Gold leaf electroscope problem

[gracy]

Homework Statement

http://www.physicstutorials.org/home/electrostatics/electroscope
Negatively charged electroscope and X-Y plates are placed like in the figure given below. If the distances between electroscope and conductor plates are equals; find the types of charges of plates for given situations.
1)If the leaves are closed a little bit
2) If the leaves rise a little bit

Homework Equations

___________

The Attempt at a Solution

Answers of 1) is
1. Both of the plates can be positively charged and have different amounts of charges. They both attract electrons of the leaves.
2. They can have different charges; however, amount of positive charge must be larger than the negative charge. Since, there must be net positive charge to attract electrons from the leaves.

Should not there be third possibility i.e
3)Both of the plates can be positively charged and have same amount of charge

similarly Answers of 2) is
1.Both of the plates can be negatively charged and have different amounts of charges. They repel electrons from top of the electroscope to the leaves.

2.They can have different types of charges, but amount of negative charge must be larger than the amount of positive charge to repel electrons from top of the electroscope to the leaves.


Should not there be third possibility i.e
3)Both of the plates can be negatively charged charged and have same amount of charge

 

[gracy]

 

[BvU]

Hello, good you rang, because I indeed missed this thread.

I have little to add, but I am quite sure you have a question. What is the question ?

 

[gracy]

Hello

Hi !

 

[gracy]

What is the question ?

Should not there be third possibility i.e
3)Both of the plates can be positively charged and have same amount of charge

 

[BvU]

Sorry for overlooking that ! Let me check.

 

[BvU]

I fully agree with you.

 

[gracy]

And what about

Should not there be third possibility i.e
3)Both of the plates can be negatively charged and have same amount of charge

quite obvious ?

 

[gracy]

In the third point,it is written that "During this process, leaves of electroscope are closed and after completion of charge sharing they rise again".
Why the leaves of electroscope were closed earlier?

 

[BvU]

Same thing. They reason correctly that some electrons move towards the leaves. The only thing that is needed for that is some negative charge near the tip sphere of the electroscope. One negative, two negative (equal or not equal), one negative and one lesser charged positive, it doesn't matter.

It looks to me as if they wanted to ask and explain a little too much for this example.

Can't find an e-mail address for this club, otherwise you could point this out to them !

 

[gracy]

Can't find an e-mail address for this club, otherwise you could point this out to them

Similarly i have found another mistake on this particular page

 

[gracy]

Similarly i have found another mistake on this particular page

 

[gracy]

Opposite charges repel?opposite !

 

[gracy]

1. Both of the plates can be positively charged and have different amounts of charges. They both attract electrons of the leaves.

it's also correct,right?

 

[BvU]

Can't keep up with you! posts cross: e.g. my #10 was in reply to your #8. and I missed your #9 (& 11, 12, 13, 14 !) For #14: yes. I thought we've had that settled already before.
For the others, give me time to read the stufff ...

 

[gracy]

(& 11, 12, 13, 14 !)

Don't take tension .#(11,12 13) are same

 

[BvU]

For me (no expert in didactics, just a student long time ago) this is again not a very helpful exercise. But what it says is correct.
One can argue about details (formally Y is + and Z is - goes too far. All that can be concluded is that Y is more + than Z).

Opposite charges repel. That's correct. But the sentence

if Y is +, then Z must be -, because opposite charges repel each other

is gibberish.

My account:
If the leaves spread, positive charge must be pushed down the electroscope. So Y has become more postitive than before. YZ is on an insulator, so the total charge on YZ is constant. THerefore Z must have become less positive (= more negative) than before. That means T has pushed away positive charge from Z. Since pushing away is repelling that means T is positively charged.

 

[gracy]

Opposite charges repel. That's correct.

I thought Opposite charges attract.

 

[BvU]

See ! You did manage to confuse me. Or rather, the PhysTut site managed that.

Of course you are right!

What happens is "YZ is on an insulator, so the total charge on YZ is constant. Therefore Z must have become less positive if Y has become more positive."

You're better than I in this, so keep up the good work !

 

[gracy]

You're better than I in this,

Not possible.Teachers always lead irrespective of the context.

 

[gracy]

You did manage to confuse me.

PhysTut site is the culprit.Don't blame me !

 

[BvU]

Don't blame you. And you must be a natural (teacher, that is)

 

[gracy]

Sir,please answer my post #9.

 

[BvU]

No sirring in PF (one of the nice features of such a forum) ! Will look. After all I admitted I missed it.

- behavior of leaves when we touch charged object A to the charged electroscope
- A has +4q charge and electroscope has -2q
- capacities of a and scope are same

A far away: leaves apart

A closer: A is pushing + charge down the electroscope (equal charges repel, remember ), so leaves come closer

A touching: -2q from electroscope and +2q from A get married and happily live ever after. Whole lot now has a total charge of +2q, nicely divided over a and the scope, probably the +1q of the scope sits aas far away as it can, in the leaves.
The +1q makes the leaves of the electroscope spread out again. But -if I'm right- not as much as the -2q at the very start.

A no longer touching: A is still pushing away the + charge from the top sphere on the scope

A far away again: scope evenly distributes +1q and spread reduces

---

I've been thinking this up from what I know. My last experience with these things was when my teacher demonstrated, a very, very long time ago. So I hope you or other bright readers correct me if I went wrong at some point !

 

[andrevdh]

There is a flow of electrons from the electroscope to the rod up to the point where
the rod is charged +2q. At this stage the electroscope has lost all of its charge
and the leaves are closed all the way down. Now the rod draws more electrons
off the electroscope and the leaves rise again until both are charged +q.

Note this can only happen if the rod is made of an conductive material.

 

[gracy]

No sirring in PF

Don't you like if someone calls you sir?sorry if that's the case.It's my habit.

 

[gracy]

Will look. After all I admitted I missed it.

Sorry for the haste I have shown.(i am behaving seriously because there are no smilies in your sentence)

 

[BvU]

Don't you like if someone calls you sir?sorry if that's the case.It's my habit.

Makes me feel old and wrinkly. I am old and wrinkly...

 

[gracy]

Ok. one more query about this gold leaf electroscope
when a negatively charged rod is brought near the cap of a gold leaf electroscope whose case is earthed then what will happen?
I think when a negatively charged rod is brought near the cap of a gold leaf electroscope cap,it will try to develop positive charge .so electrons from the cap will travel to gold leaves and in this way cap will develop induced positive charges and leaves would have induced negative charge (induced because rod has just been brought near not touched).But the question further says the cap case is earthed ,so it will be provided with electron from Earth and it will get neutral(as this is the purpose of earthing as far as I know).Then in this way the leaves would have negative charge ;which will be there forever ( permanently) I mean as it is not induced charge. I think induced charges are there for limited time as soon as we remove the inducing agent, the induced charges disappear.

 

[gracy]

I want to understand if the question says the case is earthed what actually is earthed?Is it case of cap or case of whole gold leaf electroscope?

 

[andrevdh]

For some measurements the case gets connected to earth. This somewhat screens the
electroscope from external effects and makes it a more relaible measuring device, otherwise
charges on nearby objects, for instance you, or nearby metallic objects, a lab stand, can influence
its deflection. This changes the electroscope into a capacitor with a somewhat constant
capacitance and makes it suitable for voltage measurements.

The electroscope is also used without earthing the case. As you know it can be charge by induction.
That is charging the electroscope without bringing the charged object into direct contact with the plate
or cap. In this instance the plate is earthed momentarily while holding the charged rod nearby. We then
get that the rod pulls the opposite charge from Earth onto the electroscope. The Earth is then removed
and finally the rod, leaving the electroscope with a charge opposite to that of the rod.

 

[gracy]

when a negatively charged rod is brought near the cap of a gold leaf electroscope whose case is earthed then what will happen?answer is leaves will develop induced negative charge .but i don't understand why.as the leaves are earthed how it can have any charge?earthing is for neutralisation

 

[andrevdh]

This is what I think will happen:
In this instance the case of the electroscope is earthed, not the leaves.
The rod will push the electrons off the case into Earth so that a positive
charge is induced on the case. It will also push electrons in the cap down
into the leaves so that they become negatively charged, but less than if
the case was not earthed.

 

[gracy]

 

[gracy]

In this instance the case of the electroscope is earthed, not the leaves.

This is what my teacher said on my question
My question:

when a negatively charged rod is brought near the cap of a gold leaf electroscope whose case is earthed then what will happen?answer is leaves will develop induced negative charge .but i don't understand why.as the leaves are earthed how it can have any charge?earthing is for neutralisation

Teacher answer :The case is the insulated box on top of which the cap is placed and inside which there are the leaves of metal. We need to Earth this case to ensure any charges on it which may lead to incorrect reading are neutralised.