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"what causes an object in friction with a surface to become negatively charged?"

A: In order to understand where this negative surface charge is coming from, we have to first understand the concept of friction. When two surfaces are in contact and move or rub against the other, their irregularities cause a frictional force to be exerted between the two surfaces. This force disturbs the surface atoms - more specifically the valence electrons. So when an object rubs against a surface, there is a net transfer of electrons from the surface to the object - i.e. the atoms of the surface lose electrons, while the object’s surface atoms gain it. This overall gain of electrons is what causes the object to become negatively charged!

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