De Broglie Formula Application

[Pyroadept]

Homework Statement

Estimate the wavelength of a 30 keV electron.

Homework Equations

p = h/λ
E = hf

The Attempt at a Solution

The electron has 30 keV of mass-energy.
As it is acting as a particle-wave, E = hf, so f = E/h = 30,000x1.6x10^-19/6.3X10^-34 = 7.619x10^18 Hz

Now, v = fλ, so λ = v/f, but this is where I run into a problem, as I'm not told what speed the electron is traveling at - do I assume speed of light, or is there some other way?

 

[tiny-tim]

Estimate the wavelength of a 30 keV electron.
…
Now, v = fλ, so λ = v/f, but this is where I run into a problem, as I'm not told what speed the electron is traveling at - do I assume speed of light, or is there some other way?

Hi Pyroadept!

If you can convert the eV into J, and if you know the mass of an electron, then you can calculate the speed from the energy.