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When I was learning a chapter related to Static electricity, my teacher had mentioned that when you rub your comb with DRY hair specifically, static electricity is generated due to friction. Further more she had mentioned a case in which a glass rod is rubbed with silk cloth and still static electricity is generated. However it is important to note that glass rod and silk cloth are very smooth, thereby this reduces friction considerably,then how come static electricity was produced?

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    $\begingroup$ Note that your title and body don't seem to match. $\endgroup$ – Kyle Kanos Aug 24 '15 at 3:54
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When considering whether two surfaces will have a high friction or a low friction when rubbed together, more important than whether they are individually smooth or rough is what the barrier to their passing actually is. Consider first two perfectly flat plates. Even if the two plates are made out of wood, which is rough, they slide relatively easily past each other. However, two ridged plates will not easily slide (perpendicular to the ridges) even if the material is quite smooth. The ridges must push the plates apart to pass each other, i.e. they must overcome an energy barrier of sorts.

When you get down to the atomic level and examine friction, you see that it is actually due to weak chemical bonding between the two surfaces. Each chemical bond takes a certain amount of energy to break, so it takes energy to slide two surfaces against each other. If you assume that the binding energy of each point of contact is roughly equal, then the frictional force becomes proportional to the number of contact points between the surfaces. At the atomic level, two very smooth surfaces fit very close together - there are a lot of points of contact - so the frictional force is high. Ironically, at that scale a certain amount of roughness can actually reduce the friction by reducing the number of points at which the two surfaces make significant contact.

So while it may seem to you that the glass rod and silk cloth are too smooth to create static electricity, it's only because you can't see small enough. At the size of the atom, where electrons are jumping from one molecule to another, the surfaces are still relatively very "rough," in that there's a lot of sticking together going on.

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The correlation between smoothness or roughness and static friction is not trivial. If the surfaces are perfectly flat and clean, then they could theoretically weld together. This is generally not the case as surfaces are terminated by oxygen and water generally which significantly affects molecular scale surface interactions. The chemical composition of a material and its surface termination (adsorbates) govern the molecular scale interactions. Overall the relationship between surface structure and friction depends on a combination of surface structure and atomic friction (interactions at the molecular scale). It is found that the fractality of surfaces is a key consideration in predicting their frictional behaviour.

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