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Homework Help: A couple Quantum Questions

  1. Jan 24, 2005 #1
    Hey, this is my first week in Quantum class, going pretty well so far.

    My first questoin is:

    1.) What is an eV a Unit of...give it in terms of MKS

    Well my answer is that it is a unit of energy, and for MKS...

    1 Electron*Volt = Joules

    1 electron has a charge of 1.602*10^-19 C

    1 Volt = 1 J/C


    1.602*10^-19*1 means that

    1 eV = 1.602*10^-19 J

    Is this correct reasoning?

    My next Question

    Neutrons at room Temperature have a kinetic energy of around .02 eV. What is their wavelength?

    KE = 0.2 eV

    0.2 eV = q*V

    p = h/lambda

    1 eV = 1.609*10^-19 J

    h = 6.626*10^-34 J*s * 1 ev/(1.609*10^-19 J)

    Therefore I get Planck's constant in terms of eV

    h = 4.136*10^-15 eV*s

    But how do I use the 0.2 eV in terms of the p = h/lambda?

    Thanks for any assistance!
  2. jcsd
  3. Jan 24, 2005 #2
    For the second part i can think of no other way but to get the velocity of the neutrons from the kinetic energy using

    [tex] K = m_{n}c^2(\gamma -1) [/tex] and then use the De Broglie Hypothesis about wavelength

    [tex] \lambda = \frac{h}{mv} [/tex]

    and [tex] \gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}} [/tex]
  4. Jan 24, 2005 #3

    Dr Transport

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    [tex] E = \frac{ \hbar c} {\lambda} [/tex]
  5. Jan 24, 2005 #4
    Thanks! But I'm looking at my notes, and it says that the equation applies only to massless particles?
  6. Jan 25, 2005 #5

    Dr Transport

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    [tex] E^{2} = p^{2}c^{2} + m_{0} ^{2} c^{4} [/tex] is the expression for particles with mass.
  7. Jan 26, 2005 #6


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    2.U don't need the relativistic formula.
    [tex] \lambda=\frac{h}{p}=\frac{h}{\sqrt{2mE}} [/tex]

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