What is the physical meaning of charge density on lines or on areas?

Answer 1

Every object consists of a certain amount of positive charge and a certain amount of negative charge. For neutral objects, the amount of each type of charge is equal in every tiny, or infinitesimal, portion of the object. If the object has the shape of a line, the amount of positive charge in each tiny segment of length along the line is equal to the amount of negative charge in each tiny segment of length. For a neutral three-dimensional object, such as a cube, the amount of negative charge in each small volume element of the total volume of the cube is equal to the amount of positive charge in each small volume element. All neutral objects have a charge density of zero throughout their volumes despite the fact that they have charge. The charge density describes the amount of excess charge per given region of space. For objects that are not neutral, then, the charge density is either positive or negative. A positive charge density expresses the fact that an object has a given amount of positive charge more than it has negative charge in a specific region of space. Likewise, a negative charge density means the object has a given amount of negative charge more than positive charge for a given region of space. For a line of charge, the charge density is expressed as Coulombs per meter when using SI units. For a two-dimensional object, such as a disk, the charge density using SI units is Coulombs per (meter^2). For objects that have uniform excess charge throughout their volume, the charge density is expressed as the total amount of excess charge on the body divided by the total length/ area/ volume of the body. For objects that have nonuniform charge excesses, the charge density must be expressed as a function of position (and possibly, time) within the object.