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Homework Help: Uncertainty of a particle

  1. Nov 14, 2008 #1
    [tex]\Delta E = \sqrt{(E - <E>)^{2}}[/tex]

    This is evident. However, I don't understand why

    [tex](E - <E>)^{2}[/tex] can be written [tex]E^{2} - <E>^{2}[/tex]
    Last edited: Nov 14, 2008
  2. jcsd
  3. Nov 14, 2008 #2


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    I think what you mean is < (E-<E>)^2 >=<E^2>-<E>^2. <(E-<E>)^2>=<E^2-2E<E>+<E>^2>=<E^2>-2<E><E>+<E>^2. Right?
  4. Nov 14, 2008 #3
    I'll say it once more: you have to read your textbook: you might think this is evident, but actually it's wrong. What your textbook will say is that

    [tex]\Delta E = \sqrt{<(E - <E>)^{2}>}[/tex]
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