## 1B 15 7th edition

Megan Wong 4E
Posts: 32
Joined: Fri Sep 28, 2018 12:28 am

### 1B 15 7th edition

15. Velocity of electron emitted from a metallic surface by a photon is 3.6 km/s. What is the wavelength of the ejected electron?

For this question, I tried to use the kinetic energy formula 1/2mv^2 but it doesn't work out. Why can't I use the KE formula and how do you know it's supposed to be De Broglie's equation that's used instead?

sameeksha_panda_3h
Posts: 31
Joined: Fri Sep 28, 2018 12:25 am

### Re: 1B 15 7th edition

You can't use KE for wavelength because you don't know the threshold energy to find the total energy.

Andrew Bennecke
Posts: 62
Joined: Fri Sep 28, 2018 12:15 am

### Re: 1B 15 7th edition

Since we are unable to use the Ek formula, which formula must we use in order to solve the problem?

Milena Aragon 2B
Posts: 60
Joined: Fri Sep 28, 2018 12:19 am

### Re: 1B 15 7th edition

You can use De Broglie's equation (wavelength=h/(m)(v)) since it relates wavelength to the momentum of the electron.

Andre_Galenchik_2L
Posts: 69
Joined: Fri Sep 28, 2018 12:23 am

### Re: 1B 15 7th edition

For part c I keep getting 12nm and not 8.8nm (the answer). What am I doing wrong as I am plugging in the exact formula for speed of light?

Lopez_Melissa-Dis4E
Posts: 66
Joined: Fri Sep 28, 2018 12:20 am

### Re: 1B 15 7th edition

For part c I also keep getting 12nm, it might be a typo in the book.

Elena Maneffa 1E
Posts: 33
Joined: Fri Sep 28, 2018 12:27 am
Been upvoted: 1 time

### Re: 1B 15 7th edition part C

For 1B 15 c :
The question is: What is the wavelength of the radiation that caused photoejection of the electron? So first we have to find the Energy of this wavelength. We know the threshold energy from part 1B 15 b=1.66x10^-17.

We also know that:

Energy (per photon) - threshold energy =Ek

We can rearrange this to be: Energy = threshold energy + Ek. (Additionally Ek=1/2mv^2)

So we now have:

E=threshold energy + 1/2mv^2

E=1.66x10^-17 + 1/2(9.1095x10^-31)(3.6x10^6)^2

E= 2.25x10^-17

So now we know the energy per photon. Now we have to find the wavelength that corresponds to this energy.

E=ch/wavelength

We can rearrange this to be: wavelength= ch/E.

wavelength = ch/2.25x10^-17

wavelength= 8.82x10^-9 m

:)