1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Another energy and momentum SR problem

  1. Aug 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A 100kev photon scatters from a free electron initially at res. Find the recoil velocity of the electron if the photon scattering angle is 180 degrees.


    2. Relevant equations
    Conservation of Energy, Conservation of Momentum
    (It is not allowed to use Compton's effect equation!)


    3. The attempt at a solution
    I've written:
    [tex]
    \eqalign{
    & {{hc} \over \lambda } + m_e c^2 = {{hc} \over {\lambda '}} + {{m_e c^2 } \over {\sqrt {1 - {{\bar v^2 } \over {c^2 }}} }} \cr
    [/tex]
    [tex]
    & {h \over \lambda } = {{m_e \bar v} \over {\sqrt {1 - {{\bar v^2 } \over {c^2 }}} }} - {h \over {\lambda '}} \cr} [/tex]

    but as I develop the equations on trying to solve the equations, somewhat it doesn't work out...
     
  2. jcsd
  3. Aug 3, 2008 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi zimo! :smile:

    Well, you want v, but you don't want [tex]\lambda'[/tex] …

    so just multiply the second equation by c, and add it to the first equation. :smile:
     
  4. Aug 3, 2008 #3
    OK.

    Did I write the equations correctly?
     
  5. Aug 3, 2008 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Looks ok to me.

    Why are you asking? :confused:
     
  6. Aug 3, 2008 #5
    because I get v=0.483c, in contrast to the final answer in the book, which is 0.319c.
     
  7. Aug 3, 2008 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Unless you show your full calculations, we can't see where you've gone wrong. :smile:
     
  8. Aug 3, 2008 #7
    I attached my solution. (starting with the lines indicating I) and II) )
     

    Attached Files:

    • 22.jpg
      22.jpg
      File size:
      14.5 KB
      Views:
      46
  9. Aug 4, 2008 #8
    can someone read my uploaded solution, please, and reveal my mistake?
     
  10. Aug 4, 2008 #9

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Your attachment needs to be approved by a moderator before we can view it. Be patient, attachments are usually approved fairly quickly. If it hasn't been approved by this evening I'll see if I can get someone's attention.
     
  11. Aug 5, 2008 #10

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi zimo! :smile:
    That's with (.711/.511)2 = 1.936.

    The book seems to have used .711/.511 = 1.391, giving the answer of .319

    The book seems to be wrong. :frown:
     
  12. Aug 5, 2008 #11
    No.

    I made an error during the strike-out of (u+1) on both sides, I needed to leave one (u+1) at the right side of the eqation.

    Finally, I got 0.319 as requested.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?