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Homework Help: What's wrong? Electron acceleration

  1. Feb 23, 2008 #1
    I am working on this problem but I got it wrong. I'm not sure what happened.

    1. The problem statement, all variables and given/known data

    An electron with initial velocity Vo = 2.3e5 m/s enters a region 1.0 cm long where it is electrically accelerated. It emerges with velocity v = 4.10e6 m/s. What was its acceleration, assumed constant?

    _________ m/s^2



    2. Relevant equations

    DeltaX = 1/2 (Vo + V)t

    V = Vo +at

    3. The attempt at a solution

    I thought these were my known variables:

    Vo = 2.3e5 m/s
    DeltaX = 1.0 cm
    Vf = 4.10e6 m/s

    I tried to solve for t using DeltX equation. I got 4e-9 s. Then I tried to solve for a using my new value for t and V equation. I got 9.675e14 m/s^2 which is wrong. So anyone know what happened?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 23, 2008 #2

    HallsofIvy

    User Avatar
    Science Advisor

    Vr+V0= 4100000+ 230000= 433000 or 4.33 e5 m/s. Half of that is 2.665 e5 and deltax= 1 cm= 0.01 m. t= 0.01/(2.665 e5= 3.75 e-8 s, not -9. Try your calculation again, not using scientific notation this time. That may be what is confusing you.

    Vf- V0= 4100000- 230000= 387000 or 3.87 e5 m/s. Dividing that by 3.75 e-8 gives an acceleration of 1 x 1014 m/s2. Your equations are right. Check your arithmetic.
     
  4. Feb 23, 2008 #3
    Thank you. When I solved for t, i got .000000004 and I thought it would be like totally okay to just use 4e-9. When I tried it again I got a more specific number by using SN button on my calculator. Using 4.6189e-9 for t, I got the right answer. 8.37855e14
     
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