How do the electroscope leaves behave after grounding?

[JessicaHelena]

Homework Statement

A positively charged rod is brought near the knob of an uncharged electroscope and the leaves of the electroscope diverge. As the positively charged rod is held near the knob, the electroscope is grounded with a wire. Then the grounding wire and the positively charged rod are removed. The leaves of the electroscope

a) diverge and are negatively charged
b) dierge and are positively charged
c) converge and are negatively charged
d) converge and are positively charged
e) converge and are neutral

Homework Equations

I think this is more of a conceptual question...

The Attempt at a Solution

I had thought the answer would be e), but apparently it is a) and I need help understanding why. Grounding is the process of 'removing' excess charge by transferring electrons. Since the rod is positive, the electrons of the (initially neutral) leaves are attracted to the rod and the leaves are left positive (with less electron), causing them to repel each other. Since the leaves are now positive, when we have grounding the electrons are going to be 'added' to the leaves. And (I'm not confident about this but) since grounding is a process used to neutralise objects, and while I'm not sure whether the leaves will remain diverged or start to converge, since e) is the only one with 'neutral' in it, that was what I chose for the answer...

 

[kuruman]

(e) cannot be the answer because when the positively charged rod is brought near the grounded electroscope, electrons will flow through the ground to the electroscope. When the ground is removed while the positive rod is still near the electroscope, the extra electrons are trapped and they have nowehere to go when the rod is pulled away. The electroscope would be neutral if the rod where pulled away first and then the ground removed.
Check out "Charging by induction".

 

[QuantumQuest]

First begin by what happens to the knob of the electroscope as the positively charged rod is brought near it. What sort of charge does the knob acquire? It is negative as you say and consequently leaves acquire positive charge. So far so good. Now, if we Earth the electroscope i.e. the knob, we effectively connect to a negative electric potential so what happens to the charge of the leaves is to get neutralized as you say by electrons. Now, having removed the earthing and as long as the positively charged rod is present near the knob what sort of charge is there on the knob? If you remove the positively charged rod what sort of charge does remain on the electroscope? What sort of movement of the leaves does this imply?

 

[JessicaHelena]

@kuruman —

When the ground is removed while the positive rod is still near the electroscope, the extra electrons are trapped and they have nowehere to go when the rod is pulled away.

Maybe it should be obvious to me but I'm not sure how the leaves will behave when the electrons have nowhere to go after the wire was removed first and then the rod. If the rod is removed last, wouldn't all the electrons that came through the wire have been attracted to the rod so as to leave the leaves positively charged?

@QuantumQuest — If the rod remains then the knob will be negative and removing that rod would still leave the knob negative? I'm sorry, but I'm not sure if I'm getting it... :|

 

[QuantumQuest]

If the rod remains then the knob will be negative and removing that rod would still leave the knob negative? I'm sorry, but I'm not sure if I'm getting it... :|

Indeed, as long as the rod remains close to the knob, the knob will have negative charge. Now, if you remove the rod you have only the negative charge on the knob. What happens then regarding the whole electroscope?

 

[kuruman]

What happens then regarding the whole electroscope?

More precisely, what happens to the conducting part of the electroscope which is a conducting rod with a knob attached to one end and the leaves attached to the other. When the positively charged rod is near the knob (after the grounding wire has been removed) where do you think the extra electrons will most likely be? Will they be redistributed after the rod is removed? If so how?

 

[QuantumQuest]

More precisely, what happens to the conducting part of the electroscope which is a conducting rod with a knob attached to one end and the leaves attached to the other.

Yes. that's what I mean too. I just didn't phrase it precisely enough but I think the OP got the idea ;)

 

[JessicaHelena]

So when there's positively charged rod still near the electroscope, the extra electrons are attracted to the rod and so will stay around that area, but after the rod is gone, since there's no longer a positive charge to attract the electrons, will they distribute to the leaves and make the leaves (more) negatively charged?

 

[QuantumQuest]

...but after the rod is gone, since there's no longer a positive charge to attract the electrons, will they distribute to the leaves and make the leaves (more) negatively charged?

After removing the rod - we have already removed earthing, negative charge indeed will distribute uniformly onto the leaves. Now, the question for you is what happened to the leaves of the electroscope i.e. converged or diverged when we applied earthing and what will happen when we remove the rod? The answer to this last question is what you are asked to answer.