# Definition:Density (Physics)

## Definition

Density is a physical quantity that is variously specified according to context.

Its general meaning is that of some physical quantity per unit volume, or area, or length.

### Mass Density

The mass density of a body is its mass per unit volume.

### Area Mass Density

The area mass density of a two-dimensional body is its mass per unit area.

### Linear Mass Density

The linear mass density of a one-dimensional body is its mass per unit length.

### Electric Charge Density

Let $A$ be a point in space in which an electric field acts.

Let $\delta V$ be a volume element containing $A$.

The (electric) charge density $\map \rho {\mathbf r}$ at $A$ is defined as:

 $\ds \map \rho {\mathbf r}$ $=$ $\ds \lim_{\delta V \mathop \to 0} \dfrac Q {\delta V}$ $\ds$ $=$ $\ds \dfrac {\d Q} {\d V}$

where:

$Q$ denotes the electric charge within $\delta V$
$\mathbf r$ denotes the position vector of $A$.

Thus the electric charge density is the quantity of electric charge per unit volume, at any given point in that volume:

## Also see

• Results about density in the context of physics can be found here.