# Photoelectric Effect questions to help my understanding

• Jeff97
In summary: The threshold photon energy is where photo emission 'just' starts. Many photons of that energy will just be absorbed as they meet more tightly bound electrons.

#### Jeff97

Homework Statement:: This isn't homework, I didn't know exactly where to post my question so I thought it would be safest to post here.
Relevant Equations:: N/a

My questions are as follows:

1. When the voltage is increased to a certain value the current between the two plates(emitter plate) is reduced to zero. Can someone explain to me why increasing the voltage can stop the current?

2. How did Bohr modify the Rutherford model to explain the stability of hydrogen and its spectrum.?

I was thinking it was because... The motion of the electrons in the Rutherfords model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.

So to remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.?

Jeff97 said:
This isn't homework, I didn't know exactly where to post my question so I thought it would be safest to post here.
Thank you for defaulting to the Homework Help forums when you are not sure. But since these are general questions about the photoelectric effect, your thread has been moved to the technical forums.

I'm guessing that @ZapperZ will be able to help you with some intuition on this subject...

Jeff97
Jeff97 said:
Homework Statement:: This isn't homework, I didn't know exactly where to post my question so I thought it would be safest to post here.
Relevant Equations:: N/a

My questions are as follows:

1. When the voltage is increased to a certain value the current between the two plates(emitter plate) is reduced to zero. Can someone explain to me why increasing the voltage can stop the current?

This is a reverse-bias voltage, meaning the opposite plate has a lower potential than the photocathode (where the electrons came out of). As you increase the reverse bias, more and more electrons are being repelled away until at some point, even the most energetic electrons cannot reach that plate. So you get no current.

Zz.

berkeman and Jeff97
ZapperZ said:
This is a reverse-bias voltage, meaning the opposite plate has a lower potential than the photocathode (where the electrons came out of).
To make that possibly more understandable, if the the 'collector' plate is sufficiently negative potential then the KE of the photoelectrons will not be sufficient to reach it; they will be repelled. That stopping potential tells you the threshold energy for photoemission.

It doesn't affect the actual production of the photoelectrons so forget about the atom / photon interaction - electrons will always be produced. An isolated photocathode will build up its own positive charge till electrons form a cloud outside it ('space charge'), each one falling back onto the surface as new electrons are kicked off. Net charge is zero until you provide an external path with another electrode with a positive potential or a negative potential, of insufficient magnitude to repel photoelectrons.

The threshold photon energy is where photo emission 'just' starts. Many photons of that energy will just be absorbed as they meet more tightly bound electrons. That used to confuse me when I first came across this topic so it may not be obvious to people.

Jeff97