A red blood cell typically carries an excess charge of about -2.5 10-12 C distributed uniformly over its surface. The red blood cells can be modeled as spheres approximately 7.0 m in diameter and with a mass of 9.0 10-14 kg. How many excess electrons does a typical red blood cell have? excess electrons: 1.5606 x107 electrons Does the mass of the extra electrons appreciably affect the mass of the cell? To find out, calculate the ratio of the mass of the extra electrons to the mass of the cell without the excess charge. ratio: 1.58 x10-10 What is the surface charge density on the red blood cell? Express your answer in units of C/m and electrons/m.