An initially uncharged electroscope is polarized by bringing a negatively charge

Question

An initially uncharged electroscope is polarized by bringing a negatively charged rubber rod near the bulb. (a) If the bulb end of the electroscope acquires a net charge of +2.50 pC, how many electrons are on the leaf end? What is the net charge on the electroscope (including both bulb and leafs)? (b) If you keep the rubber rod in place, you connect the bulb end of the electroscope with a ground wire to the earth. What is the sign of the net charge on the electroscope (including both bulb and leafs)?

Explanation / Answer

Part A)
The charge per electron is 1.6 X 10^-19 C

Thus 2.50 X 10^-12/1.6 X 10^-19 = 1.56 X 10^7 electrons

The net charge, however, is zero since so charge has been transferred, it just polarizes momentarily while the rod is close to the bulb

Part B)

Grounding removes electrons. Thus the electroscope will end up with a net positive charge.

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