Help: Due Tonight Electric Field Question

[rbraunberger]

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.)

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!

 

[tiny-tim]

welcome to pf!

hi rbraunberger! welcome to pf!

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

 

[rbraunberger]

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.

 

[rbraunberger]

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 =

 

[tiny-tim]

hi rbraunberger!

(have a delta: ∆ and try using the X² and X₂ icons just above the Reply box )

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

v₀ is only in the x direction

a is only in the y direction

start again! ​

 

[rbraunberger]

I thought the motion equations were (the ones we are talking about)

Vx = V0 + at
Y=Y0 + V0t + 1/2 at^2

 

[rbraunberger]

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!

 

[tiny-tim]

hi rbraunberger!

(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! ​

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

get some sleep! :zzz:​