## 2015 Midterm Question #3

Dan_Jin_1K
Posts: 15
Joined: Wed Sep 21, 2016 2:59 pm

### 2015 Midterm Question #3

Hello all,

Question 3A on midterm 2015 reads "The velocity of an electron emitted from a metallic surface by a photon is 3.6 x 10^3 km/s.

What is the wavelength of the ejected electron?"

How can one deduce from this question's wording that we should use De Broglie's equation instead of using perhaps the kinetic energy equation, and then converting that energy to wavelength?

Thanks!

Raul Hernandez
Posts: 28
Joined: Fri Jun 17, 2016 11:28 am

### Re: 2015 Midterm Question #3

This is one question that you shouldn't overthink. It gives you the velocity of the electron, correct? And then it asks you about the wavelength of the same electron, right? So, you should just see that there's no need to go back and find the energy of the photon or anything like that because the question is just asking you about the wavelength of the electron with the given velocity, and the equation that deals with velocity (well, momentum, but it can be calculated with the velocity since we know the mass of the electron) is the De Broglie equation. Does that make sense? I hope that helped somewhat.

Edgar Khachatryan 3G
Posts: 20
Joined: Fri Jul 15, 2016 3:00 am

### Re: 2015 Midterm Question #3

In order to use the kinetic equation you would need 2 of the 3 things: the energy of the photons being emitted onto the metallic surface, the threshold energy of the metallic surface, or the kinetic energy of the photons emitted(1/2mv^2).

According to the De Broglie equation, Wave Length= h/p and p=m(v).

Convert velocity to m/s from km/s and multiply to the mass of an electron (9.11x10^-31kg) and divide h(6.626x10^-34J/s).

Thanks.