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There was a section of my textbook that mentions this and I can't understand it...

Q: Is static electricity caused by friction?

A: No. It appears when two dissimilar insulating materials are placed into intimate contact and then separated, they get charged and all that is required is touching. It is only that the process of rubbing one material upon another can greatly increase the surface area.

I can't understand the answer. What does dissimilar mean here? Does it refer to different magnitudes of charge? If both the objects are electrically neutral and come in contact, how can they charge each other?

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  • $\begingroup$ That book seems to use unnecessarily confusing language... $\endgroup$
    – knzhou
    May 19 '20 at 17:57
  • $\begingroup$ @knzhou Yeah, lol $\endgroup$ May 21 '20 at 6:40
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Dissimilar means different materials, for example perspex (lucite or polymethylmethacrylate!) and polythene (polyethene). They may indeed be electrically neutral to start with, but rubbing them together will give the perspex a positive charge and the polythene a negative charge. This is because electrons are transferred from the surface of the perspex to the surface of the polythene. Why does this happen? It is perhaps best thought of as like a chemical reaction: the surface atoms of the polythene molecules have a greater affinity for electrons than the surface atoms of the perspex. To understand why in any more detail requires some serious surface chemistry.

Merely touching the dissimilar bodies together will cause a small amount of charge to be transferred, but rubbing them together will greatly increase the amount. This is because rubbing together increases the area of contact between the bodies. It will also cause a frictional force between the bodies, but this is irrelevant: rubbing together produces both a force of friction and charge transfer, but that doesn't mean that the force causes charge transfer (or vice versa!).

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  • $\begingroup$ So, if they are not rubbed together and just brought in contact and then moved apart with no friction, they would not get charged, right? $\endgroup$ May 16 '20 at 9:45
  • $\begingroup$ They would get a small charge, but as your textbook points out, much more charge will be transferred if you rub them together, because this greatly increases the true area of surface contact. $\endgroup$ May 16 '20 at 9:47
  • $\begingroup$ "All that is required is the touching". In this context, are we using touching to refer to negligible friction or no friction at all? If it's negligible friction, that would probably explain it... $\endgroup$ May 16 '20 at 9:50
  • $\begingroup$ Friction is a $force$ between two surfaces tangential to the surfaces. This force is irrelevant to the charging. What matters is the contact and the area of contact. $\endgroup$ May 16 '20 at 9:53
  • $\begingroup$ As I understand it, if the objects are just brought into contact without any rubbing (or without any friction being generated), then the electrons would not transfer between the objects and they will not get charged. If some amount of friction is generated, only then will the objects be charged. Where am I wrong? $\endgroup$ May 16 '20 at 9:55

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