Stokes' Theorem is a fundamental result in vector calculus that relates a surface integral over a surface $S$ to a line integral around the boundary curve $\partial S$ of that surface. It generalizes several theorems, including Green's Theorem and the Fundamental Theorem of Calculus, to higher dimensions.

Stokes' Theorem (Formal Statement):

Let $S$ be a smooth, oriented surface with a boundary $\partial S$, and let $\mathbf{F}$ be a vector field that is continuously differentiable on and around $S$. Stokes' Theorem states that:

$\int_S (\nabla \times \mathbf{F}) \cdot d\mathbf{S} = \oint_{\partial S} \mathbf{F} \cdot d\mathbf{r}$

where:

• $\nabla \times \mathbf{F}$ is the curl of the vector field $\mathbf{F}$,
• $d\mathbf{S}$ is a vector representing the oriented surface element on $S$,
• $\mathbf{F} \cdot d\mathbf{r}$ is the line integral of $\mathbf{F}$ along the boundary curve $\partial S$,
• $\oint_{\partial S}$ indicates a line integral around the closed boundary $\partial S$.

Key Components:

1. Surface Integral: The left-hand side, $\int_S (\nabla \times \mathbf{F}) \cdot d\mathbf{S}$, is a surface integral where you are integrating the curl of the vector field over the surface $S$.
2. Line Integral: The right-hand side, $\oint_{\partial S} \mathbf{F} \cdot d\mathbf{r}$, is a line integral of the vector field along the closed boundary curve $\partial S$.

Intuitive Explanation:

Stokes' Theorem essentially states that the total circulation of a vector field $\mathbf{F}$ around the boundary of a surface is equal to the sum of the curls of $\mathbf{F}$ over the entire surface. This ties the local property of the vector field (the curl) to the global property (the circulation around the boundary).

Example:

Consider a vector field $\mathbf{F} = (y, -x, 0)$ over a surface $S$ which is a disk of radius $R$ in the $xy$-plane, with boundary $\partial S$ being the circle $x^2 + y^2 = R^2$.

• The line integral $\oint_{\partial S} \mathbf{F} \cdot d\mathbf{r}$ computes the circulation of $\mathbf{F}$ around the boundary.
• The surface integral $\int_S (\nabla \times \mathbf{F}) \cdot d\mathbf{S}$ computes the flux of the curl of $\mathbf{F}$ through the surface.

Stokes' Theorem tells us these two quantities are equal.

Would you like more detailed examples or clarification?

Related Questions:

1. How does Stokes' Theorem relate to Green’s Theorem?
2. Can Stokes' Theorem be applied to a non-planar surface?
3. What is the physical interpretation of the curl in vector fields?
4. How is the orientation of the surface and its boundary curve determined?
5. What is the difference between Stokes' Theorem and the Divergence Theorem?

Tip:

When using Stokes' Theorem, make sure the surface $S$ is smooth, the vector field is continuously differentiable, and the orientation of both the surface and its boundary are consistent.