An infinitely long straight wire is carrying current I and another wire which is parallel to the first wire , is carrying  current 2I in the same direction.The wires  produce a magnetic field B at the midpoint. What will be the field when 2I wire is switched off?

When two parallel conductors carrying current I and 2I in same direction, then magnetic field at the midpoint isB=μ02I2πr −  μ0I2πr = μ0I2πr(Directed upwards)When current 2I is switched off, then magnetic field due to conductor carrying current I is B= μ0I2πr(Directed downwards)