Two infinite plates shown in cross-section in figure carry $\lambda$amperes of current out of the page per unit width of plate. Find the magnetic field at point $P$ and $Q$. 

$B$in the above situation is given by  $B=\frac{{\mu }_0\lambda }{2}$

At point $P,B_1$and $B_2$ are in opposite directions.Hence, $B_P=0$

At the point, $Q, B_1$and $B_2$are in same direction.

Hence,    $B_Q=2\left(\frac{{\mu }_0\lambda }{2}\right)={\mu }_0\lambda$