Chapter 14: Problem 11
In Exercises \(9-12,\) a closed curve \(C\) that is the boundary of a surface \(S\) is given along with a vector field \(\vec{F}\). Verify Stokes' Theorem on \(C ;\) that is, show \(\oint_{c} \vec{F} \cdot d \vec{r}=\iint_{S}(\operatorname{curl} \vec{F}) \cdot \vec{n} d S\). \(C\) is the curve that follows the triangle with vertices at (0,0,2),(4,0,0) and (0,3,0) , traversing the the vertices in that order and returning to \((0,0,2),\) and \(\mathcal{S}\) is the portion of the plane \(z=2-x / 2-2 y / 3\) enclosed by \(c ; \vec{F}=\langle y,-z, y\rangle .\)
Short Answer
Step by step solution
Parameterize the curve C
Compute the line integral \(\oint_{C} \vec{F} \cdot d\vec{r}\)
Compute the curl of \( \vec{F} \)
Find the normal vector \( \vec{n} \) of the surface \( S \)
Compute the surface integral \( \iint_{S} (\text{curl } \vec{F}) \cdot \vec{n} \, dS \)
Compare line integral and surface integral
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Line Integral
To compute a line integral of a vector field \( \vec{F} \) along a curve \( C \), we express it as \( \oint_{C} \vec{F} \cdot d\vec{r} \), where \( \vec{r}(t) \) is a parameterization of the curve \( C \). The vector \( d\vec{r} \) represents a differential element of the curve, often written in differential form, such as \( \langle dx, dy, dz \rangle \).
- Curves can be broken down into segments if they are piecewise smooth.
- Each segment can be parameterized with a suitable parameter \( t \) to express the curve along particular coordinates.
- Calculate the integral for each segment by substituting the parameterized expressions into \( \vec{F} \cdot \frac{d\vec{r}}{dt}\).
Surface Integral
In this scenario, we calculate the integral of a vector field over a surface \( S \), which is expressed as \( \iint_{S} \vec{F} \cdot dS \). Here, \( dS \) is the differential surface area vector on \( S \), and is usually oriented by an outward-facing normal vector \( \vec{n} \).
- The differential area \( dS \) is replaced by \( \vec{n} \, dS \), where \( \vec{n} \) is the unit normal to the surface.
- To compute this, first find the normal vector to the surface. This can often be determined from a given plane equation.
- The dot product of the vector field's curl with the surface normal is evaluated, and the integral computes the overall effect across the area.
Curl of a Vector Field
Mathematically, for a vector field \( \vec{F} = \langle P, Q, R \rangle \), the curl is given by the cross product \( abla \times \vec{F} \). Using partial derivatives, this results in:\[abla \times \vec{F} = \left( \frac{\partial R}{\partial y} - \frac{\partial Q}{\partial z}, \frac{\partial P}{\partial z} - \frac{\partial R}{\partial x}, \frac{\partial Q}{\partial x} - \frac{\partial P}{\partial y} \right)\]
- The curl component indicates how the field rotates around an axis locally.
- A non-zero curl at a point signifies that the field exhibits a circulatory or rotational flow.
- In Stokes' Theorem, the integral of the curl over a surface provides insights into the accumulated rotation bracketing the surface.
Parameterization of Curves
A curve is parameterized by assigning a vector function \( \vec{r}(t) = \langle x(t), y(t), z(t) \rangle \) that traces the path of the curve as \( t \) varies over an interval. This simplification replaces complicated coordinate descriptions with a streamlined representation:
- The parameter \( t \) often represents "time" along the path, scaling from the start point to the endpoint, usually from \( t = 0 \) to \( t = 1 \).
- Different segments of the piecewise curve are parameterized individually to fit particular characteristics of each section.
- Parameterization ensures continuity and enables analytical evaluations across diverse sections of the curve.