a link to a wikipedia page on the triboelectric effect, which in turns contains the answer to your question. From the "Cause" section:

Although the word comes from the Greek for "rubbing", τρίβω (τριβή: friction), the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away, though the imbalance will be partially destroyed by tunneling or electrical breakdown (usually corona discharge). In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules.

The triboelectric effect is related to friction only because they both involve adhesion. However, the effect is greatly enhanced by rubbing the materials together, as they touch and separate many times.

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  • $\begingroup$ Thanks! Just to make sure I understand your answer correctly: If you just make contact there would be an exchange of electrons, just significantly less than if you rubbed them. Am I right? $\endgroup$ – dfg Jan 25 '14 at 18:04
  • $\begingroup$ Yes, that is correct. $\endgroup$ – Scott Griffiths Jan 27 '14 at 5:35
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In all kinds of solids the electrons no matter how free to move are bound to atoms by electromagnetic forces, to break off electrons from the grip of rest of the atom energy needs to be supplied.

The required energy changes when one of the material has more affinity for electrons than the other as in the case of glass rod and silk. The new bonds formed release more energy than was in the original bond state in the other material and hence only certain combinations produce *easily observable * effects. When we rub the materials together we deliberstely provide the energy via friction and therefore heating, this energy is the required activation energy for electrons to jump the energy barrier, break old bonds and make new bonds in other body.

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