4.5: Gradient

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The gradient operator is an important and useful tool in electromagnetic theory. Here’s the main idea:

The gradient of a scalar field is a vector that points in the direction in which the field is most rapidly increasing, with the scalar part equal to the rate of change.

A particularly important application of the gradient is that it relates the electric field intensity \({\bf E}({\bf r})\) to the electric potential field \(V({\bf r})\). This is apparent from a review of Section 2.2; see in particular, the battery-charged capacitor example. In that example, it is demonstrated that:

The direction of \({\bf E}({\bf r})\) is the direction in which \(V({\bf r})\) decreases most quickly, and
The scalar part of \({\bf E}({\bf r})\) is the rate of change of \(V({\bf r})\) in that direction. Note that this is also implied by the units, since \(V({\bf r})\) has units of V whereas \({\bf E}({\bf r})\) has units of V/m.

The gradient is the mathematical operation that relates the vector field \({\bf E}({\bf r})\) to the scalar field \(V({\bf r})\) and is indicated by the symbol “\(\nabla\)” as follows: \[{\bf E}({\bf r}) = -\nabla V({\bf r}) \nonumber \] or, with the understanding that we are interested in the gradient as a function of position \({\bf r}\), simply \[{\bf E} = -\nabla V \nonumber \]

At this point we should note that the gradient is a very general concept, and that we have merely identified one application of the gradient above. In electromagnetics there are many situations in which we seek the gradient \(\nabla f\) of some scalar field \(f({\bf r})\). Furthermore, we find that other differential operators that are important in electromagnetics can be interpreted in terms of the gradient operator \(\nabla\). These include divergence (Section 4.6), curl (Section 4.8), and the Laplacian (Section 4.10).

In the Cartesian system:

\[\nabla f = \hat{\bf x}\frac{\partial f}{\partial x} + \hat{\bf y}\frac{\partial f}{\partial y} + \hat{\bf z}\frac{\partial f}{\partial z} \label{eDelCar} \]

Example \(\PageIndex{1}\): Gradient of a ramp function.

Find the gradient of \(f=ax\) (a “ramp” having slope \(a\) along the \(x\) direction).

Solution

Here, \(\partial f/\partial x = a\) and \(\partial f/\partial y = \partial f/\partial z = 0\). Therefore \(\nabla f = \hat{\bf x}a\). Note that \(\nabla f\) points in the direction in which \(f\) most rapidly increases, and has magnitude equal to the slope of \(f\) in that direction.

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