help with a physics problem?

L

Lenny

New member
The equation for deBroglie wavelength is simple:

L = h / p

p is the momentum (p = mv)
h is planck's constant.

First, you have to find the momentum. Since you know the energy of the electron, you can find its momentum.
K = (1/2)mv^2
2K/m = v^2
v = sqrt(2K/m)
then just multiply this velocity by the electron's mass to get the momentum
p = mv = m*sqrt(2K/m)

There's one catch: The energy is given in eV, electron volts. We need it in joules. The conversion is:
1 eV = 1.60 x 10^-19 joules

Now you can just look up Planck's constant (NOT planck's reduced constant!) and the mass of an electron, and you should be okay.
 
I have a question to do with de broglie question.
A scanning electron microscope uses an electron beam for imaging a specimen, given that each electron has a kinetic energy of 21.2 KeV, calculate the de broglie wavelength of the electrons in picometres (pm) (10 to the power of minus 12)
 
Energy is much less than rest mass of electron m=511keV, the problem is only weakly relativistic.

Use formula
E = p²/2m
to find momentum p of electron.

Once you know momentum, you know wavelenght:
λ = h/p
 
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