# A 42mT magnetic field points due west. If a proton of KE 9X10E-12 J enters this field in an upward direction,?

Find the magnetic force acting on the proton, in magnitude & direction.

### 1 Answer

- Anonymous9 years agoBest Answer
Recall that the Lorentz force on a point charge is given by F=qv x B where q is the charge (+e), v is the velocity of the point charge (not given) and B is the value of the magnetic field (42mT). The x above does not refer to multiplication but cross product which means the force is perpendicular to both the direction of the particle's motion and the magnetic field.

First you need to find the proton's velocity. Since you are given the proton's kinetic energy, using the equation for kinetic energy and looking up the mass of the proton will allow you to solve for its velocity. Then you just need to sub all of the values into the above equation.

To find the direction you need to use the right hand rule. Thumb points in the direction of positive current, fingers point in the direction of the magnetic field, and the resultant force comes outward from your palm.

Hope this helps!