The loop shown moves with a velocity $v$ in a uniform magnetic field of magnitude $B$, directed into the paper. The potential difference between points $P$ and $Q$ is $e$. Then

$e=Bvl$, where $l=\frac{L}{2}$

Induced emf e = $(\overrightarrow{v}\times \overrightarrow{B}).\overrightarrow{l} .Here \overrightarrow{v}\times \overrightarrow{B} gives the direction of induced current through the moving conductor.$

Therefore P has positive polarity and Q has negative polarity.