A 1.0-μm-diameter oil droplet (density 900 kg/m^3) is negatively charged with the addition of 30 extra electrons. It is released from rest 2.0 mm from a very wide plane of positive charge, after which it accelerates toward the plane and collides with a speed of 4.1m/s.
What is the surface charge density of the plane.
Express your answer to two significant figures and include the appropriate units.