Can electrons pass through a positively charged disc in electrostatic induction?

[Zahid Iftikhar]

TL;DR Summary

A simple explanation may be helpful regarding charging by electrostatic induction. Whether a body is positively or negatively charged if a point on the conductor is grounded.

My explantion of this electrostatic induction is that if the disc of electroscope is ground, electrons will flow from the ground and neutralize the disc, leaving the electroscope negatively charged after removal of the ground, but the book says it should be postively charged. As per book, the electrons from the gold leaves will flow to the ground. But it is not convincing as they will have to pass through the positively charged disc. I will be grateful if some scholar helps to explain. High regards.

 

[.Scott]

Perhaps the easiest way of looking at this is that the negatively charged rod will push electrons away and force a positive charge on the disc - just because that disc is the nearest charge holder. So initially the electrons get pushed to the leafs. But when a path is provided, those electrons will escape to ground.

 

[kuruman]

That's a good question. However, you are ignoring the negative charges on the rod in your analysis. Look at the modified drawing below. In (a) there is an excess of negative charges enclosed by the gray bubble. This excess would move to Earth if there is a path in order to neutralize the total charge enclosed. That path is established in (b) and the excess charges move into the Earth. Now the grounding wire is removed with the negatively-charged rod in place. This traps the positive charges on the electroscope so that when the negative rod is removed, the electroscope has a net positive charge.

 

[Zahid Iftikhar]

Perhaps the easiest way of looking at this is that the negatively charged rod will push electrons away and force a positive charge on the disc - just because that disc is the nearest charge holder. So initially the electrons get pushed to the leafs. But when a path is provided, those electrons will escape to ground.

Thanks, sir for your kind time. My reservation on the provision of path, in this case ground attached to the disc, is that it should attract electrons from the ground, instead of gold leaves. Stated simply, electrons from leaves cannot come to the disc as long as positively charged source remains close to the disc.

 

[kuruman]

Stated simply, electrons from leaves cannot come to the disc as long as positively charged source remains close to the disc.

Why exactly is that the case? What do you think is the role of the positive source? If anything, the positive charges would attract the negative charges.

 

[Zahid Iftikhar]

That's a good question. However, you are ignoring the negative charges on the rod in your analysis. Look at the modified drawing below. In (a) there is an excess of negative charges enclosed by the gray bubble. This excess would move to Earth if there is a path in order to neutralize the total charge enclosed. That path is established in (b) and the excess charges move into the Earth. Now the grounding wire is removed with the negatively-charged rod in place. This traps the positive charges on the electroscope so that when the negative rod is removed, the electroscope has a net positive charge.

View attachment 333044

Thank you very much for your precious time sir. I could not understand what you mean by gray bubble here. Did you mean the disc at the end of the rod? Further what is this object covering the whole electroscope? What is it for? Regards

 

[kuruman]

Thank you very much for your precious time sir. I could not understand what you mean by gray bubble here. Did you mean the disc at the end of the rod? Further what is this object covering the whole electroscope? What is it for? Regards

Look at the modified drawing I attached in post #3. It's the volume enclosed by the gray closed area (you may think of it as Gaussian surface) that encloses all the charges in the vicinity of the electroscope. The total charge inside this volume is negative. This means that if there is a connection to Earth, the excess negative charges will leave the Earth until the total charge inside this volume is neutralized.

 

[anuttarasammyak]

The Earth can be treated as a huge conductor. After connected to the system of disk and leaves, the Earth takes the role of electrostatic induction from the system. So excess electrons in leaves are of no use. They come back to their home position of disk. Thus caused neutrralization of disk harms electric induction made in the whole Earth, so electrons in disk are drawn to the Earth.

 

[.Scott]

Stated simply, electrons from leaves cannot come to the disc as long as positively charged source remains close to the disc.

Let me provide a more complete description of this system. Let's say that the negatively charged rod would normally cause 2 billion electrons to flee the plate - but once the leaves charge up they push back and allow only 1 billion electrons to actually move. So, relative to ground, the plate still holds a negative voltage. Can you see the how it discharges now?

 

[Zahid Iftikhar]

Thank you very much for this wonderful explanation. It is really very helpful. Regards

 

[Zahid Iftikhar]

The Earth can be treated as a huge conductor. After connected to the system of disk and leaves, the Earth takes the role of electrostatic induction from the system. So excess electrons in leaves are of no use. They come back to their home position of disk. Thus caused neutrralization of disk harms electric induction made in the whole Earth, so electrons in disk are drawn to the Earth.

High regards for your kind reply. It was very useful.