Photoelectric Effect: Which Answer is Correct?

[science.girl]

Homework Statement

A photon of wavelength 250 nm ejects an electron from a metal. The ejected electron has a de Broglie wavelength of 0.85 nm.

(a) Calculate the kinetic energy of the electron.

Homework Equations

KE = .5mv^2

The Attempt at a Solution

Which answer is correct?

ANSWER1: E = .5mv^2 and v = h/m$$\lambda$$
Therefore, E = .5m(h/$$\lambda$$)^2

Substituting values (including $$\lambda$$ = .85*10^(-9)), you get 3.34*10^(-19) J

ANSWER2: wavelength=h/p=h/rad(2mK)
250e-9=6.63e-34/(2*9.11e-31K)
K=6.87e-4 Joules

 

[diazona]

The first one is correct, I think. But the second method will also give you the correct answer if you do it properly - you just made a couple of little mistakes (one algebraic, one plugging in the wrong number).

 

[science.girl]

Ah! Thank you very much!