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We know that a charged glass rod can positively charge a neutral object when it goes into contact with that object. But wouldn’t the glass rod become electrically neutral by gaining electrons from the object in contact? Is there a limit to the number of neutral objects that can be charged by a charged object through contact?

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    $\begingroup$ If your glass rod originally had charge Q unbalanced, then later after they touch, the pith ball AND the glass rod also has to have charge Q unbalanced, just spread out between the two things in some complicated way. That is what it means for charge to be conserved. $\endgroup$ Apr 25, 2023 at 3:07
  • $\begingroup$ related physics.stackexchange.com/questions/278285/… $\endgroup$
    – anna v
    Apr 25, 2023 at 3:27

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Charge is conserved. When charge flows between objects, the total amount of charge stays the same. Only the location of the charge changes. Charge can't be moved around in packages smaller in magnitude than the electron charge $e$. Dividing the total charge by the electron charge gives you the absolute upper limit of neutral objects (each so small or so strongly bound to their electrons they only donate one electron before being repelled by the positive charge of the rod) that could be positively charged by the charged rod.

In Coulombs, $e \approx -1.6 \times 10^{-19} C$. If the USA's AP Physics curriculum uses realistic values, the triboelectric charge on a small glass rod rubbed with a silk cloth is on the order of $10^{-6} C$, meaning that it's missing approximately six trillion electrons (which have moved to the silk cloth, giving it $-10^{-6} C$ of charge). So in principle, about six trillion neutrally charged objects could each donate one electron and go on their way with a positive charge of $-e$.

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