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When a glass rod is rubbed with a silk cloth, both get charged: The silk gets positively charged and the rod gets negatively charged.

My question is the following: How/why do these objects return to a more or less neutral state after a certain amount of time. The glass rod is negatively charged, that means there is a net surplus of electrons. Where do they go?

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If you put your rod in a ultra high vacuum it will stay charged almost forever, but since you probably keep it exposed to air, this is where the electron excess slowly migrates (and the same for the electron defect in the silk). Since the charge exchange requires an hit between an air molecule and a spot of the rod where an electron excess is present, and not always this hit is effective, the discharge is quite a slow process; however humidity, so water molecules, can considerably speed it up.

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    $\begingroup$ when a scale is rubbed with hair and when brought near pieces of paper, then paper being nuetral is still attracted towards scale. Why? $\endgroup$ Mar 27 '14 at 9:44
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    $\begingroup$ @MurtuzaVadharia Because scale induces opposite charge in paper and thus, attracts it. $\endgroup$ Mar 27 '14 at 10:13
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    $\begingroup$ @MurtuzaVadharia Paper isn't really neutral. It's very close to being so, but there is slight polarization to the charge. Since paper is very light, this is enough to lift it up. In fact, the charge migrates inside the paper - when attracted to a net negative object, the electrons will try to get away from it, leaving a deficiency closer to the rod, which will make it weakly positive, leading to attraction. There's also more examples of similar principles, eg. you can attract individual water molecules with magnetic fields, since it doesn't have uniform charge. $\endgroup$
    – Luaan
    Mar 27 '14 at 12:56
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    $\begingroup$ explain in detail about the hit and discharge said in the above answer. In detail. $\endgroup$ Mar 31 '14 at 8:42

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