Charge density

The electric charge density is a physical quantity from electrodynamics, which describes a charge distribution. Since there are both positive and negative charges, both positive and negative values ​​are also possible for the charge density.

Since charges can be distributed to surfaces or approximately along a thin wire, the charge density can be described:

• The charge per volume ( space charge density ρ )
• The charge per unit area ( surface charge density σ )
• The charge per length ( line charge density λ ).

The surface charge density achievable is limited by corona discharge into the surrounding air as the maximum field strength of 105 V / m is exceeded:

This contributes negatively charged any square centimeter, the excess charge of 1.8 x 10-10 As, which corresponds to 1.1 × 109 free moving electrons. Approximately one million times more electrons are bound to the atomic cores of the metal surface (see also induction # Number of participating electrons).

Similar sizes

One with the surface charge density σ corresponding quantity is the electric flux density (also electrical excitation, dielectric displacement or dislocation density ), and in a vertically standing on the surface considered vector; other hand, is a scalar σ ( and under certain circumstances equal to the amount ).

Not to be confused with the charge density are also the charge carrier density, ie the number of protons, electrons, etc. per room, area or unit length, and calculated in the density functional theory electron density.

Definition

The definition of the space charge density is similar to the mass density:

Where Q is the electric charge and V is the volume.

In the surface and the line charge density is derived accordingly the areas A and the length l:

Discrete charge distribution

Is the charge in a volume of discrete carriers (such as electrons), then the charge density can be expressed with the aid of the delta function:

With

• Of the load and
• The location of the -th charge carrier.

All carry the same charge carrier ( electrons in case of the same of the negative electron charge: ), we can simplify the above formula with the aid of the carrier density:

Electric potential

The electrical potential is dependent according to the Poisson equation of electrostatics

Only from the charge density. Herein, the permittivity.

Points of zero charge

The point of zero charge ( PZC ) (German point of zero charge ) is reached when the charge density of a surface is zero. This concept originates from the physical chemistry and is relevant for the adsorption of substances to particles or surfaces.

For suspension of the particles in PZC is the point at which the zeta potential is zero. This may be the case for example for a specific pH. Away from the PZC, the particles are charged, therefore repel each other electrically and so are less likely to agglomerate into flocs or aggregates together. The lack of charge at the PZC also leads to a reduction of solubility / hydration in water.

Knowledge of the PZC is useful in assessing the mobility of solutes or particles, which may play a role, among other things for the risk assessment of pollutants.

A similar concept is the isoelectric point.

• Electrical size
• Electrostatics
• Physical size style