General Science: Charge density

Saturday, December 12, 2009

Charge density

The linear, surface, or volume charge density is the amount of electric charge in a line, surface, or volume. It is measured in coulombs per metre (C/m), square metre (C/m²), or cubic metre (C/m³), respectively. Since there are positive as well as negative charges, the charge density can take on negative values. Like any density it can depend on position. It should not be confused with the charge carrier density. As related to chemistry, it can refer to the charge distribution over the volume of a particle, molecule, or atom. Therefore, a lithium cation will carry a higher charge density than a sodium cation due to its smaller ionic radius.

Homogeneous charge density

For the special case of a homogeneous charge density, that is one that is independent of position, equal to $ρ q,0$ the equation simplifies to:

$Q=V\cdot \rho_{q,0}$

The proof of this is simple. Start with the definition of the charge of any volume:

$Q=\int\limits_V \rho_q(\mathbf r) \,\mathrm{d}V$

Then, by definition of homogeneity, $\rho_q(\mathbf r)$ is a constant that we will denote $ρ q,0$ to differentiate between the constant and non-constant forms, and thus by the properties of an integral can be pulled outside of the integral resulting in:

$Q=\rho_{q,0} \int\limits_V \,\mathrm{d}V = \rho_{q,0} \cdot V$

so,

$Q=V \cdot \rho_{q,0}$

The equivalent proofs for linear charge density and surface charge density follow the same arguments as above.

Discrete charges

If the charge in a region consists of $N$ discrete point-like charge carriers like electrons the charge density can be expressed via the Dirac delta function, for example, the volume charge density is:

$\rho(\mathbf{r})=\sum_{i=1}^N\ q_i\delta(\mathbf{r} - \mathbf{r}_i)\,\!$ ;

where $\mathbf{r}\,\!$ is the test position， $q_i\,\!$ is the charge of the ith charge carrier, whose position is $\mathbf{r}_i\,\!$ .

If all charge carriers have the same charge $q$ (for electrons $q = - e$ ) the charge density can be expressed through the charge carrier density $n(\mathbf r)$ : Again, the equivalent equations for the linear and surface charge densities follow directly from the above relations.