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[ "article:topic", "Maxwell\u2019s equations", "Gauss\'s Theorem", "authorname:tatumj", "showtoc:no" ]
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15.2: Maxwell's First Equation

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  • Maxwell's first equation, which describes the electrostatic field, is derived immediately from Gauss's theorem, which in turn is a consequence of Coulomb's inverse square law.  Gauss's theorem states that the surface integral of the electrostatic field \(\textbf{D}\) over a closed surface is equal to the charge enclosed by that surface. That is

    \[ \int_{\text{surface}} \textbf{D} \cdot \boldsymbol{d\sigma} = \int_{\text{volume}} \rho \, dv \tag{15.2.1} \label{15.2.1}\]

    Here \(\rho\) is the charge per unit volume.

    But the surface integral of a vector field over a closed surface is equal to the volume integral of its divergence, and therefore

    \[ \int_{\text{surface}} \text{div}\, \textbf{D}\, dv = \int_{\text{volume}} \rho \, dv \tag{15.2.2} \label{15.2.2}\]


    \[\text{div} \textbf{D}  = \rho, \tag{15.2.3} \label{15.2.3}\]

    or, in the nabla notation, 

    \[\nabla \cdot \textbf{D}  = \rho. \tag{15.2.3} \label{15.2.4}\]

    This is the first of Maxwell's equations.