AP2 appendix A p 291 question 6 - charging electroscope

[sdt99]

I am trying to understand the answer to this question.   If a negatively charged rod is brought near the knob at the top of the electroscope then the electrons in the electroscope will be pushed to the end with the leaves (the knob will assume a net positive charge).    If the aim is to finish with a positively charged electroscope then a grounding wire / finger would have to be applied near the leaf end of the electroscope to give the excess electrons a path to earth.

The given answer does not specify which part of the electroscope is grounded, but since only the knob is accessible, the rest being in the glass bulb, the implication is that the knob is grounded.   It seems to me that grounding the knob would cause electrons to flow from earth onto the positively knob and when the ground is broken and the rod removed the result would be a negatively charged electroscope.

[FizziksGuy]

Page 78 in the book talks about charging by induction.  This section should help you out.  It doesn't matter which part of the metal rod is grounded, since it's a conductor.

[sdt99]

Thanks for the reply,

But if I ground the end of the electroscope / metal rod that has an induced positive charge then electrons will flow from the earth to balance that net positive charge.  When the negatively charged rod is removed from the electroscope the electroscope will then have a net negative charge - the opposite of what was asked for in the question.

[FizziksGuy]

Connecting to ground provides an infinite supply of electrons (or can be an infinite sink of electrons).  If you ground a metal rod, and hold a negative rod near an end, negative charges will be repelled from the rod into the ground.  If you then disconnect the rod from the ground, no charges can enter or leave the rod, so you're left with a net positive charge on the rod.