A proton traveling due north enters a region that contains both a magnetic field and an electric field. The electric field lines
point due west. It is observed that the proton continues to travel in a straight line due north. In which direction must the magnetic field lines point
In the electromagnetic system of force if the direction of motion of proton does not changes it means that the electric and magnetic forces are such a ways that they are cancelling each other's effect.
Since, electric field lines will exert a force on the proton to the west, hence, the magnetic force must force it to the east. It is well known that magnetic force acts perpendicular to the direction of magnetic field. magnetic field should point upward direction.
