- #1
FranzDiCoccio
- 342
- 41
Hi,
I was wondering about saturation current in the photoelectric effect. It is clear to me that for a sufficiently large accelerating potential all of the electrons are gathered by the collecting electrode. Since it is all of them, there cannot be more, and the current won't change if the accelerating potential is increased further.
What I'm not entirely clear about is what happens when the current is not saturated. My intuition is that some of the electrons simply won't make to the collecting electrode, but I find answers to related questions that invoke the speed of the electron (kinetic energy) and the fact that the current is the quantity of charge per unit time.
The latter explanation sort of confuses me. The perhaps naive way I'm picturing this phenomenon makes me think that electron speed does not really matter, unless it is in the wrong direction (so velocity rather than speed matters).
I guess that not all electrons are necessarily emitted towards the collecting electrode, so some of them could simply "miss the target". If an accelerating potential is switched on, the electrons trajectories are bent towards the electrode. If the potential is too weak some electrons might still escape, though.
At some point the acceleration towards the electrode is so strong that basically all of the electrons are pushed to the collecting electrode, no matter how unfortunate the direction of their initial velocity.
Does this make sense? Or else one should really take into account the different initial velocities of the electrons, and the current being "quantity of charge per unit time"? In that case the saturation would come about because the final velocities of the accelerated electrons would be much larger than any of their initial values, and so the latter would not matter. But for some reason this does not feel right.I expect that the "slope" of the ramp towards saturation is affected by geometric factors, such as the area and shape of the collector and its distance from the target. For instance, the farther apart the electrodes are, the more "stretched" the saturation is, because only electrons emitted within a small solid angle would reach the collector.
So which one is it? The current is less than its saturation value because
1) some electrons won't reach the collector
2) all electron reach the collector, but with different velocities?
ZapperZ's replies in this post and in this other post seem to point to 1), if I get them right.
If it was possible to "wrap" the collector around the target I guess that saturation would be achieved for an arbitrarily small positive potential.
Also, do collisions with residual atoms in the bulb matter? Simplified explanation claim that there is a vacuum in the bulb, but I guess that this is true only up to some degree, in reality.
Thanks a lot for any insight
Franz
I was wondering about saturation current in the photoelectric effect. It is clear to me that for a sufficiently large accelerating potential all of the electrons are gathered by the collecting electrode. Since it is all of them, there cannot be more, and the current won't change if the accelerating potential is increased further.
What I'm not entirely clear about is what happens when the current is not saturated. My intuition is that some of the electrons simply won't make to the collecting electrode, but I find answers to related questions that invoke the speed of the electron (kinetic energy) and the fact that the current is the quantity of charge per unit time.
The latter explanation sort of confuses me. The perhaps naive way I'm picturing this phenomenon makes me think that electron speed does not really matter, unless it is in the wrong direction (so velocity rather than speed matters).
I guess that not all electrons are necessarily emitted towards the collecting electrode, so some of them could simply "miss the target". If an accelerating potential is switched on, the electrons trajectories are bent towards the electrode. If the potential is too weak some electrons might still escape, though.
At some point the acceleration towards the electrode is so strong that basically all of the electrons are pushed to the collecting electrode, no matter how unfortunate the direction of their initial velocity.
Does this make sense? Or else one should really take into account the different initial velocities of the electrons, and the current being "quantity of charge per unit time"? In that case the saturation would come about because the final velocities of the accelerated electrons would be much larger than any of their initial values, and so the latter would not matter. But for some reason this does not feel right.I expect that the "slope" of the ramp towards saturation is affected by geometric factors, such as the area and shape of the collector and its distance from the target. For instance, the farther apart the electrodes are, the more "stretched" the saturation is, because only electrons emitted within a small solid angle would reach the collector.
So which one is it? The current is less than its saturation value because
1) some electrons won't reach the collector
2) all electron reach the collector, but with different velocities?
ZapperZ's replies in this post and in this other post seem to point to 1), if I get them right.
If it was possible to "wrap" the collector around the target I guess that saturation would be achieved for an arbitrarily small positive potential.
Also, do collisions with residual atoms in the bulb matter? Simplified explanation claim that there is a vacuum in the bulb, but I guess that this is true only up to some degree, in reality.
Thanks a lot for any insight
Franz