Find the interaction force of two coils with magnetic moment `p_(1m) = 43.0 mA.m^(2)` and `p_(2m) = 6.0 mA.m^(2)` and coilnear axis if the separation between the coils is equal to `l = 20 cm` which exceeds considerably their linear dimensions.

Explanation

`P = P_(2m) (del)/(del t) [(mu_(0))/(4pi) (3 vec(p_(1m)) . vec(r) vec(r) - vec(p_(1m)) r^(2))/(r^(2))]`
`P_(2m) (del)/(del l) [(mu_(0))/(2pi) (P_(1m))/(l^(3))] = (-3)/(2) (mu_(0) P_(1m) p_(2m))/(pi l^(4)) = 9 nN`