Vector Calculus in General and Orthogonal Coordinate Systems

Vector calculus in general orthogonal coordinate systems extends the familiar operations of gradient, divergence, and curl beyond Cartesian coordinates to coordinate systems where axes remain mutually perpendicular (orthogonal) but may be curved or scaled differently. Orthogonal coordinate systems are defined by the property that their coordinate surfaces intersect at right angles, ensuring the basis unit vectors are mutually perpendicular at every point.

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Key points about vector calculus in orthogonal coordinate systems:

Orthogonality means the dot product of distinct basis vectors is zero, and each unit vector has length one. Mathematically, the basis vectors $e _i$ satisfy

$$ e _i \cdot e j=\delta{i j} $$

where $\delta_{i j}$ is the Kronecker delta ( 1 if $i=j, 0$ otherwise). This simplifies defining vector operations.
Common orthogonal coordinate systems include:
- Cartesian coordinates $(x, y, z)$ with constant unit vectors.
- Cylindrical coordinates ( $r, \theta, z$ ), useful for problems with axial symmetry.
- Spherical coordinates $(r, \theta, \phi)$, suited for radial symmetry.
Scale factors (metric coefficients), often denoted $h_u, h_v, h_w$, are introduced because coordinate lines may be curved and unit vector lengths may vary with position. These scale factors relate infinitesimal changes in coordinates to actual distances:

$$ d s^2=h_u^2 d u^2+h_v^2 d v^2+h_w^2 d w^2 $$

Vector calculus operators adapt to include these scale factors to correctly represent gradients, divergences, curls, and Laplacians in the curvilinear framework.
Explicit forms of vector calculus operators in orthogonal coordinates depend on the scale factors. For example, the gradient of a scalar function $f$ is:

$$ \nabla f=\frac{1}{h_u} \frac{\partial f}{\partial u} e _u+\frac{1}{h_v} \frac{\partial f}{\partial v} e _v+\frac{1}{h_w} \frac{\partial f}{\partial w} e _w $$

Divergence and curl expressions similarly involve derivatives weighted by scale factors and the Jacobian (volume element).
Application and significance:
- Orthogonal systems simplify complex vector calculus problems by exploiting problem symmetries.
- Examples include solving Laplace's equation, Helmholtz equation, and Maxwell's equations in physics.
- Coordinate transformations between Cartesian and orthogonal coordinates enable easier boundary condition application and integral evaluations on curved surfaces.
General approach to use vector calculus in such systems involves:
1. Expressing position vectors and basis vectors in terms of the coordinate variables.
2. Computing scale factors from derivatives of position vectors.
3. Adapting vector calculus operators by incorporating scale factors.
4. Applying integral theorems in their adapted forms valid for the coordinate system.

The "Non-Cartesian Coordinate Systems" section provides a comprehensive exploration of curvilinear coordinate systems beyond the familiar Cartesian, emphasizing their foundational principles (basis vectors, transformations, orthogonality), practical applications in visualizing fields and physical phenomena (e.g., flux, charge distributions), and analytical methods for verifying their properties and transformations, ultimately deepening the understanding of how vector fields behave in diverse spatial representations.

The "Non-Cartesian Coordinate Systems" section provides a comprehensive exploration of curvilinear coordinate systems beyond the familiar Cartesian, emphasizing their foundational principles (basis vectors, transformations, orthogonality), practical applications in visualizing fields and physical phenomena (e.g., flux, charge distributions), and analytical methods for verifying their properties and transformations, ultimately deepening the understanding of how vector fields behave in diverse spatial representations.