A group of students floated a charged invisible tape above another charged tape. They determined that the approximate charge on the floating tape was about 1.2e-08 C. The floating tape was 16 cm long and 1.25 cm wide.
The ratio of the number of excess electronic charges to the number of molecules on the surface of the tape is the fraction of the molecules on the surface of the tape that have gained (or lost) an extra electronic charge e = 1.6e-19 C. What is this fraction? To estimate this, assume that molecules in the tape are arranged in a cubic lattice, as indicated in the accompanying figure, and that the diameter of a molecule in the tape is about 3e-10 m.