Question

The diagram shows an electron, with charge e, mass m, and velocity v, entering a uniform electric field of strength E.

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20%289702%29/9702_s09_qp_1.pdf

Scroll down to Question No.) 29
The direction of the field and the electron’s motion are both horizontal and to the right.
Which expression gives the distance x hrough which the electron travels before it stops momentarily?

the options and diagram is given in the link above
the answer is D but I want to know why is it so?

Answer 1

Well.. thats simple 1D kinematics

whats the acceleration (retardation in this case) experienced by the electron?

Answer 2

there is no number given

Answer 3

tell me the expression!

Answer 4

uhm that's the thing, I don't why they are using kinetic energy in this

Answer 5

forget about kinetic energy.. :P
just answer my question..

what is the acceleration ? (in terms of E, e, and m)

Answer 6

I looked through my book: the only thing that is coming in my mind is E=F/Q or V/d
I don't know how you relate that with F=ma though :/

Answer 7

you already wrote two equations

E = F/q
and F = ma
using these equations can't u relate them? :D

Answer 8

*hint eliminate F * shhhhshhhhhhh..

Answer 9

uhm.....F= E/Q aaand F= Ma soooo......

Answer 10

sooooooo... ?

Answer 11

wait.. F is NOT E/q

E = F/q

Answer 12

wait I'm looking for distance right?

Answer 13

no u are looking for acceleration.. !!! plz focus here.. ll walking you through this.. but i can't help you if you don't follow me step by step :O

Answer 14

I'm trying >___<

Answer 15

so elimate F from the two equations

here

E = F/Q => F = EQ
and F = ma

so what is a ???

Answer 16

so acceleration is Force/ Mass right?

Answer 17

If that's what we are looking for right?

Answer 18

i want a in terms of (E, m and Q).. using those two equations..

Answer 19

so If I make F as the subject in the E= F/Q formula the F is E x Q over mass?

Answer 20

or may be I just screwed it up

Answer 21

T_T

Answer 22

WHAT?!.. JUST EQUATE THE TWO EQUATIONS.. :O .. u are telling me you can't do that?!? :O

Answer 23

F= ma
F = EQ

THEREFORE

ma = EQ

NOW WHAT IS 'a' ????

Answer 24

if you don't answer this right.. ll kil you!

Answer 25

EQ/m!!!!

Answer 26

Okay! Okay!

Answer 27

=_=

Answer 28

THERE.. was that so hard? :P

now Q = e

therefore a = Ee/m

now the electron is going to the right..
but which direction does it experience the force.. hence which direction is the acceleration?
left or right?

Answer 29

it's going along in the direction of the electric field so it's right?

Answer 30

uh-oh don't tell me I got it wrong agian??? o_O

Answer 31

if that is the case.. then it should speed up .. and keep going.. why would it ever stop and turn back?

Answer 32

so there is a force opposing it to make it stop

Answer 33

yes.. so acceleration is directed towards ...... ?

Answer 34

left

Answer 35

ok
so we can call it negative

now u know initial velocity = v
u know acceleration = -eE/m
u know final velocity = 0 (cause it stops)

can u calculate the distance? x ?

Answer 36

do we use one of the equation of motion like this one?

v^2 = u^2 + 2as? (where u is the initial velocity and s is the displacement)

Answer 37

yea.. precisely!

Answer 38

*applauds* :P

Answer 39

=_='''' thank you miss genius

Answer 40

o.O .. u now made the biggest mistake of your life in your final reply

i dunno who you are, but m gonna find you , m gonna hunt you.. and i WILL KILL YOU!

Answer 41

what?? lol hahaha I complemented you >xD

Answer 42

i meant the 'miss' part

ITS MR.
now did you get the answer?

Answer 43

ohhh Ooops sorry sir ^.^'

Answer 44

uhm......all my friends and me are dumb in maths (hence physics) so I didn't get it when you said "equate two equations" it's a utter shame (I know lol) but who do you equate two equations?

Answer 45

*what do you mean by saying equate

Answer 46

meaning eliminating a variable

liek if x = 2y and y = 3z

then i can elimnate y

x = 2(3z) => x = 6y

Answer 47

btw.. did you get the answer? how many times should i ask :(

Answer 48

I got it don't worry, don't worry :D

Answer 49

x = 6z * sorry :D

Answer 50

now i wanted to tell you how to do it using energy..

u wanna know? or you are fine with acceleration method? :P

Answer 51

Oooohh I see

Answer 52

oh I don't mind please go ahead :)

Answer 53

well.. u use the concept of conservation of energy..


what i have drawn is equipotential line

when the electron moves down the field ( its moving towards the lower potential region)

since E = V/d
V = Ed
and since i already took intiially as V = 0, the line to the right x distance away is at a potential -Ex

so far so good?