Chapter 14: Problem 2
Sketch the vector field \(\mathbf{F}=\langle x, y\rangle.\)
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Chapter 14: Problem 2
Sketch the vector field \(\mathbf{F}=\langle x, y\rangle.\)
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Let \(R\) be a region in a plane that has a unit normal vector \(\mathbf{n}=\langle a, b, c\rangle\) and boundary \(C .\) Let \(\mathbf{F}=\langle b z, c x, a y\rangle\) a. Show that \(\nabla \times \mathbf{F}=\mathbf{n}\) b. Use Stokes' Theorem to show that $$\operatorname{area} \text { of } R=\oint_{C} \mathbf{F} \cdot d \mathbf{r}$$ c. Consider the curve \(C\) given by \(\mathbf{r}=\langle 5 \sin t, 13 \cos t, 12 \sin t\rangle\) for \(0 \leq t \leq 2 \pi .\) Prove that \(C\) lies in a plane by showing that \(\mathbf{r} \times \mathbf{r}^{\prime}\) is constant for all \(t\) d. Use part (b) to find the area of the region enclosed by \(C\) in part (c). (Hint: Find the unit normal vector that is consistent with the orientation of \(C\).)
Prove the following identities. Assume that \(\varphi\) is \(a\) differentiable scalar-valued function and \(\mathbf{F}\) and \(\mathbf{G}\) are differentiable vector fields, all defined on a region of \(\mathbb{R}^{3}\). $$\nabla(\mathbf{F} \cdot \mathbf{G})=(\mathbf{G} \cdot \nabla) \mathbf{F}+(\mathbf{F} \cdot \nabla) \mathbf{G}+\mathbf{G} \times(\nabla \times \mathbf{F})+\mathbf{F} \times(\nabla \times \mathbf{G})$$
Consider the vector field \(\mathbf{F}=\frac{-y}{x^{2}+y^{2}} \mathbf{i}+\frac{x}{x^{2}+y^{2}} \mathbf{j}+z \mathbf{k}\) a. Show that \(\nabla \times \mathbf{F}=\mathbf{0}\) b. Show that \(\oint_{C} \mathbf{F} \cdot d \mathbf{r}\) is not zero on a circle \(C\) in the \(x y\) -plane enclosing the origin. c. Explain why Stokes' Theorem does not apply in this case.
The potential function for the gravitational force field due to a mass \(M\) at the origin acting on a mass \(m\) is \(\varphi=G M m /|\mathbf{r}|,\) where \(\mathbf{r}=\langle x, y, z\rangle\) is the position vector of the mass \(m\) and \(G\) is the gravitational constant. a. Compute the gravitational force field \(\mathbf{F}=-\nabla \varphi\). b. Show that the field is irrotational; that is, \(\nabla \times \mathbf{F}=\mathbf{0}\).
Let \(\mathbf{F}\) be a radial field \(\mathbf{F}=\mathbf{r} /|\mathbf{r}|^{p},\) where \(p\) is a real number and \(\mathbf{r}=\langle x, y, z\rangle .\) With \(p=3, \mathbf{F}\) is an inverse square field. a. Show that the net flux across a sphere centered at the origin is independent of the radius of the sphere only for \(p=3\) b. Explain the observation in part (a) by finding the flux of \(\mathbf{F}=\mathbf{r} /|\mathbf{r}|^{p}\) across the boundaries of a spherical box \(\left\\{(\rho, \varphi, \theta): a \leq \rho \leq b, \varphi_{1} \leq \varphi \leq \varphi_{2}, \theta_{1} \leq \theta \leq \theta_{2}\right\\}\) for various values of \(p\)
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