# Einstein's Photoelectric effect expirement

1. Jul 5, 2011

### Makveger

First check out this picture (from the book I'm reading)

http://imageshack.us/photo/my-images/838/25090635.png/

It says that the collector's voltage is decreasing , can you explain why?

also it says "few photoelectrons can overcome this potential difference", since the potential difference is high so the metal surface should be having a higher voltage than the collector so what prevents photoelectrons from reaching the collector?

2. Jul 5, 2011

### silmaril89

Do you mean to say why is the collector's voltage becoming increasingly negative? The point is this, you are extracting electrons from some metal at the bottom of the the picture (photoelectrons). Let's start the voltage at 0. If we do that the photoelectrons will not have any force on them and will be detected as a current. If we begin to make the voltage increasingly negative, which really just means we are going to be getting a lot of electrons to start collecting at the collector (from the battery). This will create a force on the photoelectrons. If the voltage is strong enough it will create such a force on the photoelectrons as to cause no current to be detected from them, because they will not be able to move towards the collector. If we slowly increase this voltage we will get less and less photoelectrons (this is dependent on the energy they have). A very high energy electron can move further through a potential difference than a low energy photon.

Basically the experiment is being used to find the maximum energy of the photoelectrons that are being expelled from the metal.

3. Jul 5, 2011

### Drakkith

Staff Emeritus
The collectors voltage is decreasing because fewer electrons are reaching the collector when you increase the voltage between the collector and the emitter. The metal itself isn't being charged. (I can't see the picture from here at work, but the experiment I've heard of originally had a metal mesh that was charged I believe. The electrons would go through the holes in the the mesh and reach the collector, but as the mesh had its voltage increased fewer electrons could overcome the potential until finally none could.)