# Homework -- wavelength associated with a neutron moving with energy of 2ev

1. Oct 26, 2015

### as123

1. The problem statement, all variables and given/known data
Calculate the de - Broglie wavelength associated with a neutron moving with energy of 2ev .( mass of proton mp= 1.67*10^-27 kg )

2. Relevant equations
Wavelength= nh/ 2π
KE = eVs = 1/ 2mv^2
eVs=hf
Wavelength= v*f

3. The attempt at a solution
To calculate the wavelength at first I required velocity that I got from second equation.
I substituted mass of proton since its equal to that of neutron.
Then I got the frequency from third equation and put it in equation fourth.
But I didn't get the required answer.
De- Broglie wavelength is associated with first equation . so I am confused with it
I will really appreciate your help.

Last edited by a moderator: Oct 26, 2015
2. Oct 26, 2015

### Mister T

Weird that they give you the mass of the proton. Likely it's a typo. You can use that value as the mass of the neutron as it's essentially the same.

Look up the definition of the deBroglie wavelength.

$\frac{nh}{2\pi}$ is the angular momentum of an electron in the Bohr atom, it's not a wavelength.

$hf$ is the energy of a photon, a massless particle that moves at speed c. It's not the energy of a massive particle such as a neutron that travels at a speed less than c.

$\frac{v}{f}$ is the wavelength of a wave of frequency $f$ moving at speed $v$.

Last edited: Oct 26, 2015
3. Oct 27, 2015

### as123

[Moderator's note: Fixed a broken quote tag]
Oo. Thank you. I got it
de-broglie wavelength is h/ mv and v=
√2E/m
Putting the values I got the answer as
2.02*10^-11m . but the answer given is 6.4*10^-13 . I think I messed up somewhere. I want some help.thank you.

Last edited by a moderator: Oct 27, 2015
4. Oct 27, 2015

### Staff: Mentor

Can you check your question source to confirm the given energy of the neutron? Was the value given in eV or keV?

5. Oct 27, 2015

### as123

The energy is given in eV
I checked two sources. But I got the correct answer with 2kev .
Thank you very much for the hint.