Question

A positive charge 'q' of mass 'm' is moving along the x-axis a uniform magnetic field B for time delta t so that the charge reverses its direction crossing the y-axis at a distance d. Then find B and delta t

Answer 1

@samigupta8 @ganeshie8

Answer 2

Is ∆t=πm/Bq
And B=2mv/qd

Answer 3

how?

Answer 4

Are they right??

Answer 5

B is correct but delta t is not..

Answer 6

can you tell me how did you solve for b?

Answer 7

*B

Answer 8

Sure!! Just a sec i m trying to figure out mistake in ∆t

Answer 9

ok

Answer 10

@samigupta8 ?

Answer 11

just tell me this how will the radius of its path be d/2 ?

Answer 12

and how can we say v is perpendicular to B?

Answer 13

U simply check it from the options..
If there is no perpendicular velocity to B then we can't have the circular motion of the particle.

Answer 14

And will you pls tell of which jee paper is this question from ..
I m unable to find it in my book..
Pls do tell so that i may confirm my ans..

Answer 15

i know .. thats why i asked how is B perpendicular to v and how can it reverse the direction?

Answer 16

Why not ??
B has a great knack to do all these things straightaway...

Answer 17

Okk i will tell you how??

Answer 18

can you pls explain a bit more conceptually ?

Answer 19

I got the ans
It is ∆t=πd/2v

Answer 20

oh thats right! can you pls explain a bit more conceptually ?

Answer 21

Okk priyar u know that ven particle will enter magnetic field and due to that it will be feeling a magnetic force on it...

Answer 22

yeah! F= q(vxB)

Answer 23

This magnetic force will cause it to move it in a circular path..
Now we want that particle should just cross the y axis at d distance from y axis so that means centre must be at distance d/2 from origin..
Ryt??

Answer 24

ok but the part which i am not getting is how is it a circular path and how B reverse the direction of v

Answer 25

Magnetic force is the reason for its circular motion

Answer 26

U analyse the situation by luking at the concept that when it will reach distance d then force is acting towards centre and so the particle must have the velocity opposite in direction to what it had initially at the start of motion

Answer 27

the magnetic force must be acting in the opp dir. right?

Answer 28

@samigupta8

Answer 29

Opposite direction to what??

Answer 30

opp to its direction of motion... only then it can reverse the dir. by 180 degrees right?

Answer 31

How ..
F=q(v*B)
How can force be ever opposite to direction of motion for a particle in uniform magnetic field

Answer 32

that is what i came to ask you.. so how is this situation possible?

Answer 33

i got the values of B and delta t if i take it to be circular.. but why circular?

Answer 34

U r not trying to understand..
Force direction is always towards centre of circular path so just do that and let go the particle at distance d from origin on y axis ..
U will see what i want to tell by this ??

Answer 35

Draw a diagram for that in ur notebook and see the diff

Answer 36

ok thanks

Answer 37

U get it??

Answer 38

think so

Answer 39

Great!!

Answer 40

there can be 2 possibilities of circular motion right? either it can move in a circular motion and go further away from the origin or it can come in a circle towars origin... in what dir. must B act for the charge to come towards origin? @samigupta8

Answer 41

+j right?

Answer 42

It can be in any direction k or -k

Answer 43

how? we need the charge to come toward origin right?

Answer 44

How can u direct your force in k direction by assuming magnetic field to be in y direction?? We want it to cross y axis and not z axis

Answer 45

ya... i was keeping the axes diff for understanding.. and forgot to change it according to this Q..

Answer 46

anyway, is there two possibilities of circular motion or not?

Answer 47

Yep there can be bt we want it to be like that mentioned in question..
So go as per they suggest in the question

Answer 48

then how can u say particle will cross y axis no matter what direction B acts...??

Answer 49

I asked you to consider the case when particle is moving such that it completes a half circle in magnetic field

Answer 50

thats not what i m asking...

Answer 51

Thanks anyway! @samigupta8

Answer 52

@samigupta8 i got it!

Answer 53

B can act only in one direction for this to be possible

Answer 54

Wohooo!!! That is ??

Answer 55

that is -k!!!!

Answer 56

Even k would do ..
Consider it too!!

Answer 57

i considered it.. but it gives the other possibility that i told you

Answer 58

Then the particle would cross a distance d in negative y direction.. That's it!!

Answer 59

no.. it will move farther away from x- axis.. as i told you before.. those two cases.. remember?!

Answer 60

Both the possibility in k and -k certainly don't give the possibility of that what u told me bcoz that's the case when magnetic field is restricted to a certain region..and that's not mentioned here..
So consider it to be present throughout the motion of particle