Question

What should be the mass of a 5 nC charge be so that it is balanced in the space between two parallel plates carrying equal and opposite charges of σ = 8.1 x 10-4 C/m2 ?

Answer 1

$$mg=qE=\frac{q\sigma}{\epsilon_0}\\9.8m=\frac{(5\cdot10^{-9})(8.1\cdot10^{-4})}{(8.85\cdot10^{-12})}\\m=0.047\ kg$$