Coulombs

[marshall4]

What charge is required to suspend a 1.7*10^-13kg oil drop in an electric field of 2.5*10^5 N/C?

[himanshu121]

Draw the Free Body Diagram
Apply \Sigma F = \vec F_m_a_g + \vec F_w_t = 0
As in case of equilibrium

Also apply the charge quantization