Test 2 Question 3c

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kevinav
Posts: 32
Joined: Fri Sep 28, 2018 12:16 am

Test 2 Question 3c

Postby kevinav » Wed Oct 31, 2018 4:31 pm

Question 3, part c on test 2 asks the following:
In an X-ray photoelectron experiment, chromium metal is irradiated with light of wavelength 9.020x10^(-11)m.
c)Calculate the wavelength of the ejected electron.

From part a we know that the work function is 2.18x10^(-15) J and we know from the graph that the kinetic energy of the ejected electron is 2.00x10^(-17) J.
I used the equation E=hc/λ, and converted to λ=hc/E. I plugged in the constants and I used the given kinetic energy from the graph as E. Is this not the correct equation to use? What would be the correct answer? My answer was λ= 9.94x10^(-9) and I got 0 points for this problem.

pauline_le_3I
Posts: 21
Joined: Fri Sep 28, 2018 12:25 am

Re: Test 2 Question 3c

Postby pauline_le_3I » Wed Oct 31, 2018 5:57 pm

The ejected electron will have a velocity, which means you have to use the DeBroglie equation to find it, not the energy of light equation. Sometimes they can get confused with eachother because they both have wavelength in the equations, but because you know the electron has mass and a velocity, you have to use the DeBroglie equation. So, the problem gives you the kinetic energy of the electron, which is 2.00x10^(-17) J, so you plug that into KE=1/2mv^2 to find v(velocity). KE is the kinetic energy, m is the mass of an electron which is given on the first page of the test. Then, once you find the velocity, you plug that into the Debroglie equation wavelength=h/(mv), where h is Planck's constant, m is, again, the mass of an electron, and v is the velocity you just calculated. This should give you the right wavelength for the ejected electron.


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