# Homework Help: Consider a particle moving through magnetic field(T/F)

1. Nov 17, 2016

### gkamal

1. The problem statement, all variables and given/known data

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. Nov 17, 2016

### Student100

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.

3. Nov 17, 2016

### Simon Bridge

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.

4. Nov 17, 2016

### Student100

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.

5. Nov 17, 2016

### gkamal

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 :(

6. Nov 17, 2016

### Student100

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.

7. Nov 17, 2016

### gkamal

Yeah I guess you're right but well there's nothing we can do about it.Anyways , thanks for the help.

8. Nov 17, 2016

### Student100

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.