Along straight wire has fixed negative charge with a linear charge density of magnitude 5.1 nc/m. the wire is to be enclosed by a coaxial, thin-walled, nonconducting cylindrical shell of radius 1.9 cm. the shell is to have positive charge on its outside surface with a surface charge density σ that makes the net external electric field is zero. calculate σ. express your answer in terms of 10-8.