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Stopping potential

  1. Jul 3, 2015 #1
    In the photoelectric effect experiment..it says - the stopping potential should be more negative (-V) than the cathode plate to stop the current...but
    i) should not it be equal to the potential of the cathode plate...?? i mean if it is equal, the there would be no potential difference between the plate and currrent flow should stop...and the electron flow would stop..
     
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  3. Jul 3, 2015 #2

    sophiecentaur

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    The Potential you need has to be negative enough with respect to the photo-electrode to repel even the fasted electrons. That means it has to be lower than the plate potential. If the potentials are equal (your suggestion) then electrons will find themselves on the 'catcher' and they will not return across the gap but find their way round the circuit because that's the 'easiest path'.
    The Kinetic Energy from the photo electrons will actually be dissipated in the resistance of the connecting wire and components.
     
  4. Jul 4, 2015 #3
    ohh thanks for helping...
     
  5. Jul 4, 2015 #4

    ZapperZ

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    Initially, I was going to leave it as it is, since the OP appears to "understand" the answer that was given. However, I still think that what was written is rather unclear, and a couple of important points might have been missed here.

    1. The cathode is usually grounded, so it is at zero potential. So making the anode to be at the same potential as the cathode means that everything is grounded.

    2. The photoelectrons are "born" with a range of kinetic energy, i.e. they are already moving! If everything is at the same potential, there will be nothing to stop these electrons from reaching all parts of the system, including the anode, and thus, registering a current. So how would this be the "stopping potential"?

    Zz.
     
  6. Jul 4, 2015 #5
    Thanks for your explanation ZapperZ.

    I have another question..if I apply stopping potential..the electrons emitted from cathode stops "reaching" anode...but still they are generating from cathode...right...?? So,what happens...??
    They are just emitted from the surface and become motionless ??
     
    Last edited: Jul 4, 2015
  7. Jul 4, 2015 #6

    ZapperZ

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    They get pushed away from the anode. Where they end up depends on their initial directions and energies. They don't become motionless. Remember, there is an external field due to the anode being at a certain potential. So these electrons can't just sit there.

    Zz.
     
  8. Jul 4, 2015 #7

    sophiecentaur

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    If you put an insulated plate next to the photoelectron emitter, the photoelectrons will accumulate on that plate, making its potential more and more negative (wrt the emitter) when the potential reaches a 'stopping potential', electrons will not hit the plate (repelled) and may just return to the emitter plate. that's ok but there is no way of telling when this condition has been reached and measuring a zero current using a variable bias potential will tell you when it has.
    Same 'automatic' thing happens for an insulated photo electron emitter. Electrons are kicked off, leaving the emitter with a positive potential. Eventually, an equilibrium condition will be reached when the space charge, around the emitter reaches a maximum value.
     
  9. Jul 5, 2015 #8
    As long as the incident photons have the sufficient energy,electron will come out. Now they cannot go near the insulated plate. So the electrons come back to the emitter (positive potetial). That means both emission and recombination goes on simultaneouly at a constant rate and thus space charge reaches the maximum value...???
     
  10. Jul 5, 2015 #9

    ZapperZ

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    I've done photoemission experiments on insulators, and so have others if one judges by the number of papers published on this. There are ways to minimize such charging effects, depending on the nature of the insulator. I suggest you not follow or extend this line of discussion. The introduction of insulators in all of this is an unnecessary complication, and from what I can tell already, it is distracting you from understanding the straightfoward physics in a standard and simple photoelectric effect experiment.

    Zz.
     
  11. Jul 5, 2015 #10

    sophiecentaur

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    Yes. That will be the equilibrium state.
     
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