Some confusion about photoelectric effect 
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*In photoelectric effect, we give negative voltage to stop the flow of electrons, but I don't understand what negative potential is and in which direction it is applied.


*Recently, I have performed the experiment in my college laboratory. I became more confused because, in the apparatus I used, we actually increased the potential, but did not give any negative potential and, to my surprise, the current was negative. Why did these counter observations arose? Did I do the experiment in a wrong way or is it correct in some other sense?
 A: Let us take the simple apparatus of the photoelectric experiment:

Here two things can happen:

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*We apply potential (positive) wrt cathode, to the anode, the electrons are attracted towards the anode. We can think in Laymen's terms that an anode is being devoid of electrons, so the positive charge attracts the negative electrons. Here we can see the photocurrent.


*If we apply negative potential wrt cathode to anode, we see the current lowering down, as now electrons are repelled by the negative potential anode. In laymen's terms, the anode is now electron-rich, which repels the incoming photoelectrons. The potential at which photocurrent completely stops is stopping potential.
Now negative potential is nothing hard to think of. We define positive potential to be: work done on unit charge on moving it from infinity to some point in space under influence of some electric field. For negative potential we reverse it: work done on unit charge on moving it from some point in space to infinity under influence of some electric field.
Hope it helps!
