### Kinetic energy equal to zero?

Posted:

**Fri Oct 13, 2017 9:56 am**How do you know when to set the kinetic energy equal to zero? What's the key information they give you? thanks!

Created by Dr. Laurence Lavelle

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https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=16&t=22165

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Posted: **Fri Oct 13, 2017 9:56 am**

How do you know when to set the kinetic energy equal to zero? What's the key information they give you? thanks!

Posted: **Fri Oct 13, 2017 10:52 am**

Hi! From what I know, if the work function (threshold energy to remove electrons) is equal to the energy of the photon then no electrons are ejected which means there’s no excess energy and kinetic energy equals 0. I believe that the problems will usually tell you that no electrons were ejected or there wasn’t any excess energy in which you’d be able to tell KE=0. Hope that helped!

Posted: **Fri Oct 13, 2017 11:04 am**

If you need to find the specific amount of energy that would be needed to eject an electron from a metal that's when you know to set it equal to zero. Or if you know the energy(or are given enough information to calculate it yourself) that it took to eject an electron but need to find the work function (threshold energy).

Ek= hv - work function. So lets say they wanted you to find the frequency (v) of the wave that is needed to eject an electron and have a kinetic energy of zero. You would set hv=work function and solve for v. If you didn't know the threshold energy they would have had to give you the wavelength probably. If you needed to know the threshold energy you could use v=c/λ and solve for E=hv and then set Ek = 0. Then you would have hv=work function (or h(c/λ)=work function).

Side note: If you knew the kinetic energy of the electron was above zero that means there was an excess amount of energy absorbed by the electron.

We also have the equation Ek=(1/2)mv^2 Basically you would use this equation whenever you needed to solve for the mass or velocity of the electron. If the kinetic energy was known to be zero though then we already know the velocity should be zero so setting this equation equal to zero I dont think would have much application that I can think of off the top of my head.

Hope what I said makes sense, someone correct me if I made a mistake because it is very possible that I missworded something.

Is there a specific question that you were wondering about from the book?

Ek= hv - work function. So lets say they wanted you to find the frequency (v) of the wave that is needed to eject an electron and have a kinetic energy of zero. You would set hv=work function and solve for v. If you didn't know the threshold energy they would have had to give you the wavelength probably. If you needed to know the threshold energy you could use v=c/λ and solve for E=hv and then set Ek = 0. Then you would have hv=work function (or h(c/λ)=work function).

Side note: If you knew the kinetic energy of the electron was above zero that means there was an excess amount of energy absorbed by the electron.

We also have the equation Ek=(1/2)mv^2 Basically you would use this equation whenever you needed to solve for the mass or velocity of the electron. If the kinetic energy was known to be zero though then we already know the velocity should be zero so setting this equation equal to zero I dont think would have much application that I can think of off the top of my head.

Hope what I said makes sense, someone correct me if I made a mistake because it is very possible that I missworded something.

Is there a specific question that you were wondering about from the book?

Posted: **Sun Oct 15, 2017 2:00 pm**

I was confused about this concept at first too. It can be confusing to think that an ejected electron(e-) can have a kinetic energy of zero. Like the other responses have been saying, when the work function/threshold energy is equal to the energy of the photon, the kinetic energy of the ejected e- will be zero. Even though the kinetic energy is actually zero, there is a positive charge on the electron detector that detects ejected electrons, causing the e- to still move off the metal surface.

When the photon doesn't exceed the work function/threshold energy, there will also be no kinetic energy because the electron is not ejected from the metal surface. The formula for these calculations is: E(photon) - threshold E = Ek(kinetic energy)

When the photon doesn't exceed the work function/threshold energy, there will also be no kinetic energy because the electron is not ejected from the metal surface. The formula for these calculations is: E(photon) - threshold E = Ek(kinetic energy)

Posted: **Mon Oct 16, 2017 1:33 am**

If the energy of the photon is equal to the threshold energy, then there will be no excess energy. All kinetic energy from the electron is attributed to excess energy of the incoming photon. If there's no kinetic (excess) energy, then no electron will be ejected. Therefore, when you're looking at word problems, be mindful of whether or not the question mentions if an electron was ejected or not. The same goes with whether or not they mention if the photon had just enough or more than energy energy than the work function. No electrons being ejected means that the kinetic energy is zero.