Two infinitely long parallel wires having linear charge densities `lambda_(1)` and `lambda_(2)` respectively are placed at a distance of `R` metres. The force per unit length on either wire will be `(k= 1/(4piepsilon_(0)))`

Explanation

Electric field at wire B due to charge on wire A will be
`E_A=lambda_a/(2piepsilon_0a)`
Force per unit length acting on wire B will be ,
F=Charge on wire x `E_A`
`=lambda_b . E_A`
`rArr F=(lambda_a lambda_b)/(2piepsilon_0a)`