A 17.0 g piece of Styrofoam carries a net charge of - 7.00 μC and floats above the center of a large horizontal sheet of plastic that has a uniform charge density on its surface. What is the charge per unit area (in C/m2) on the plastic sheet? (Include the sign of the value in your answer.) Hint

Given that,

Mass of styrofoam = 17g=0.017kg

Net charge is q = -7.00 μC

Electric field of a sheet is given as

E = σ / 2εo

σ = charge density

εo = permittivity of free space = 8.85*10^-12

Also electric field is given as

E=F/q

The force acting on the body is its weight which is given as

F=mg,

Where g=9.81

F=0.017*9.81

F=0.16677N.

Therefore,

E=F/q

E=0.16677 / - 7*10^-6

E=-23824.29N/C

Then,

E = σ / 2εo

Then, σ = E * 2εo

σ=-23824.29*2*8.85*10^-12

σ = - 4.22*10^-7C/m²

σ = - 42.2*10^-6 C/m²

σ = - 42.2 μC/m²

The charge density is - 42.2 μC/m²