A small amber bead with a mass of 12.6 g and a charge of - 0.646 µC is suspended in equilibrium above the center of a large, horizontal sheet of glass that has a uniform charge density on its surface. Find the charge per unit area on the glass sheet (in µC/m2).

Answer: - 1.68 μC/m^2

Explanation: In order to solve this problem we have to consider the equilibrium between the gravititional and electric forces on the small amber bead.

In this sense we have:

Fg-Fe=0 where Fe = q * E field from the plane (σ/ε0)

m*g-q*σ/ε0

replacing the values

σ=m*g*ε0/q = (12.6*10^-3*9,8*8.85*10^-6) / 0.646 = - 1.68 μC/m^2