Question

force on a charge in magnetic and electric field question.

Answer 1

@Mashy

Answer 2

This is basically cross fields.. and lorentz force acting on it

Answer 3

i know that. but how to solve it?

Answer 4

hmmm m not sure.. cause the force keeps varying.. and so does the acceleration :-/

Answer 5

ok.. decompose the forces.. that ll do the trick

Answer 6

see due to E, F=qE in +j direction. and due to B, F=qvB in -k direction.

Answer 7

correct.. !! precisely!!

Answer 8

so net force= sqrt ((qE)^2 + (qvb)^2 and net acc= [sqrt ((qE)^2 + (qvb)^2]/m

Answer 9

no no.. don't do that.. cause the net force will keep changing.. !!..

Answer 10

wait. so if you seperate them.. then in both cases, the initial velocity would be zero right? :-/

Answer 11

initial velocity is Vo no?

Answer 12

but thats in i direction.. we have j and k direction here..

Answer 13

see the magnetic field does no work.. hence the magnetic field will not change its speed.. !!! its the electric field that ll do it

Answer 14

so answer should be a :D

Answer 15

hmm. right. what next?

Answer 16

the answer is a. did you just see there is no B in it?

Answer 17

yea logical analysis.. the other three are wrong.. cause they HAVE B in it.. B will not affect the speed.. but ll try to derive it.. wait

Answer 18

yea i got it!

Answer 19

i have a problem in the logic now. when i say there magnetic field doent do any work, does that mean it doesnt accelerate the particle too? it applies a force so that it moves in a circle no?
HOW?

Answer 20

yea.. but it doesn't change its SPEED.. the acceleration is perpendicular to the velocity vector.. so regardless of how big or how small the field is.. that ll only affect the radius of the circle.. but not speed!

Answer 21

you want the derivation?

Answer 22

oh wait try deriving yourself :P

Answer 23

oh, it doesnt change the speed, but it changes the velocity. hmm.
i tried it myself.
i should use v=u+at right?

Answer 24

yup.. correct

and yes.. you should use that equation.. but remember you are using that in j direction.. so initial velocity for j direction is zero.. ! final velocity is such .. that when you consider both the j direction and the initial i direction it would turn out to be 3v..

Answer 25

umm. i dont think i got that. initial velocity in y direction is 0. final is 3V. so it shouldnt matter if there is a initial velocity in x direction?

Answer 26

does matter.. you can't just add the velocities now can you? .. so the final velocity in y direction is such that.. when you take vector sum of v and the final velocity of j direction .. it turns out to be 3v

Answer 27

ohho. so sqrt [v^2 + X^2]=3V^2 where X is the final velocity in X direction --> X=sqrt8 V
and then i spply v=u+at.
hmm. i got it.

Answer 28

apply*

Answer 29

= 9 v^2 :P..

Answer 30

wait you took sqr root.. so its 3v

Answer 31

oh yeah. :P
i dont have any more questions atm. but be ready for more. :P
thank you again.

Answer 32

lol ok.. no prob :D..

Answer 33

cant believe m solving jee sums though :d

Answer 34

haha. you can be a physics professor in an iit :D

Answer 35

i have to go. bbye.