Photoelectric Effect: Grade 12 Questions Answered

[mustafallica]

ok what i don't understand is that, after a certain amount of time, won't the piece of metal loose all its electrons becoming completely positive!, what will happen then, just beomes +vly charged, am in grade 12 and my teacher couldn't give me a satisfying answer, thanks

 

[interested_learner]

Assuming no outside source of electrons, as you knock off more and more electrons, the metal becomes more and more positively charged. This means that the photons have to have more and more energy to knock off any more electrons. Pretty soon even cosmic rays (the photons with the most energy), won't even be able to knock off any more electrons and the whole thing comes to a halt. Keep in mind that the electricmagnetic field is VERY strong. It doesn't take much of an imbalance to bring the whole thing to a halt.

Normally, if you want electrons to continue to fly off the metal, you have to supply an outside voltage

 

[mustafallica]

cool, thanks bud

 

[free_electron]

Hmmm I posted a similar question just today. The electrons must eventually return since the metal is positively charged. I've been looking for more detailed discussion of how soon they return and how far the holes left behind can drift.

 

[ZapperZ]

No they don't. The cathode is grounded, so none of the photoelectrons have to "return". If you look at a standard photoelectric setup, it is the anode that is biased to a positive potential. This is all you need to collect the photoelectrons.

Zz.

 

[free_electron]

No they don't. The cathode is grounded, so none of the photoelectrons have to "return". If you look at a standard photoelectric setup, it is the anode that is biased to a positive potential. This is all you need to collect the photoelectrons.

Zz.

Dear Zz,

Thanks for your response. I see what you mean. If the metal is grounded, electrons can "return" through ground rather than directly. Or another way to think of it is the holes also leave the metal through ground?

What if the metal is floating (e.g.,placed on insulator)?

 

[ZapperZ]

Dear Zz,

Thanks for your response. I see what you mean. If the metal is grounded, electrons can "return" through ground rather than directly. Or another way to think of it is the holes also leave the metal through ground?

What if the metal is floating (e.g.,placed on insulator)?

Then you get the charging effect and after a while, you'll stop getting any photoelectrons coming out, because the effective work function would have increased beyond the energy of the incoming photons.

People who do photoemission spectroscopy on insulating material have to deal with this all the time. I certainly did.

Zz.

 

[free_electron]

Then you get the charging effect and after a while, you'll stop getting any photoelectrons coming out, because the effective work function would have increased beyond the energy of the incoming photons.

People who do photoemission spectroscopy on insulating material have to deal with this all the time. I certainly did.

Zz.

Great you're the right person to ask then! :)