The magnetic flux density at a distance d from the current carrying wire is given by:
B=μo I /2πd
I is the current through the wire,
d is the distance away from wire
μo is the permeability of free space.
here dQ=dP so |BQ|= |BP |
as new B'P =0 BP should be equal to = -B
using right hand grip rule for conductor to get B'P=0 we can find direction of current flow and direction of horizontal part of B'Q. then we can say the direction of B'Q.
new B'Q = horizontal B + vertical B
= sqroot2 *B