Homework Help: Direction of a magnetic field

1. Mar 24, 2009

ehilge

1. The problem statement, all variables and given/known data
A positively charged particle moving in the direction of the positive x axis feels no magnetic force when it passes through point P. When moving in the direction of the positive z axis, the particle experiences a magnetic force in the positive y direction when it passes through point P. Thus the magnetic field B at point P is directed along:.

2. Relevant equations

F=q(v x b)

right hand rule

3. The attempt at a solution
Since the particle feels no force when traveling in the direction of positive x, (v x b) must be 0 and the only way this happens is if the angle between the is the angle between the velocity and B field is 0 or 180. So the B field must be parallel to the x axis. Now the question states when the particle moves along the + z axis, it feels a force in the positive y direction. Due to the right hand rule, I believe that this must mean the b field is in the -x direction. Fingers in the direction of the velocity (+z), thumb in the direction of the force (+y), curl fingers towards the B field, which then must be the -x direction.

However, the answer is not the -x axis. Could you please show me where I went wrong in my reasoning and help me find the correct answer?
Thanks!

2. Mar 24, 2009

LowlyPion

3. Mar 24, 2009

Redbelly98

Staff Emeritus
Your description of how you solved this is correct, but for some reason you came up with -x instead of +x.

I am just guessing, either you used your left hand by mistake, or you are picturing the x,y,z axes incorrectly.

If the axes are set up properly, then +x × +y = +z

EDIT
LowlyPion beat me to it.

4. Mar 24, 2009

ehilge

Yup, after reviewing that, it appears that I definitely used a left hand coordinate system. Thanks for your help.