Photoelectric Effect: Grade 12 Questions Answered

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Discussion Overview

The discussion revolves around the photoelectric effect, specifically addressing questions related to the behavior of electrons in a metal when exposed to light, the implications of charging, and the conditions under which photoelectrons are emitted. The scope includes conceptual understanding and technical details relevant to high school physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether a metal will lose all its electrons and become positively charged over time, seeking clarification on the implications of this charge.
  • Another participant explains that as electrons are knocked off, the metal becomes more positively charged, requiring photons with increasingly higher energy to eject additional electrons until no more can be emitted.
  • A different participant suggests that electrons must eventually return to the positively charged metal, prompting further inquiry about the dynamics of electron movement and charge distribution.
  • One participant clarifies that in a standard photoelectric setup, the cathode is grounded, meaning photoelectrons do not need to return directly to the metal, as they can be collected by a positively biased anode.
  • A follow-up question arises about the scenario where the metal is not grounded, leading to a discussion about the charging effect and its impact on photoelectron emission, particularly in the context of photoemission spectroscopy.

Areas of Agreement / Disagreement

Participants express differing views on whether electrons must return to the metal after being emitted and the effects of grounding on this process. The discussion remains unresolved regarding the specifics of electron behavior in grounded versus floating scenarios.

Contextual Notes

There are assumptions about the behavior of electrons and the conditions under which photoelectrons are emitted that are not fully explored. The discussion also touches on the implications of grounding and the effective work function, which may depend on specific experimental setups.

mustafallica
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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
 
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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
 
cool, thanks bud
 
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.
 
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.
 
ZapperZ said:
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)?
 
free_electron said:
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.
 
ZapperZ said:
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! :)
 

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