when to use photoelectric effect or debroglie?

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Alyssa-Marie Dones
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

when to use photoelectric effect or debroglie?

Postby Alyssa-Marie Dones » Sat Nov 07, 2015 2:33 am

On the recent midterm we had a question in which we were supposed to find the wavelength of an ejected electron given it's velocity. I personally used Kinetic Energy= 1/2mv^2 and once I found the kinetic energy I used wavelength= hc/E to find the wavelength. I assumed that using this way would be okay because this is what I did for some hw problems for the photolectric effect, but upon asking some of my friends and looking at the book (this was question #33 from chapter 1 by the way) they used the de broglie equation.

I understand that it was quicker way to answer the question, but can someone provide an explanation why it needed to be answered using debroglie?

Thank you!!

Bryce Bentley 1l
Posts: 25
Joined: Fri Sep 25, 2015 3:00 am

Re: when to use photoelectric effect or debroglie?

Postby Bryce Bentley 1l » Sat Nov 07, 2015 2:56 pm

I'm pretty sure the De brogolie equation had to be to show your understanding of the equation and its use. Also I'm not sure if your way worked but your method should only be using for EM radiation. The debroglie is used to tell how noticeable the wavelength characteristics of a moving object is, which included small particles like electrons or large objects like cars. I hope this answers your question.

JasmineAlberto4J
Posts: 73
Joined: Fri Sep 25, 2015 3:00 am

Re: when to use photoelectric effect or debroglie?

Postby JasmineAlberto4J » Sat Nov 07, 2015 4:11 pm

the de broglie equation is used for things with rest mass. An electron has rest mass so in finding its wave length you use de broglie. A photon doesn't not have rest mass so you use photoelectric equations, E=Φ+1/2 mv^2 and λ= hc/E

Vanessa A 3F
Posts: 29
Joined: Fri Sep 25, 2015 3:00 am

Re: when to use photoelectric effect or debroglie?

Postby Vanessa A 3F » Sat Nov 07, 2015 8:08 pm

Using the De Brogile equation to find the wavelength of an electron is the only correct way to do so.
There are some equations, like E=hc and c=lambda*velocity, that are only to be used when you're dealing with light. And there are some equations that are only to be used with massive objects, things, like electrons, that have a mass.

For example,
is an equation that is used to find the wavelength of light only.

An electron, a person, a baseball, a proton, a neutron, etc. are all objects with mass.
We cannot use the light equation for massive objects, we have to use the De Brogile equation, lambda= h/p = h/(mass x velocity), to find their wavelengths.
The best way to remember this is to double check if the wavelength you're looking for is of a light or an object with mass. This will ensure you pick the correct equation to use.


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