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Homework Help: Speed of a 796eV electron

  1. Apr 2, 2010 #1
    1. The problem statement, all variables and given/known data

    What is the speed of a 796eV electron?

    2. Relevant equations

    E = hc/lambda
    E = hv

    3. The attempt at a solution

    I'm not sure how to get a value for meters... I know if I take h (plank's constant) and divide the electron, I will get 4.14E-15 ev*s / 796 eV = 5.20E-18 seconds, but that is just a time, i don't know how to get the distance for that time...
     
  2. jcsd
  3. Apr 2, 2010 #2

    rock.freak667

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    Homework Helper

    That energy is entirely kinetic.

    Ek=1/2 mv2
     
  4. Apr 2, 2010 #3
    If 796 = 1/2 * 9.11E-31 * v^2, I get v to be 4.18E16 m/s which is wrong...i'm not sure what i'm doing wrong...
     
  5. Apr 2, 2010 #4

    Borek

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    Staff: Mentor

    Watch your units.
     
  6. Apr 2, 2010 #5
    I tried converting eV to joules, which gave me 1.27E-16 joules, which gave me 8.36E6 m/s but that is wrong too. I think the units for mass (kg) are correct, I'm not sure what I have wrong...
     
  7. Apr 2, 2010 #6

    Borek

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    Staff: Mentor

    Check your math.
     
  8. Apr 2, 2010 #7
    For:
    796 = 1/2 * mv^2 (where m = 9.11E-31) I keep getting 4.18E16 for v

    I don't understand what part I'm doing wrong, I've done it about ten times...
     
  9. Apr 2, 2010 #8

    Borek

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    Staff: Mentor

    Show how you solved for v.

    --
     
  10. Apr 2, 2010 #9
    eV = 1/2 mv^2
    eV / (1/2m) = v^2
    v = sqrt (eV / 1/2m)

    Am I doing this wrong?
     
  11. Apr 2, 2010 #10

    Borek

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    Staff: Mentor

    So far OK.

    Seems to me like somehow you are misplacing 2 when calculating value.
     
  12. Apr 2, 2010 #11
    I'm using
    m = 9.11E-31
    eV = 796

    796 = 1/2*9.11E-31 v^2
    796 = 4.55E-31 v^2
    1.748E33 = v^2
    v = 4.18E16

    Don't know where I'm wrong...
     
  13. Apr 2, 2010 #12

    Borek

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    Staff: Mentor

    Seems like my guess was wrong. You have ignored my earlier remark about using correct units. Sigh.
     
  14. Apr 2, 2010 #13
    I didn't ignore it, I don't understand where my units are wrong. eV, m = kg

    If you would tell me which part I'm doing wrong I could try and fix it.
     
  15. Apr 2, 2010 #14
    Nevermind, I got it myself. I was using the wrong value to convert eV. Thanks anyway
     
  16. Apr 3, 2010 #15

    Borek

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    Staff: Mentor

    You have to convert eV to J, from what you wrote you were assuming 796 [eV] = 1/2 9.11E-31 [kg] * x2 [m/s]2. eV is NOT kg*m2*s-2, it is about 1.602e-19 kg*m2*s-2 (or 1.602e-19 J).
     
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