Kinetic Energy vs De Broglie's wavelength equation

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Edward Tang 2E
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Kinetic Energy vs De Broglie's wavelength equation

Postby Edward Tang 2E » Tue Oct 27, 2020 6:13 pm

The velocity of an electron that is emitted from a metallic surface by a photon is 3.6×10^3 km/s
(a) What is the wavelength of the ejected electron?
(b) No electrons are emitted from the surface of the metal until the frequency of the radiation reaches
2.50×10^16Hz How much energy is required to remove the electron from the metal surface?
(c) What is the wavelength of the radiation that caused photoejection of the electron?
(d) What kind of electromagnetic radiation was used?

I've had this confusion for a while but this question really brought it up. In part a, we have to use the de broglie's wavelength relation (lambda=h/mv). In part C, we have to use work function and kinetic energy to calculate the total energy for the incoming photon. My question is 1) why can't we use kinetic energy for part a) and equate 1/2mv^2 = E = hv to figure out the frequency of the electron and therefore the wavelength and 2) why can't we use the wavelength's we've figured out in part a) to calculate the energy of the in-flight electron and add that to the work function to get the total energy of the incoming photon(h'*c/lambda + work function). Put simply: why isn't 1/2mv^2 equal to h*v for an ejected electron? Professor probably discussed this somewhere I just missed it.

I'd guess it has something to do with the uncertainty of momentum of electron but in that case why can we use kinetic energy at all to calculate the total energy of the incoming photon?

Jordan Tatang 3L
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Re: Kinetic Energy vs De Broglie's wavelength equation

Postby Jordan Tatang 3L » Tue Oct 27, 2020 6:19 pm

Hi! I think in short the answer to your question is that you can't use E=hv for any object with mass. That equation is for a photon which doesn't have any mass while the de broglie equation is for objects with mass and momentum.

Edward Tang 2E
Posts: 45
Joined: Wed Sep 30, 2020 10:00 pm
Been upvoted: 1 time

Re: Kinetic Energy vs De Broglie's wavelength equation

Postby Edward Tang 2E » Tue Oct 27, 2020 6:22 pm

Jordan Tatang 3L wrote:Hi! I think in short the answer to your question is that you can't use E=hv for any object with mass. That equation is for a photon which doesn't have any mass while the de broglie equation is for objects with mass and momentum.


Ok that's what im missing. That answers everything thank you so much.


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