81. A charged ball weighing 300 g is suspended from a thread. When a ball charged with the same charge with a radius of 2 cm was brought up to it at a distance of 40 cm from the bottom, the tension force of the thread decreased by 4 times. Determine the surface density of the electric charge on the presented ball.

Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)