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Consider a particle moving through magnetic field(T/F)

  1. Nov 17, 2016 #1
    1. The problem statement, all variables and given/known data
    hrkn48.jpg



    2. Relevant equations

    F= qvb


    3. The attempt at a solution

    I tried FTTT and FTFT but both were wrong , i know that the acceleration is in the same direction as the force , but how come the other 3 are not all true.By the right hand rule shouldn't the force be perpendicular to B and since accleration and forc are parallel the 2nd should be true right?.Also, since by the right hand rule again the force , velocity and magnetic field should all be perpendicular no? thus the 3rd and 4th should be true.I don't know what I am doing wrong here please help
     
  2. jcsd
  3. Nov 17, 2016 #2

    Student100

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    You should write the whole equation, ##\vec{F}=q\vec{v}\times\vec{B}## or ##|F|=q|v||B|sin(\theta)##, can you reason which one could be false now?

    I don't really like this question, it's quite ambiguously worded for my taste.
     
  4. Nov 17, 2016 #3

    Simon Bridge

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    Your equation is incorrect - the proper equation is ##\vec F = q\vec v\times \vec B##
    The vector cross product means that force, velocity, and field, are perpendicular to each other.
    If ##\vec v## and ##\vec B## make angle ##\theta## between them, then the magnitude of the force is given by: ##F=qvB\sin\theta##

    1 is false because the force and acceleration are always in the same dircetion - well done.
    This means that 2 is true becuase equation and 1. which you got, well done.
    3 is true because equation ... which I think you said, so well done again.
    4 ... you did not explain your reason here.
     
  5. Nov 17, 2016 #4

    Student100

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    I also want to continue on my mini-rant for a moment, don't use the right hand rule - it's pedagogic nonsense. Put the things in a matrix and be done with it. Here, if you draw a few examples of possible velocities and magnetic fields, it should become obvious which one is wrong.
     
  6. Nov 17, 2016 #5
    So i guess 4 is false because the velocity and magnetic field could actually go in the same direction ? and also Student100 , i would like to do it in a matrix but i have not yet reached linear algebra so i have no idea how to do that yet :(
     
  7. Nov 17, 2016 #6

    Student100

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    Okay, I just don't like that it's taught is all, you should have linear algebra before delving into this stuff, just my opinion.

    Yes, your answer should be okay now.
     
  8. Nov 17, 2016 #7
    Yeah I guess you're right but well there's nothing we can do about it.Anyways , thanks for the help.
     
  9. Nov 17, 2016 #8

    Student100

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    For sure, it isn't your fault, they just make these things harder than they really are by insuring students don't have the prerequisites to do it first.
     
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