Charge Densities Just as the mass density reveals how concentrated the matter is in a material, the charge density reveals how concentrated the electrostatic charges are in the material. There are three types of charge density you will encounter in electric-field problems: The volumetric charge density , or amount of charge per unit volume, is given the Greek letter rho ( ρ ). The surface charge density , or amount of charge per unit area, is given the Greek letter sigma ( σ ).The linear charge density , or amount of charge per unit length, is given the Greek letter lambda ( λ ). In terms of calculation, they are: = Q/ V = Q/A = Q/ l It is very important that you know these well, but it is equally necessary that you know the volumes and surface areas of cylinders and spheres. Here are two corny phrases that may help you remember the volumes if you don’t already know them: “If the pie r esembles a r od, l eave.” “If the pie is r eally r ound, r eturn for thirds .” The first is
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