4.10: The Laplacian Operator

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The Laplacian \(\nabla^2 f\) of a field \(f({\bf r})\) is the divergence of the gradient of that field:

\[\nabla^2 f \triangleq \nabla\cdot\left(\nabla f\right) \label{m0099_eLaplaceDef} \]

Note that the Laplacian is essentially a definition of the second derivative with respect to the three spatial dimensions. For example, in Cartesian coordinates,

\[\nabla^2 f= \frac{\partial^2 f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2} + \frac{\partial^2 f}{\partial z^2} \label{m0099_eLaplaceScalar} \]

as can be readily verified by applying the definitions of gradient and divergence in Cartesian coordinates to Equation \ref{m0099_eLaplaceDef}.

The Laplacian relates the electric potential (i.e., \(V\), units of V) to electric charge density (i.e., \(\rho_v\), units of C/m\(^3\)). This relationship is known as Poisson’s Equation:

\[\nabla^2 V = - \frac{\rho_v}{\epsilon} \nonumber \]

where \(\epsilon\) is the permittivity of the medium. The fact that \(V\) is related to \(\rho_v\) in this way should not be surprising, since electric field intensity \(({\bf E}\), units of V/m) is proportional to the derivative of \(V\) with respect to distance (via the gradient) and \(\rho_v\) is proportional to the derivative of \({\bf E}\) with respect to distance (via the divergence).

The Laplacian operator can also be applied to vector fields; for example, Equation \ref{m0099_eLaplaceScalar} is valid even if the scalar field “\(f\)” is replaced with a vector field. In the Cartesian coordinate system, the Laplacian of the vector field \({\bf A} = \hat{\bf x}A_x + \hat{\bf y}A_y + \hat{\bf z}A_z\) is

\[\nabla^2 {\bf A} = \hat{\bf x}\nabla^2 A_x + \hat{\bf y}\nabla^2 A_y + \hat{\bf z}\nabla^2 A_z \nonumber \]

An important application of the Laplacian operator of vector fields is the wave equation; e.g., the wave equation for \({\bf E}\) in a lossless and source-free region is

\[\nabla^2{\bf E} + \beta^2{\bf E} = 0 \nonumber \]

where \(\beta\) is the phase propagation constant.

It is sometimes useful to know that the Laplacian of a vector field can be expressed in terms of the gradient, divergence, and curl as follows: \[\nabla^2 {\bf A} = \nabla\left(\nabla\cdot{\bf A}\right) - \nabla\times\left(\nabla\times{\bf A}\right) \nonumber \]

The Laplacian operator in the cylindrical and spherical coordinate systems is given in Appendix B2.