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Homework Help: Change of measurement units

  1. Oct 27, 2005 #1
    how do you change the measurement unit for an electron mass from kg to MeV/c^2?
     
  2. jcsd
  3. Oct 27, 2005 #2

    andrevdh

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    use Einstein's famous equation
    [tex]E=mc^2[/tex]
    and use the conversion for joules per electron volt.
     
  4. Oct 27, 2005 #3
    thanks!!! :)
     
  5. Oct 28, 2005 #4

    andrevdh

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    It is a pleasure.
     
  6. Oct 30, 2005 #5
    @@a i'm still having trouble doing the calculations~
    could you write out the whole equation to transfer the mass of an electron from kg to Mev?
     
  7. Oct 30, 2005 #6
    You know the mass of the electron and you know the speed of light. Just plug those into [tex]E=mc^2[/tex]. You will have an energy value in terms of joules. Since you also know that one electronvolt is equivalent to [tex]1.602*10^-^1^9[/tex] joules, you can find the energy value in terms of electronvolts instead. Rearrange the original expression and you can reexpress the mass in terms of [tex]MeV/c^2[/tex].
     
    Last edited: Oct 31, 2005
  8. Oct 31, 2005 #7

    andrevdh

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    It is amazing how much confusion such a deceptively simple formula can generate. First calculate the rest mass of an electron:
    [tex]\begin{split*} E_e = m_ec^2 \\
    \ = 9.11 \times 10^{-31} \times (3.0 \times 10^8)^2 \\
    \ = 8.2 \times 10^{-14} \ joule \\
    \ = 512 \ keV \\
    \ = 0.512 \ MeV \\
    \ m_ec^2=0.512 \ MeV\end{split*} [/tex]
    Now get the mass in the "strange" new units:
    [tex]m_e=0.512 \ MeV/{c^2}[/tex]
     
    Last edited: Nov 1, 2005
  9. Oct 31, 2005 #8
    thank you very much!!! :)
     
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