l g of cork with a charge 1 μC floats motionless 1 cm above a large uniformly charged plane near the surface of the earth. The surface charge density of the plane, assuming it to be an infinite sheet, is

The cork floats motionless if the weight of the cork is equal to electrostatic force due to uniformly charged plane near the earth's surface, i.e.,mg=Fe=QENow, electric field E due to large plane isE=σ2ε0 mg=σ2ε0Q σ=2ε0mgQ=2×8.85×10-12×10-3×9.81×10-6 =17.3×10-8C/m2