De Broglie Wavelength Calculation for an Electron with 120 eV Energy

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SUMMARY

The de Broglie wavelength of an electron with 120 eV energy is calculated using the formula λ = h/p, where p = sqrt(2MeE). The calculated wavelength is 1.1E-10 m, which aligns with the expected result. The discussion clarifies that the equation E = hf, which leads to λ = hc/E, is valid only when considering total energy, not kinetic energy. For non-relativistic speeds, the momentum and energy relationship differs, making the use of the kinetic energy formula essential for accurate de Broglie wavelength calculations.

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  • Understanding of de Broglie wavelength concepts
  • Familiarity with kinetic energy calculations
  • Knowledge of Planck's constant (h) and its application
  • Basic principles of relativistic physics
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  • Learn about relativistic effects on particle momentum
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Homework Statement


Calculate the de Broglie wavelengths of an electron with energy 120 eV ...

Homework Equations


lambda = h\p where p = sqrt(2*Me*E)

The Attempt at a Solution


E=1.6E-19*120ev..
Then sub into equation and I get 1.1E-10m for the wavelength, which is the answer quoted.

The question that concerns me is why can you not use E=hf, where rearranged gives lambda=(hc/E) which gives
a different answer...
Thanks
 
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E=hf is valid if E is the total energy, (E=mc2). The formula p = sqrt(2*Me*E) is valid for speeds much less than c, and E means the kinetic energy.
 
ehild said:
E=hf is valid if E is the total energy, (E=mc2).

Note that you can only use this for the de Broglie wavelength the way done in the OP if the particle is relativistic and thus has momentum essentially equal to its energy. In other words, when the velocity is close to c - otherwise the relation between wavelength and frequency changes.
 

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