Photoelectric effect

[jaiezou]

Hey guys,

I was wondering why the metal surface should be negatively charged?

Is it because  as electrons leave the surface when enough energy is given to them the surface becomes positive and so the other electrons become attracted to the positive charge of the metal and will not leave the surface thus no photoemission?

Thanks

[Calebark]

Hey there!

You're spot on with the first part. If you're taking Chemistry, you might learn about effective nuclear charge and the shielding effect (idk about the HSC course, I was VCE haha), which explains this in a lot more detail. I'm not overly sure for the clean part, but I'd wager that it's because if there's any foreign object on the surface of the metal (say, dust), then the light could hit the dust instead of the metal plate, which would not cause the photoelectric effect to occur, and would skew your results. Whenever you're conducting an experiment, you want to remove as much outside interference as possible.

[Fizzycyst]

The cathode is charged negatively such that the electrons accelerate away from the cathode, resulting in a flow of charge (i.e., current)
Usually you would say “clean metal surface in a vacuum” just to make it clear that the metal has not oxidised. Which of course, is a problem with the active / reactive metals 👍