Source: High school physics (Chinese)
Problem Sets:
Problem
A movable conducting wire slides to the right with velocity $v$ along a rectangular circuit. A magnetic field is present in region $A$, to the left of the wire. As the wire moves, the induced current it produces is directed upward in the wire (as shown).
The induced current in the moving wire is driven by the magnetic force on its charge carriers, $\vec{F} = q\vec{v}\times\vec{B}$. For the conventional current to flow upward in the wire, this force on the positive carriers must point upward. With $\vec{v}$ to the right, $\vec{v}\times\vec{B}$ points upward only if $\vec{B}$ is directed into the page. Equivalently, by Lenz's law: as the wire moves right the area of region $A$ grows, and an upward current in the wire circulates counterclockwise around region $A$, producing an out-of-page field that opposes an increasing into-page flux -- consistent with $B$ into the page.