A solenoid of length 0.4 m and having 500 turns of wire carries a current of 3 A . A thin coil having 10 , turns of wire and of radius 0.01 m carries a current of 0.4 A . The torque required to hold the coil in the middle of the solenoid with its axis perpendicular to the axis of solenoid.

Which of the following graph represents the variation of magnetic flux density $B$ with distance $r$ for a straight long wire carrying an electric current?

A current carrying conductor experiences maximum force in magnetic field when the angle between current element and direction of magnetic field is

Two parallel conductors carry current in opposite directions as shown in the figure. One conductor carries a current of $$20 \mathrm{~A}$$. Point $$C$$ is a distance $$d / 2$$ to the right of a 20 A current. If $$d=18 \mathrm{~cm}$$ and $$i$$ is adjusted so that the magnetic field at $$C$$ is zero, the value of the current $$i$$ is