What is the magnetic force on the particle

[sugz]

Homework Statement

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

Homework Equations

F= qv x B

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?

 

[berkeman]

shouldn't the magnetic field be zero?

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?

 

[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!

 

[berkeman]

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!

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?

 

[sugz]

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