Photoelectric effect for a positively charged plate in vacuum In the link below, to demonstrate photoelectric effect, an electroscope is positively charged, then a UV light is shed on the top plate of the electroscope which causes the repelling force of the gold leaves disappear. This implies some electrons are added to the metal plate and neutralised its charge (the electroscope was positively charged). Where did these electrons come from?
One can say, the electrons come from air atoms, if this is correct, what will happen if we perform the same experiment in vacuum?
Here is the video of the experiment (performed in the presence of air): https://www.youtube.com/watch?v=v-1zjdUTu0o
 A: The plate would have to have initially been negatively charged. When the UV light is shined on the plate, negative photoelectrons leave it so that the plate overall becomes neutral (negative charge leaving a surface induces a positive charge resulting in electrical neutrality if the surface was already negatively charged).
By saying the plate was first positively charged would result in it being more positive after photoelectric emission. This means it is not possible for the electroscope to become neutral. If the person in the video states that it has positive charge to begin with, then this person misspoke.
There are no electrons coming from the air and if you did the same experiment in a vacuum, you'd get the same result.
A: The discoveries by Hertz led to a series of investigations by Hallwachs,[28][29] Hoor,[30] Righi[31] and Stoletov[32][33] on the effect of light, and especially of ultraviolet light, on charged bodies. Hallwachs connected a zinc plate to an electroscope. He allowed ultraviolet light to fall on a freshly cleaned zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged.
