Static Electricity!!! Need help!!!?

Explain, briefly:


a) When an uncharged metal plate held by an insulating handle is brought into contact with the cap of a negatively charged leaf electroscope the divergence of the leaf is decreased.


b) As the same metal is then brought near the cap again, the leaf begins to diverge.


c) When the plate again makes contact with the cap the divergence is of the same value as in the final stage of (a), above.

Static Electricity!!! Need help!!!?
a) some of the charge in the leaves of the electroscope move into the metal plate when it touches. The leaves of the electroscope now have less charge and repel less, so they diverge less.


b) The plate (now negatively charged) brought close, but not touching the electroscope, pushes electrons away from itself, they move toward the leaves and make them diverge.


c) When the plate touches, electrons move so that they are evenly divided between the electroscope and the plate. The leaves diverge less.
Reply:a) As there is contact it implies that electrons flowed from highly -vely charged electroscope to the cap. Hence it is no longer neutral but is becoming -vely charge.





b) Like charge repels. (both are -vely charged). There is induction and no contact.





c) The same "number" of electrons which was given at the beginning is still the same since there was no contact. Hence there will be the same effect.
Reply:a) Electrons move from the electroscope to the metal plate until the charge between them becomes equal. Now they both have a negative charge. Part of the charge went to the plate and part remained in the electroscope. Since the charge in the electroscope decreased, the distance between the leaves decreased.





b) When the two are brought in close proximity, the free electrons repel each other and move towards the opposite ends from where the two objects are close to each other. In this case, the free electrons in the plate stay there but move towards the end furthest away from the electroscope. And the electrons in the electroscope also stay put, but move towards the opposite end, the end with the leaves, increasing the negative charge between the leaves, causing the leaves spread further apart again.





c) When the two materials touch again, you revert back to the same conditions as in "a" - all the free electrons become evenly dipersed between the materials with an overall negative charge so the leaves do still repel each other, just not so much as when the electrons were repeled and concentrated towards the opposite ends as in "b," or when the leaves had the higher negative charge that you had in the beginning.

balsam