A Cartesian coordinate system is characterized by three mutually perpendicular axes, usually named x, y, and z. As shown in Figure C.1, a point P can be expressed as
where i, j, and k are unit vectors parallel to the three axes. The scalars x, y, and z are the Cartesian coordinates of the point P.
Figure C.1. Cartesian coordinates.
The gradient operator ▽ has the following form in Cartesian coordinates.
In other coordinate systems in which a point P has coordinates u, v, and ...