Looking for some help with static electricity

[shanktank]

hey all,
i'm new to the forum...just finished up the momentum and energy unit and now were on to static electricity...i was hopin i could get a bit of help with a couple questions i have:

3. A negatively charged rubber rod is momentarily touched to the knob of a neutral electroscope and then removed.
...i've gotten to the e) part of this question.
e) How could you prove that the electroscope has a negative charge?


4. A positively charged glass rod is held near the knob of a neutral electroscope.
d) Explain what happens to the leaves?
...well i figure the knob has a negative charge, the leaves have a positive charge...thus, the leaves should come together?
e)Explain what happens if the knob is grounded?
I have no idea with this one...
f) Explain what happens to the electroscope if the ground is removed and then the glass rod is removed?
Again no idea...

and finally
8 a) It is observed that a charged rubber rod will attract a small piece of paper. Explain how this occurs?
...I assume because the charged rubber rod is either negatively or positively charged, and the paper is the oppositely charged?
b) Eventually the rubber rod will "spit" the piece of paper away and will not attract anymore. Explain why this occurs?
...Once the electrons are done transferrin in the rubber rod then the chargin is done or something? haha not to sure...

Thanks for the help, never done anythin with this topic, and I'm doing it through correspondance and teachers seem to be impossible to get a hold of on the phone..
Shane

 

[Hootenanny]

Welcome to PF shanktank,

4. A positively charged glass rod is held near the knob of a neutral electroscope.
d) Explain what happens to the leaves?
...well i figure the knob has a negative charge, the leaves have a positive charge...thus, the leaves should come together?

It is indeed true that the leaves will both have a positive charge. You need to ask yourself - "what do like charges do?"

-Hoot

 

[shanktank]

well like charges repel...so the negative charge on the knob has nothin to do with the leaves then...

 

[Hootenanny]

well like charges repel...so the negative charge on the knob has nothin to do with the leaves then...

The rod has a positive charge and therefore produces a electic field, which exerts a force on any charged particles. When the rod is brought close to the knob, it attract the electrons in the electroscope to the surface of the knob, so electrons are 'pulled' up from the leaves and the 'stem' to the knob at the top. Thus leaving behind a positive charge on the leaves. Don't forget only electrons can move, the positive ions are fixed in a metal lattice.

Does that make sense?

-Hoot

 

[shanktank]

ya ok that makes sense

 

[shanktank]

any ideas with 8?

 

[Hootenanny]

Same process as the electroscope, think about induced charges and whether a charge can flow. Remember it is only the electrons that can move.