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posted by  Ayiti on 4/18/2007 12:11:30 PM  |  status: Live  

PHYS Chp 28 / P07

Course Textbook Chapter Problem
Calculus Based Physics Fundamentals of Physics Extended (7th) by Halliday, Resnick, Walker 28 7P
Question Details:
In Figure 28-33, an electron accelerated from rest through potential difference 1.7 kV enters the gap between two parallel plates having separation 20.0 mm and potential difference 170 V. The lower plate is at the lower potential. Neglect fringing and assume that the electron's velocity vector is perpendicular to the electric field vector between the plates.

F28_33
Figure 28-33

What uniform magnetic field allows the electron to travel in a straight line in the gap? T  
 
________________________________
 

I found that:

First, 
E = V2/d
   = 170 / 20E-3
   = 8500 V/m
 
Now, 
(1/2)mV2 = qV1
(1/2)(9.11E-31)(V2) = (1.6E-19)(1700)
(4.555E-31)(V2) = 2.72E-16
V2 = (2.72E-16)/ (4.555E-31)
V   = (24.44E6) m/s
 
Then,
B = E/V
   = 8500 / 24.44E6
   = (3.478E-4) T (+k)

 This is coming back as incorrect. Please work it out so that I can see where I'm going wrong.

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posted by shorty13 on 4/18/2007 1:01:34 PM  |  status: Live
Asker's Rating: Helpful   
Ayiti's comment:
"Thanks, I over looked that."
Response Details:
All your work is correct except that you left out a negative sign. The electron has a negative charge, so when you put this sign in you will get B = -0.348T z.
hope this helps
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