Dimension analysis of de Broglie equations

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You have the units of momentum wrong. It is $[\frac{\textrm{kg} \textrm{m}}{\textrm{s}}]$. From there it is just simple cancellation.

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David Z
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David Z

Updated on September 14, 2020

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  • David Z
    David Z about 3 years

    One form of one of the de Broglie's equations is this:

    $\lambda = \frac{2\pi\hbar}{p}$

    Units:

    $\lambda = [m]$

    $\hbar = [Js]$

    $p = [\frac{kg m}{s}]$

    $J=[Nm]$

    How can one show with dimension analysis that the right side is equal to the left side, having unit meter?

    EDIT: Sorry for having used the wrong units. But I actually had them right at some point when trying this out. But I didn't "see" the last step. If someone finds this post later and have that problem this might help them,

    $[m]=\frac{[Js]}{\frac{kgm}{s}}=m\frac{Ns^2}{kgm}=m\frac{N}{kg\frac{m}{s^2}}=[m]$

    This follows because N is the unit for force, and $kg\frac{m}{s^2}$ is force by Newton's laws.

    • Emilio Pisanty
      Emilio Pisanty about 11 years
      Use the identity $\textrm{N}=\textrm{kg m s}^{-2}$.
    • David Z
      David Z about 11 years
      Althalos, it would probably be more useful if the material you edited into the question were instead posted as an answer, since you basically answered your own question. It's perfectly fine (and encouraged, when appropriate) to answer your own questions and even accept those answers.
  • bobuhito
    bobuhito about 11 years
    by the way, your p has incorrect units (you should only have first power for m and s)