- The following diagram shows a proton moving with velocity v 0 ,
initially parallel to, and above, a long, straight wire. If the current in
the wire is I as shown, what is the direction of the magnetic force on
the proton?
Here’s How to Crack It
Using the second right-hand rule, we see the magnetic field B produced by the
current points out of the page above the wire (where the proton is located). Using
the first right-hand rule for a proton moving the right, feeling a magnetic field out of
the page, we see that FB points downward, toward the wire.