# Homework Help: Direction of magnetic field and physics principle?

1. Nov 16, 2014

### A123

1. The problem statement, all variables and given/known data
A proton travelling at a speed of 3.00*10^6 m/s north through a magnetic field experiences a force of 1.50*10^-14N up. What is the magnitude and direction of the magnetic field? Include the physics principle

2. Relevant equations
Fm=qvB

3. The attempt at a solution
I solved for a B of 3.13*10^-2T but my problem lies in applying the hand rules to find the direction of the field. Which of the directions, north or up would be referring to the piece of paper that the question is on? And would the physics principle be unbalanced forces?

2. Nov 16, 2014

### TSny

Hello, A123. Welcome to PF!

I guess you are meant to assume that the magnetic field is perpendicular to the velocity of the proton. Otherwise, there are an infinite number of possible answers to the question. (Why?)

With that assumption, you can answer the question. You don't need to worry about "referring to the piece of paper that the question is on". Imagine that you go outside and that you face north. Since the proton is traveling north, you can think of the proton as moving away from you parallel to the ground. The force is "up", which means that the force points vertically upward (perpendicular to the ground). You should be able to use the right hand rule to determine the direction of the field.

I'm not sure what is meant by "include the physics principle".

3. Nov 16, 2014

### A123

Thanks that makes sense. I just overthought it way to much