The gold leaf electroscope and the photoelectric effect

[zoya76]

The theory I have read
When you charge an electroscope with a negatively charged rod, electrons are repelled to the bottom of the electroscope and the gold leaf rises. Then when you shine light above the fundamental frequency on the top plate of the electroscope, electrons are liberated from the plate and the leaf falls.

What I don't understand
Where are these electrons liberated too? Where do they go? And are they dragged up from the gold leaf before being liberated somewhere? If they are liberated from the plate itself (too somewhere) then surely the plate will get even more positively charged and therefore the leaf would stay elevated. Am I missing something?

 

[collinsmark]

Hello zoya76,

Welcome to Physics Forums!

Maybe the problem statement is implying that the electrocsope is charged by touching the top plate with the negatively charged rod. That would give a net negative charge to the whole kit and kaboodle (i.e. the top plate and the gold leaf). This would cause the gold leaf to rise due to its net negative charge, and the top plate would maintain a negative charge too (that is, until you neutralize the whole thing by blasting some electrons off the top plate by shining a high-frequency light on it).

 

[Dadface]

The theory I have read
When you charge an electroscope with a negatively charged rod, electrons are repelled to the bottom of the electroscope and the gold leaf rises. Then when you shine light above the fundamental frequency on the top plate of the electroscope, electrons are liberated from the plate and the leaf falls.

What I don't understand
Where are these electrons liberated too? Where do they go? And are they dragged up from the gold leaf before being liberated somewhere? If they are liberated from the plate itself (too somewhere) then surely the plate will get even more positively charged and therefore the leaf would stay elevated. Am I missing something?

I think that initially the electrons are liberated into the surroundings and form an electron cloud which expands under mutual repulsion as more electrons are added.I have a vague memory of something called the Richardson Dushman equation which quantifies the cloud formed by thermionic emission.Try googling to see if this equation or some sort of variation of it can be applied to photoelectric emission and answer your question in greater depth.

 

[zoya76]

Thanks guys. It makes sense now. I think I'll have to use this forum more frequently!