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Homework Help: I dont understand where i went wrongplease help

  1. Dec 7, 2004 #1
    a velocity selector consists of magnetic and electric fields. if B= 0.015 find the value of E such that a 750 eV electron is moving along the + x- axis and is undeflected.

    this is what i did..

    the magnetic force=electric force so qvB=qE or vB=E
    K= .5 mv^2 and r= mv/qB so k =[.5r^2q^2B^2]/m and if you simplify you get:
    r= [(2km)^.5]/qB

    r= {[2*750*1.6*E-19*9.11E-31]^.5}/1.6E-19*.015= 1.38E-16

    if v=rqm/m then v = [1.38E-6*1.6E-19*.015]/9.11E-31 = 3.64E3

    since E= vB then E= 3.64E3*0.15= 5.45E1

    but this is not the answer
    the answer is 244 KV/m

    someone please help its driving me mad and i have a test soon
  2. jcsd
  3. Dec 7, 2004 #2

    Doc Al

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    Staff: Mentor

    No. [itex]K = 1/2 m v^2[/itex], so [itex]v = \sqrt{2K/m}[/itex]. Thus [itex]E = B \sqrt{2K/m}[/itex].
  4. Dec 7, 2004 #3
    thanks..i appreciate it
  5. Dec 7, 2004 #4


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    Science Advisor
    Homework Helper

    Well,Doc,for her sake,let's hope she got where your equation with E,B and v came from.If not,i'm willing to explain to her everything in detail.Yet,by the look of her last post,she aint't got a clue. :rolleyes:
  6. Dec 7, 2004 #5
    listen buddy.. if i know that E=vB then i know where he got the equation from

  7. Dec 7, 2004 #6


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    Science Advisor
    Homework Helper

    Yo,pal, :wink: u definitely didn't show it.Else u wouldn't have gotten yourself in messy & useless calculations.My guess... :tongue2:
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