Answer
When the direction of the magnetic field is direction 4, the top side of the wire is at a higher potential than the bottom side of the wire.
Work Step by Step
When the direction of the current is parallel or antiparallel with the direction of the magnetic field, there is no force produced so the electric potential is zero. The electric potential is zero for direction 1 and direction 2 of the magnetic field.
When the direction of the magnetic field is in direction 3 or direction 4, the direction of the current is perpendicular to the magnetic field. This will produce a force and result in a non-zero electric potential.
When the current is moving to the right, the electrons, which are the charge carriers, are moving to the left. When the direction of the magnetic field is direction 4, then the force on the electrons is directed downward. This results in a situation where the top
side of the wire is at a higher potential than the bottom side of the wire.
