# Gold leaf electroscope problem

No sirring in PF
Don't you like if someone calls you sir?sorry if that's the case.It's my habit.

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
Homework Helper
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...

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.

Last edited:
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
Homework Helper
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.

Last edited:
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
Homework Helper
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.

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.

The picture I posted on my post number #34 is not given in the question i think this is better suited for situation described in the question.

I am really confused.

andrevdh
Homework Helper
In order to investigate with an electroscope you have to handle it. You can transfer charge to the case in the process. Earthing it would cause such charge to flow away to earth.