Pre-Quantum Electrodynamics
The Three-Dimensional Delta Functionc.m.dd.3d
\begin{equation}
\delta^{(3)} ({\bf r} - {\bf r}') = \delta (x - x') \delta (y - y') \delta (z - z')
\label{Gr(1.96)}
\end{equation}
\begin{equation}
\int d\tau f({\bf r}) \delta^{(3)} ({\bf r} - {\bf a}) = f({\bf a})
\label{Gr(1.97)}
\end{equation}
Resolution of divergence of \(\hat{\bf r}/r^2\) paradox:
\begin{equation} {\boldsymbol \nabla} \cdot \left(\frac{\hat{\bf r}}{r^2} \right)= 4\pi \delta^{(3)} ({\bf r}). \label{Gr(1.99)} \end{equation}More generally,
\begin{equation} \boxed{ {\boldsymbol \nabla} \cdot \left(\frac{{\bf r} - {\bf r}'}{|{\bf r} - {\bf r}'|^3} \right)= 4\pi \delta^{(3)} ({\bf r}). } \tag{divdel}\label{divdel} \end{equation}Since
\begin{equation} {\boldsymbol \nabla}_1 \left(\frac{1}{r_{12}}\right) = -\frac{\hat{\bf r}_{12}}{r_{12}^2} \tag{div1or}\label{div1or} \end{equation}we have that
\begin{equation} {\boldsymbol \nabla}^2 \left( \frac{1}{|{\bf r} - {\bf r}'|} \right) = -4\pi \delta^{(3)} ({\bf r} - {\bf r}') \tag{Lap1or}\label{Lap1or} \end{equation}
Created: 2024-02-27 Tue 10:31