Definition 9.5.1. Charge Density.
Charge density can be found by dividing the small amount of charge \(dq\) in a portion of an object by the size of that portion of the object. If the small portion is one-dimensional (say, the \(x\)-direction), you would call it linear charge density
\begin{equation*}
\lambda = \frac{dq}{dx}
\end{equation*}
if it is two-dimensional, you would call it surface charge density
\begin{equation*}
\sigma = \frac{dq}{dA}
\end{equation*}
and if it three dimensional, you would call it volume charge density
\begin{equation*}
\rho = \frac{dq}{dV}
\end{equation*}
