1.33 Part A

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Rachel Wang
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1.33 Part A

Postby Rachel Wang » Mon Oct 16, 2017 10:42 pm

Here's the question:
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?

I tried doing this question two ways: once using the de Broglie equation, which gave me the right answer (2.0*10^-10m), and once using Ek = 1/2mv^2, which gave me the wrong answer (3.366*10^-8m).
I thought both of these methods would theoretically give me the same answer.

Is there a way to tell which equation is the right one to use?

Chem_Mod
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Re: 1.33 Part A

Postby Chem_Mod » Mon Oct 16, 2017 10:55 pm

The problem is asking for wavelength. The former equation you used (the correct one) results in wavelength using the given velocity. The Ek equation results in energy, not wavelength. Even though it uses velocity, it does not tell you anything about the wavelength of the electron.

John Huang 1G
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Re: 1.33 Part A

Postby John Huang 1G » Mon Oct 16, 2017 11:00 pm

I would use the Ek formula to solely determine the kinetic energy of the emitted electron. The de Broglie Equation allows you to determine the wavelength.

Abigail Urbina 1K
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Re: 1.33 Part A

Postby Abigail Urbina 1K » Tue Oct 17, 2017 2:36 pm

Use the equation that directly includes wavelength, since that is the quantity you are looking for. The Ek formula should be utilized when you are looking for mass, velocity, or excess energy electron

Madeline Musselman 3H
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Re: 1.33 Part A

Postby Madeline Musselman 3H » Wed Oct 18, 2017 10:46 pm

I ran into this situation as well. I found it helpful to write all of the given information with their variables and then apply to the equation that yields your desired answer. Since you are trying to determine wavelength the Ke formula wouldn't yield a wavelength value, so therefore you would use de Broglie's equation. Writing EVERYTHING out is very helpful.


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