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Consider an infinite thin plane sheet of positive charge.Let the point P at a distance r from the sheet.The electric field intensity applying Gauss's law can be determined from the figure shown                                                                                     

Let S is the area of each flat surface of the cylinder,then 

Along  ds₁ and ds₂ , ? = 0

But q = ?_s where ? is the charge density on both sides

Note : If ? is positive, E will be along the outward drawn normal.

           If ? is negative ,E will be along the inward drawn normal.