# voltage question (photoelectric effect)

• February 20th, 2008, 03:09 PM
scientist91
voltage question (photoelectric effect)
I learn about photoelectric effect. Can somebody tell me why:
eU_z=E_k,max
What is eU_z?
• February 24th, 2008, 06:28 AM
I dont know what's "eU_z=E_k,max"
if you can express it in words, maybe you'll get the answer.
• February 24th, 2008, 11:28 AM
scientist91
Quote:

I dont know what's "eU_z=E_k,max"
if you can express it in words, maybe you'll get the answer.

http://galileo.phys.virginia.edu/cla...2/photoelr.gif
Uz is the voltage which drives the electrons back to the cathode. eUz is the negative work of the cathode. Ek,max is the maximum kinetic energy.
• March 13th, 2008, 03:44 AM
scientist91
• March 13th, 2008, 11:00 AM
Harold14370
The maximum energy an electron can pick up is determined by the energy of the photon that kicks it off the surface of the metal. So if you know how much electrical potential energy it takes to keep all the electrons on the surface (i.e., when the current is zero), then that is an indication of the photon energy.
• March 13th, 2008, 12:47 PM
scientist91
Quote:

Originally Posted by Harold14370
The maximum energy an electron can pick up is determined by the energy of the photon that kicks it off the surface of the metal. So if you know how much electrical potential energy it takes to keep all the electrons on the surface (i.e., when the current is zero), then that is an indication of the photon energy.

Why it says maximum kinetic energy of the electrons?
• March 13th, 2008, 01:40 PM
Harold14370
I suppose an electron could come off at an angle so its velocity toward the collector plate would be less than the maximum which it would have if it came off directly toward it. Or maybe it bounced off another electron and lost some of the energy it got from the photon.
• March 13th, 2008, 02:13 PM
scientist91
Quote:

Originally Posted by Harold14370
I suppose an electron could come off at an angle so its velocity toward the collector plate would be less than the maximum which it would have if it came off directly toward it. Or maybe it bounced off another electron and lost some of the energy it got from the photon.

What they mean by maximum kinetic energy? The maximum kinetic energy that the electron has while moving (with maximum speed)?
• March 13th, 2008, 02:44 PM
Harold14370
Okay, let's say you have a basket of grasshoppers. The sides of the basket are h meters high and the grasshoppers each have a mass of 1 gram. Then you can say it takes 1/1000 kg*9.8 m/sec/sec* h m = 9.8 h millijoules of energy for a grasshopper to jump out of the basket. You then keep increasing the height of the basket until most of the grasshoppers are hitting the side of the basket and falling back in. Only the ones that jump straight up can make it out. When you have increased the height of the basket walls until no grasshoppers can jump out, then you have found the maximum kinetic energy of a grasshopper.
• March 13th, 2008, 03:44 PM
scientist91
Quote:

Originally Posted by Harold14370
Okay, let's say you have a basket of grasshoppers. The sides of the basket are h meters high and the grasshoppers each have a mass of 1 gram. Then you can say it takes 1/1000 kg*9.8 m/sec/sec* h m = 9.8 h millijoules of energy for a grasshopper to jump out of the basket. You then keep increasing the height of the basket until most of the grasshoppers are hitting the side of the basket and falling back in. Only the ones that jump straight up can make it out. When you have increased the height of the basket walls until no grasshoppers can jump out, then you have found the maximum kinetic energy of a grasshopper.

So the electrons couldn't have more kinetic energy with some voltage U_z implied, until the current stops, right?
What will be Ek_min? (minimum kinetic energy)
• March 13th, 2008, 04:25 PM
Harold14370
I think you are getting it, although I wouldn't have said it that way. I understand English is not your native language. The experiment is not designed to find the minimum kinetic energy, so we can't really say. I don't know why you would want to know that.
• March 13th, 2008, 04:58 PM
scientist91
Quote:

Originally Posted by Harold14370
I think you are getting it, although I wouldn't have said it that way. I understand English is not your native language. The experiment is not designed to find the minimum kinetic energy, so we can't really say. I don't know why you would want to know that.

I want to compare Ek_max with Ek_min.
When we are at Ek_max.
Does it mean that electron have some maximum kinetic energy at some voltage?
• March 19th, 2008, 09:42 AM
scientist91
Quote:

Originally Posted by Harold14370
I think you are getting it, although I wouldn't have said it that way. I understand English is not your native language. The experiment is not designed to find the minimum kinetic energy, so we can't really say. I don't know why you would want to know that.

Am I thinking correctly, how you will think?
• March 19th, 2008, 09:58 AM
Harold14370
I'm afraid I don't follow you. I've explained it as best I can. The maximum voltage at which the current will still flow is of interest because it tells you how much energy the electron has received from the photon. It is related to the wavelength of the light. The minimum really has no significance to the photoelectric effect. The minimum is not really defined in this experiment. The more you turn thte voltage down, the more current will flow.
• March 23rd, 2008, 02:37 AM
scientist91
Quote:

Originally Posted by Harold14370
I'm afraid I don't follow you. I've explained it as best I can. The maximum voltage at which the current will still flow is of interest because it tells you how much energy the electron has received from the photon. It is related to the wavelength of the light. The minimum really has no significance to the photoelectric effect. The minimum is not really defined in this experiment. The more you turn thte voltage down, the more current will flow.

Its ok, you explained very good... But I asked you is my way of thinking correct?
• March 23rd, 2008, 04:41 AM
Harold14370
Quote:

Originally Posted by scientist91
Its ok, you explained very good... But I asked you is my way of thinking correct?

No, I don't think it is because you want to compare Ek_max with Ek_min and there is no definition of Ek_min.
• March 23rd, 2008, 03:58 PM
scientist91
Quote:

Originally Posted by Harold14370
Quote:

Originally Posted by scientist91
Its ok, you explained very good... But I asked you is my way of thinking correct?

No, I don't think it is because you want to compare Ek_max with Ek_min and there is no definition of Ek_min.

No, I said for some eU_z , the electrons have the most possible kinetic energy, it can't be greater than that kinetic energy...