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## VECTOR CALCULUS

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VECTOR CALCULUS

13.8

Stokes’ Theorem

In this section, we will learn about:

The Stokes’ Theorem and

using it to evaluate integrals.

STOKES’ VS. GREEN’S THEOREM

- Stokes’ Theorem can be regarded as a higher-dimensional version of Green’s Theorem.
- Green’s Theorem relates a double integral over a plane region D to a line integral around its plane boundary curve.
- Stokes’ Theorem relates a surface integral over a surface S to a line integral around the boundary curve of S (a space curve).

INTRODUCTION

- The figure shows an oriented surface with unit normal vector n.
- The orientation of Sinduces the positive orientation of the boundary curve C.

INTRODUCTION

- This means that:
- If you walk in the positive direction around Cwith your head pointing in the direction of n, the surface will always be on your left.

STOKES’ THEOREM

- Let:
- S be an oriented piecewise-smooth surface bounded by a simple, closed, piecewise-smooth boundary curve C with positive orientation.
- F be a vector field whose components have continuous partial derivatives on an open region in that contains S.
- Then,

STOKES’ THEOREM

- The theorem is named after the Irish mathematical physicist Sir George Stokes (1819–1903).
- What we call Stokes’ Theorem was actually discovered by the Scottish physicist Sir William Thomson (1824–1907, known as Lord Kelvin).
- Stokes learned of it in a letter from Thomson in 1850.

STOKES’ THEOREM

- Thus, Stokes’ Theorem says:
- The line integral around the boundary curve of Sof the tangential component of F is equal to the surface integral of the normal component of the curl of F.

STOKES’ THEOREM

Equation 1

- The positively oriented boundary curve of the oriented surface S is often written as ∂S.
- So,the theorem can be expressed as:

STOKES’ THEOREM, GREEN’S THEOREM, & FTC

- There is an analogy among Stokes’ Theorem, Green’s Theorem, and the Fundamental Theorem of Calculus (FTC).
- As before, there is an integral involving derivatives on the left side of Equation 1 (recall that curl F is a sort of derivative of F).
- The right side involves the values of F only on the boundaryof S.

STOKES’ THEOREM, GREEN’S THEOREM, & FTC

- In fact, consider the special case where the surface S:
- Is flat.
- Lies in the xy-plane with upward orientation.

STOKES’ THEOREM, GREEN’S THEOREM, & FTC

- Then,
- The unit normal is k.
- The surface integral becomes a double integral.
- Stokes’ Theorem becomes:

STOKES’ THEOREM, GREEN’S THEOREM, & FTC

- This is precisely the vector form of Green’s Theorem given in Equation 12 in Section 12.5
- Thus, we see that Green’s Theorem is really a special case of Stokes’ Theorem.

STOKES’ THEOREM

- Stokes’ Theorem is too difficult for us to prove in its full generality.
- Still, we can give a proof when:
- S is a graph.
- F, S, and C are well behaved.

STOKES’ TH.—SPECIAL CASE

Proof

- We assume that the equation of Sis: z = g(x, y), (x, y) Dwhere:
- g has continuous second-order partial derivatives.
- D is a simple plane region whose boundary curve C1 corresponds to C.

STOKES’ TH.—SPECIAL CASE

Proof

- If the orientation of S is upward, the positive orientation of C corresponds to the positive orientation of C1.

STOKES’ TH.—SPECIAL CASE

Proof

- We are also given that:
- F = P i + Q j + R kwhere the partial derivatives of P, Q, and R are continuous.

STOKES’ TH.—SPECIAL CASE

Proof

- S is a graph of a function.
- Thus, we can apply Formula 10 in Section 12.7 with F replaced by curl F.

STOKES’ TH.—SPECIAL CASE

Proof—Equation 2

- The result is:
- where the partial derivatives of P, Q, and Rare evaluated at (x, y, g(x, y)).

STOKES’ TH.—SPECIAL CASE

Proof

- Suppose
- x =x(t) y =y(t) a ≤t ≤b
- is a parametric representation of C1.
- Then, a parametric representation of Cis: x =x(t) y =y(t) z =g(x(t), y(t)) a ≤t ≤b

STOKES’ TH.—SPECIAL CASE

Proof

- This allows us, with the aid of the Chain Rule, to evaluate the line integral as follows:

STOKES’ TH.—SPECIAL CASE

Proof

- Next, we use the Chain Rule again, remembering that:
- P, Q, and R are functions of x, y, and z.
- z is itself a function of x and y.

STOKES’ TH.—SPECIAL CASE

Proof

- Four terms in that double integral cancel.
- The remaining six can be arranged to coincide with the right side of Equation 2.
- Hence,

STOKES’ THEOREM

Example 1

- Evaluate where:
- F(x, y, z) = –y2i + x j + z2k
- C is the curve of intersection of the plane y + z = 2 and the cylinder x2 + y2 = 1. (Orient C to be counterclockwise when viewed from above.)

STOKES’ THEOREM

Example 1

- The curve C (an ellipse) is shown here.
- could be evaluated directly.
- However, it’s easier to use Stokes’ Theorem.

STOKES’ THEOREM

Example 1

- There are many surfaces with boundary C.
- The most convenient choice, though, is the elliptical region S in the plane y + z = 2 that is bounded by C.
- If we orient S upward, C has the induced positive orientation.

STOKES’ THEOREM

Example 1

- The projection D of S on the xy-plane is the disk x2 + y2≤ 1.
- So, using Equation 10 in Section 12.7 with z =g(x, y) = 2 – y, we have the following result.

STOKES’ THEOREM

Example 1

STOKES’ THEOREM

Example 2

- Use Stokes’ Theorem to compute where:
- F(x, y, z) = xz i + yz j + xy k
- S is the part of the sphere x2 + y2 + z2 = 4 that lies inside the cylinder x2 + y2 =1 and above the xy-plane.

STOKES’ THEOREM

Example 2

- To find the boundary curve C, we solve: x2 + y2 + z2 = 4 and x2 + y2 = 1
- Subtracting, we get z2 = 3.
- So, (since z > 0).

STOKES’ THEOREM

Example 2

- A vector equation of C is:r(t) = cos t i + sin t j + k 0 ≤t ≤ 2π
- Therefore, r’(t) =–sin t i + cos t j
- Also, we have:

STOKES’ THEOREM

- Note that, in Example 2, we computed a surface integral simply by knowing the values of F on the boundary curve C.
- This means that:
- If we have another oriented surface with the same boundary curve C, we get exactly the same value for the surface integral!

STOKES’ THEOREM

Equation 3

- In general, if S1 and S2 are oriented surfaces with the same oriented boundary curve Cand both satisfy the hypotheses of Stokes’ Theorem, then
- This fact is useful when it is difficult to integrate over one surface but easy to integrate over the other.

CURL VECTOR

- We now use Stokes’ Theorem to throw some light on the meaning of the curl vector.
- Suppose that C is an oriented closed curve and v represents the velocity field in fluid flow.

CURL VECTOR

- Consider the line integral and recall that v ∙T is the component of vin the direction of the unit tangent vector T.
- This means that the closer the direction of v is to the direction of T, the larger the value of v ∙T.

CIRCULATION

- Thus, is a measure of the tendency of the fluid to move around C.
- It iscalled the circulation of v around C.

CURL VECTOR

- Now, let: P0(x0, y0, z0) be a point in the fluid.
- Sa be a small disk with radius a and center P0.
- Then, (curl F)(P) ≈ (curl F)(P0) for all points P on Sa because curl F is continuous.

CURL VECTOR

- Thus, by Stokes’ Theorem, we get the following approximation to the circulation around the boundary circle Ca:

CURL & CIRCULATION

- Equation 4 gives the relationship between the curl and the circulation.
- It shows that curl v ∙n is a measure of the rotating effect of the fluid about the axis n.
- The curling effect is greatest about the axis parallel to curl v.

CURL & CIRCULATION

- Imagine a tiny paddle wheel placed in the fluid at a point P.
- The paddle wheel rotates fastest when its axis is parallel to curl v.

CLOSED CURVES

- Finally, we mention that Stokes’ Theorem can be used to prove Theorem 4 in Section 12.5:
- If curl F = 0 on all of , then F is conservative.

CLOSED CURVES

- From Theorems 3 and 4 in Section 12.3, we know that F is conservative if for every closed path C.
- Given C, suppose we can find an orientable surface S whose boundary is C.
- This can be done, but the proof requires advanced techniques.

CLOSED CURVES

- Then, Stokes’ Theorem gives:
- A curve that is not simple can be broken into a number of simple curves.
- The integrals around these curves are all 0.

CLOSED CURVES

- Adding these integrals, we obtain: for any closed curve C.

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