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### Course: Multivariable calculus > Unit 5

Lesson 6: Stokes' theorem (articles)# Stokes' theorem examples

See how Stokes' theorem is used in practice.

## The formula (quick review)

Stokes' theorem is a tool to turn the surface integral of a curl vector field into a line integral around the boundary of that surface, or vice versa. Specifically, here's what it says:

Let's go through each term:

is a three-dimensional vector field.${\mathbf{\text{F}}}(x,y,z)$ , also often written as$\text{curl}{\textstyle \phantom{\rule{0.167em}{0ex}}}{\mathbf{\text{F}}}$ . It is the three-dimensional curl of$\mathrm{\nabla}\times {\mathbf{\text{F}}}$ , which is a vector field.${\mathbf{\text{F}}}$ is a surface in three dimensions.${S}$ represents a function that gives unit normal vectors to${\hat{\mathbf{\text{n}}}}$ .${S}$ is the boundary of${C}$ .${S}$ is oriented using the right-hand rule, meaning if you point the thumb of your right hand in the direction of a unit normal vector${C}$ near the edge of${\hat{\mathbf{\text{n}}}}$ and curl your fingers, the direction they point indicates the direction you should integrate around${S}$ .${C}$

## Example 1: From a surface integral to line integral

**Problem**

Let ${S}$ be the half of a unit sphere centered at the origin that is above the $xy$ plane, oriented with outward facing unit normal vectors. Let $\overrightarrow{\mathbf{\text{v}}}(x,y,z)$ be the vector field defined as follows:

Compute the following surface integral:

**Solution**

Remember, Stokes' theorem relates the surface integral of the curl of a function to the line integral of that function around the boundary of the surface. This means we will do two things:

**Step 1**: Find a function whose curl is the vector field$y\hat{\mathbf{\text{i}}}$ **Step 2**: Take the line integral of that function around the unit circle in the -plane, since this circle is the boundary of our half-sphere.$xy$

**Concept check**: Find a vector field

There are multiple ways to do this, but one in particular will make our lives easiest. In the one I'm thinking of, the $\hat{\mathbf{\text{i}}}$ and $\hat{\mathbf{\text{j}}}$ components are $0$ , while the $\hat{\mathbf{\text{k}}}$ component is non-zero. Can you find it?

The surface ${S}$ is defined to be the portion of the unit sphere above the $xy$ -plane. The boundary of this hemisphere is the unit circle on the $xy$ -plane.

**Concept check**: Both of the following parameterize the unit circle on the

*outward-facing*unit normal vectors? ("Correspond" in the sense that we can apply Stokes' theorem.)

**Concept check**: Let

## Example 2: Wind through a butterfly net

**Problem**

Suppose you have a butterfly net with a square-shaped rim, and the wind is blowing through the net. Think about the square rim positioned in space on the $yz$ -plane such that its four corners are at the following four points:

Furthermore, let the net be some surface emerging from this rim in the positive $x$ -direction.

Suppose the velocity vector field for the wind is given by the following function:

Assuming the air has a uniform density of $1{\textstyle \phantom{\rule{0.167em}{0ex}}}\text{kg}/{\text{m}}^{3}$ , how much air passes through your net per unit time? Specifically, suppose air going from the inside of the net to the outside counts positively towards this sum, and air going from the outside to the inside counts negatively.

**Step 1: Dissecting the question**

Before anything, we need to compose our thoughts and piece together how this physics-sounding problem is a Stokes' theorem question.

**Concept check**: What is this question really asking about?

**Concept check**: More specifically, which of the following integrals represents the answer to the question? Let

Really, this is all just a way to give a physical interpretation to a surface integral through a vector field.

**Step 2: Applying Stokes' theorem**

What might feel weird about this problem, and what suggests that you will need Stokes' theorem, is that the surface of the net is never defined! All that is given is the boundary of that surface: A certain square in the $yz$ -plane.

If we can find a way to express ${\mathbf{\text{F}}}(x,y,z)$ as the curl of some other vector field, say $\mathbf{\text{G}}(x,y,z)$ , we will be able to apply Stokes' theorem to this problem as follows:

This is analogous to performing the integral $\int f(x){\textstyle \phantom{\rule{0.167em}{0ex}}}dx$ in single-variable calculus, where you have to find a new function with the property ${g}^{\prime}(x)=f(x)$ , which then lets you compute the integral based on the boundary values. In this case, we are looking for the "anti-curl" of ${\mathbf{\text{F}}}$ , so to speak, which will let us compute the surface integral based on the values of this anti-curl function on the boundary of the surface.

Unlike single-variable calculus, not all vector fields ${\mathbf{\text{F}}}$ have such an anti-curl function. Luckily for us, this particular function is one of the special ones that do.

**Concept check**: Find a vector field

**Step 3: Compute the line integral**

Given this construction for $\mathbf{\text{G}}$ , the final step is to compute the right-hand-side line integral in our core equation:

In this context, the curve ${C}$ represents the $2\times 2$ square in the $yz$ -plane with vertices at the following four points:

Before computing the line integral around this square, it needs to be oriented in a way that aligns with the orientation of the butterfly net surface ${S}$ .

**Concept check**: Given that the butterfly net lies in the positive

**Concept check**: Our construction of

Given this, and given the orientation of the square ${C}$ that you just specified, finish the problem by computing the following line integral:

## Summary

- Stokes' theorem can be used to turn surface integrals through a vector field into line integrals.
- This only works if you can express the original vector field as the curl of some other vector field.
- Make sure the orientation of the surface's boundary lines up with the orientation of the surface itself.

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