# What is the magnetic force on the particle

1. Apr 19, 2015

### sugz

1. The problem statement, all variables and given/known data
A particle of mass M and charge Q>0 moves in a region of space where the magnetic field has a constant magnitude B and a downward direction. What is the magnetic force on the particle at an instant when it is moving horizontally toward the north with a speed v?

(a) QvB toward the east
(b) zero
(c) QvB toward the west
(d) QvB upward
(e) QvB toward the south
2. Relevant equations

F= qv x B

3. The attempt at a solution
My understanding of the question is that the magnetic field and the velocity are parallel and in the opposite direction, so shouldn't the magnetic field be zero?

2. Apr 19, 2015

### Staff: Mentor

I think you mean "magnetic force", not "magnetic field" there, correct? (simple typo)

But in this problem the B-field and the velocity v are not parallel or anti-parallel. Which way is B pointing? Which way is qv pointing? Using the right-hand rule, which way is the force pointing?

3. Apr 19, 2015

### sugz

Isn't the speed in the positive y direction and the magnetic field in the negative y direction?

And I did mean magnetic force, thank you!

4. Apr 19, 2015

### Staff: Mentor

Well, it says the B-field is pointing "down", which I would interpret as -z in a 3-D coordinate system. The qv is in the "north" direction, which I would interpret as +y (with +x pointing to the east). What do you get with this interpretation of their 3-D description?

5. Apr 19, 2015

### sugz

Oh okay, that makes sense! I guess it was the wording was what confused me!