# Homework Help: The direction of proton in a magnetic field

1. Jan 19, 2013

### ASidd

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

A positive charge is moving straight ahead in a magnetic field going out of page. What is the direction of force on the positively charged particle?

3. The attempt at a solution
I know that you use the right hand rule where fingers show direction of force. But I'm confused because my fingers keep pointing up in one direction and then down in the other so I'm very confused.

2. Jan 19, 2013

### dikmikkel

So the magnetic force is $\vec{F} = q\vec{v}\times \vec{B}$
q is the positive charge. I do not know what straight ahed means, I assume that you have a sketch. Your thumb on the right hand shall be in the direction of the velocity and your index finger along the magnetic field, then your middle finger will give the force direction since the charge is positive.
Alternatively you could define your vectors and calculate a crossproduct i.e. define a coordinate system.