I dont understand where i went wrong

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The discussion revolves around calculating the electric field (E) required for a 750 eV electron to remain undeflected in a velocity selector with a magnetic field (B) of 0.015 T. The original poster attempts to derive E using various equations, including the relationship between electric and magnetic forces, but arrives at an incorrect value. Other participants clarify that the kinetic energy (K) should be used to find the velocity (v) first, which then leads to the correct calculation of E. The correct formula for E is derived from E = B * sqrt(2K/m), which results in the expected answer of 244 kV/m. The conversation highlights the importance of correctly applying physical principles in calculations.
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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
thanks!
 
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agirlsrepublic said:
the magnetic force=electric force so qvB=qE or vB=E
Right.
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
No. K = 1/2 m v^2, so v = \sqrt{2K/m}. Thus E = B \sqrt{2K/m}.
 
thanks..i appreciate it
 
Doc Al said:
Right.

No. K = 1/2 m v^2, so v = \sqrt{2K/m}. Thus E = B \sqrt{2K/m}.

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:
 
listen buddy.. if i know that E=vB then i know where he got the equation from

:rolleyes:
 
agirlsrepublic said:
listen buddy.. if i know that E=vB then i know where he got the equation from

:rolleyes:

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

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