PQ and RS are long parallel conductors separated by certain distance. M is the midpoint between them (see the figure). The net magnetic field at M is B. Now, the current 2 A is switched off. The field at M now becomes

Step-by-step solution

Magnetic field at midpoint M in first case is B = B _ PQ - B _ RS ( B _ PQ . and B _ RS are in opposite directions) aligned &= 4 ₀ 4 d - 2 ₀ 4 d &= 2 ₀ 4 d aligned When the current 2 ~A is switched off, the net magnetic field at M is due to current 1 ~A B ^ = ₀ 2 1 4 d = B