Finding KE when given de Broglie wavelength

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Homework Help Overview

The discussion revolves around calculating the kinetic energy of a proton given its de Broglie wavelength of 10 femtometers (fm). Participants are exploring the relationship between wavelength, mass, and velocity, as well as the conversion of units in the context of kinetic energy calculations.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of velocity from the de Broglie wavelength and the subsequent determination of kinetic energy. There are questions about unit conversions, particularly whether the wavelength should be expressed in nanometers instead of meters.

Discussion Status

The discussion is active, with participants acknowledging similar mistakes regarding unit conversions. There is a focus on ensuring that all inputs are in SI units for consistency in calculations.

Contextual Notes

Participants are addressing the need for clarity in unit conversions, specifically the transition from femtometers to meters, and the implications for calculating velocity and kinetic energy. There is no consensus yet on the correct approach, but a collaborative effort to clarify assumptions is evident.

vspectra
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[SOLVED] Finding KE when given de Broglie wavelength

Homework Statement


What is the kinetic energy, in MeV, of a proton with a de Broglie wavelength of 10fm?
1 MeV = 10^6 eV
1 fm = 10^-15m
1 eV = 1.602 x 10^-19 J
h = 6.63 x 10^-34 Js
wavelength = 10 x 10^-15 m = 10^-5 nm
Mass proton = 1.673 x 10^-27 kg


Homework Equations



wavelength = h/mv
KE = (1/2)(m)(v^2)


The Attempt at a Solution



I feel like I did this right, but I still got the incorrect answer and I can't seem to notice what I did wrong.

v = (6.63 x 10^-34) / (10^-5)(1.673 x 10^-27) = .03962 m/s
K = .5(1.673 x 10^-27)(.03862^2) = 1.316 x 10^-30 J = 8.2 x 10^-12 eV
= 8.2 x 10^-18 MeV
 
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Welcome to PF,
vspectra said:
v = (6.63 x 10^-34) / (10^-5)(1.673 x 10^-27) = .03962 m/s
K = .5(1.673 x 10^-27)(.03862^2) = 1.316 x 10^-30 J = 8.2 x 10^-12 eV
= 8.2 x 10^-18 MeV
Shouldn't that be 10*10-15? :wink:
 
Hootenanny said:
Welcome to PF,

Thanks.

Hootenanny said:
Shouldn't that be 10*10-15? :wink:



I tried that the first time I did the problem, and realized 10 x 10^-15 is in meters. Wouldn't I need the wavelength to be in nm? So it would be 1 x 10^-5 nm.
 
Last edited:
vspectra said:
I tried that the first time I did the problem, and realized 10 x 10^-15 is in meters. Wouldn't I need the wavelength to be in nm? So it would be 1 x 10^-5 nm.
Why would you want it in nm? If you want your output velocity to be in m/s, then all your inputs must be in SI units.
 
Ah, you're right, silly me. I was looking at another example which was basically the same question, and to me it looked like they converted from angstroms to nanometers for wavelength. It was actually converted to meters.

Thanks!
 


Hey thanks both of u :) i made the same mistake :p
 

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