Given the wavelength of the electron, λ = 3.0 x 10^{-7} m

Find the electron's momentum, p = ?

Use the de Broglie formula λ = h/p , where Planck's constant h = 6.63 x 10^{-34} J⋅s

Re-arrange to isolate p: p = h/λ

Substitute and solve: p = (6.63 x 10^{-34} J⋅s) / (3.0 x 10^{-7} m) = 2.21 x 10^{-27} J⋅s/m

Note: a more common combination of units for momentum within the SI system is kg⋅m/s (p=mv tells you that), so you could also correctly answer that p = 2.21 x 10^{-27} kg⋅m/s .

Interesting aspect to the problem is that you were not given the speed of the electron, and you did not even need to look up and use the mass of the electron. In fact, a proton, a baseball, or a covid virion with that same wavelength would have the same momentum calculated above.

Michael K.

09/26/21