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Help: Due Tonight! Electric Field Question

  • #1

Homework Statement


An electron is traveling through a region between two metal plates in which there is a constant electric field of magnitude E directed along the y direction as sketched in the figure below. This region has a total length of L, and the electron has an initial velocity of v0 along the x direction. (Use the following as necessary: e, E, L, m, and v0.)
17-p-044.gif


1. By what distance is the electron deflected when it leaves the plates?

2. What is the final velocity (x-direction) of the electron?


Homework Equations


F=qE
Motion equations


The Attempt at a Solution



The questions I have answered.

t= L/v0

F=eE

a=(eE)/m

(F=Electric Force, e=electron charge, E=electric field, m=mass of electron, L=length, t=time)

For the final velocity Vx, I tried to work with the motion equations. Vx=V0+at Vx being final velocity in the x direction, V0 being initial velocity, a is acceleration, and t is time. I plugged in the equations I worked out in the problem for t and a getting Vx=V0 + (eE/m)(L/V0), but the systems says this is an incorrect answer.

For the Distance deflected I also used a motion equation (delta)y=V0 t + 1/2a(y direction) t^2 I worked this down to deltaY= 1/2 (eE/m)(L/v0)^2 and am unsure where to go now.


Thanks for any help!
 
Last edited:

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
25,832
250
welcome to pf!

hi rbraunberger! welcome to pf! :smile:

i'm not sure what your question is, but the important thing is to find the aceleration …

once you have that, you can treat it much as you would projectile motion under gravity :wink:
 
  • #3
Thanks, I found the acceleration and was trying to do that....look at my work now...I edited my post since you replied. I have this online homework system and it is not taking my answers....not sure if my logic is just way off.
 
  • #4
Here is a list of all the questions....I am only having trouble with d and first part of e

(a) How long does it take the electron to travel the length of the plates?
t =

(b) What are the magnitude and direction of the electric force on the electron while it is between the plates?
magnitude F =
direction

(c) What is the acceleration of the electron?
magnitude a =
direction

(d) By what distance is the electron deflected when it leaves the plates?
Δy =

(e) What is the final velocity of the electron?
vx =
vy =
 
  • #5
tiny-tim
Science Advisor
Homework Helper
25,832
250
hi rbraunberger! :smile:

(have a delta: ∆ and try using the X2 and X2 icons just above the Reply box :wink:)
For the final velocity Vx, I tried to work with the motion equations. Vx=V0+at Vx being final velocity in the x direction, V0 being initial velocity, a is acceleration, and t is time. I plugged in the equations I worked out in the problem for t and a getting Vx=V0 + (eE/m)(L/V0), but the systems says this is an incorrect answer.

For the Distance deflected I also used a motion equation (delta)y=V0 t + 1/2a(y direction) t^2 I worked this down to deltaY= 1/2 (eE/m)(L/v0)^2 and am unsure where to go now.
hmm … you're throwing v0 and a around like confetti :redface:

v0 is only in the x direction

a is only in the y direction

start again! :smile:
 
  • #6
I thought the motion equations were (the ones we are talking about)

Vx = V0 + at
Y=Y0 + V0t + 1/2 at^2
 
  • #7
ok....I removed the a from the X and Vo from the Y and got the answers correct......are the motion equations different in this way with electric fields? My book sucks!!!!
 
  • #8
tiny-tim
Science Advisor
Homework Helper
25,832
250
hi rbraunberger! :smile:

(just got up :zzz: …)
ok....I removed the a from the X and Vo from the Y and got the answers correct......are the motion equations different in this way with electric fields? My book sucks!!!!
your book does not suck!! :rolleyes:

this is exactly the same as with projectiles in a gravitational field, isn't it?

you have to remember to deal with the x and y components separately, and that means using only the x or y component of each vector (velocity or acceleration or force) as the case may be …

in the wrong direction, that component is obviously zero :smile:

get some sleep! :zzz:​
 

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