Gold leaf electroscope problem

[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.

 

[gracy]

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.

 

[gracy]

I am really confused.

 

[andrevdh]

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.