State Gauss's law in electrostatics. Use this law to derive an expression for the electric field due to an infinitely long straight wire of linear charge density A cm^–1.

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Gauss’s law in electrostatics: It states that total electric flux over the closed surface S in vacuum is  times the total charge (q) contained in side S.
                     
Electric field due to an infinitely long straight wire.
Let an infinitely long line charge having linear charge density X. Assume a cylindrical Gaussian surface of radius r and length 1 coaxial with the line charge to determine its electric field at distance r.
By symmetry, the electric field E has same magnitude at each point of the curved surface S₁ and is directed radially outward. So angle at surfaces between  is zero, and angle of  with  at  are .

Total flux through the cylindrical surface,