57. At a distance of r1 = 10 cm from an infinitely long charged thread there is a point charge Q = 1 μC. When this charge was moved to a distance of r2 = 2 cm in the direction perpendicular to the thread, work A = 1 mJ was completed. Determine the linear density τ of the filament in question.

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