The number of mobile charge carriers (electrons, ions, etc.) per unit volume is equal to the charge carrier density in a conductor. The charge density at any site is equal to the charge carrier density multiplied by the particle's elementary charge. Thus, a material with an equal distribution of charge has equal numbers of positive and negative charges distributed throughout.

Equal distribution of charge means that if you were to divide the surface area of a piece of material into small regions and then count the number of electrons in each region, they would be exactly equal. If there are more positive than negative charges, the material has an excess of positive charges or an excess of negative charges. If it has an excess of positive charges, we say that it is positively charged; if it has an excess of negative charges, it is negatively charged.

Equal distribution of charge is important in understanding **some properties** of materials. For example, materials with **equal distribution** of charge will not polarize either positively or negatively when exposed to light or other forms of radiation. This is because there are no locations where the number of **positive charges** is greater than the number of negative charges; thus, there is no place where the material can gain or lose electrons and become polarized.

Also, materials with **equal distribution** of charge do not attract or repel one another.

Charge density in electromagnetism is the quantity of electric charge per unit length, surface area, or volume. Because electric charge can be either positive or negative, charge density can be either positive or negative. The magnitude of charge density is expressed as electrons per cm-3 or electrons per cm-2.

Free charge density is the amount of charge that exists without any net charge being located anywhere in space. In **other words**, free charge density equals the total number of electrons minus the total number of holes across **all energy levels**.

The term "free charge" does not imply that there are any free particles such as electrons or photons present. It only means that no overall charge is present. If we had a sample that was completely void of electrons then it would have a free charge density of positive ions.

Ions are always present in some form in matter. They just happen to be concentrated in **certain regions** of space. This occurs because when atoms share electrons they become positively charged and thus need more negative charges around them to balance out **their charge**. Ions are usually produced by exposing material to radiation or applying **an electrical field**.

Matter becomes conducting when electrons are able to move through it freely; therefore, conductivity is related to the degree to which electrons are bound within atoms and molecules.

Charge density is determined by charge distribution and can be positive or negative. Depending on the nature of the charge, the charge density formula can be expressed as: I Linear charge density; l=ql, where q is the charge and lis the length across which it is dispersed. Cm-1 is the SI unit. For ions, the charge density is the number of ions per unit volume.

For electrons, the charge density is the ratio of the electron charge to the volume of the nucleus (or atom) if we consider only the outermost electrons. If we include all the electrons, then the charge density is the total electronic charge divided by the volume of the crystal.

In ionic crystals, the anions and cations are not necessarily uniformly distributed throughout the crystal. Instead, there may be regions within the crystal that contain many anions or many cations, respectively. In **these regions**, the anions and cations are said to be "dense". The average densities of an ionic crystal are its dense regions combined with its void spaces between **those regions**. Densities higher than expected for a random arrangement of ions are called "hyperdense" while lower ones are "hypodense". Hyperdense and hypodense regions may occur due to lattice defects such as vacancies or interstitials. Within an ionic crystal, hyperdense regions tend to contain more anions than cations while hypodense regions tend to contain **more cations** than anions.

What exactly is the distinction between charge density and current density? Charge density seems to be a scalar, whereas current density appears to be a vector, yet both are microscopic characteristics. Current density is the amount of electric current that travels per unit cross-section area and is measured in amperes per square meter. Charge density is the number of electrons or positive charges divided by the volume they occupy, which is also called the charge carrier concentration. This means that if we divide the number of electrons by the volume they occupy, we get charge density. Current density is usually expressed in amps per square meter, but it can also be expressed in amperes per millimeter squared (A/mm2) or even amp-seconds per square meter (A/s2).

Charge density and current density can be used together with the equation for electric field strength to determine the nature of **the electromagnetic field** at **a given point**. If we assume that the charge carriers are electrons, then the charge density will be negative and the current density will be positive. In this case, the right-hand side of the equation for electric field strength would be negative, indicating **an electric field** that pulls on the electron cloud.

If we assumed that the charge carriers were positive ions, then the charge density would be positive and the current density would be negative. In this case, the right-hand side of the equation for electric field strength would be positive, indicating an electric field that pushes back on the ion cloud.

The mobility of charge carriers in a current-carrying conductor is defined as the net average velocity with which free electrons travel towards the positive end of a conductor under the effect of an applied external electric field. In other words, it is the ratio of the charge on an electron to the distance it travels in time t.

In general, the higher this value, the better the conductor is for transporting electricity. But there are limitations: The maximum mobility that can be achieved in any material at room temperature is about 100,000 cm2/V-s. A human hair has a mobility of about 1000 cm2/V-s. So even a metal wire with a diameter as small as 10-5 cm would be almost completely immobile if all the charges were transported by electrons.

The reason why the mobility of electrons is so low is because they must follow the path taken by the atoms in **the crystal structure** of the material they are traveling through. If there were no obstacles in their way, electrons would be able to travel much faster than what we actually see them doing. But since this isn't the case, they will always need **some time** to find **an alternative path** when one is blocked. This is called "electron scattering" and it becomes more frequent as the material's quality increases.