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Effects of an electric field on a motion of an electron

by wendo
Tags: effects, electric, electron, field, motion
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wendo
#1
Jul7-07, 10:26 AM
P: 15
1. The problem statement, all variables and given/known data

An electron is projected at an angle of 31.7 above the horizontal at a speed of 8.4010^5 m/s in a region where the electric field is E = 383j N/C. Neglecting the effects of gravity, calculate the time it takes the electron to return to its initial height.

2. Relevant equations


I first found the electric force that the E field exerts on the electron by using F=qE.

Then I found the acceleration of the electron with a=(q/m)E

Thought perhaps electron would undergo motion similar to a projectile, so then first found the y component of the velocity sin 31.7=opp/8.40x10^5s.

Then applied vf=vi + at solving for t (with vf=0 because it will be when the electron has all potential energy at its highest point)


3. The attempt at a solution

F=383x(-1.6x10^-16C)
=-6.13x10^-14N

a=-6.13x10^-14N/9.11x10^-31kg (mass of electron)
=-6.73x10^16m/s^2

sin 31.7=opp/8.40x10^5m/s
opp= 4.41x10^5m/s

vf=vi +at
0=4.41 + -6.73x10^16m/s^2t
t=6.55x10^-12s (this would be the time to reach max height, so the amount of time to get back down to its initial height would be double this)

so t= 1.31x10^-11s
But this answer is not right... and I'm not show how else to relate t to an electric field. Can anyone point out what i'm doing wrong here??

Thanks SOOO much!! :)
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Dick
#2
Jul7-07, 10:43 AM
Sci Advisor
HW Helper
Thanks
P: 25,228
Uh, the exponent of the electron charge is -19, not -16. But there is nothing wrong with your method.
wendo
#3
Jul7-07, 10:57 AM
P: 15
omg... wow... I can't that was all it was!!

THANK YOUUU DICK!!! :D


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