The magnetic field at a distance 'r' from a long straight wire carrying current I is 0.4 tesla. The magnetic field at a distance 2r will be

In the cyclotron, as radius of the circular path of the charged particle increases, ($\omega$ = angular velocity, V = linear velocity)

Two planar concentric rings of metal wire having radii '$r_1$' and '$r_2$' (with $r_1 > r_2$) are placed in air. The current 'I' is flowing through the coil of larger radius. The mutual inductance between the coils is given by ( $\mu_0$ = permeability of free space)

Two coils having self-inductance $L_1 = 75$ mH and $L_2 = 48$ mH are coupled with each other. If the mutual inductance of the coils is 37.2 mH, then coefficient of coupling will be