Chapter 17: Problem 42
Find the average value of the temperature function \(T(x, y, z)=100-25 z\) on the cone \(z^{2}=x^{2}+y^{2},\) for \(0 \leq z \leq 2\)
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Chapter 17: Problem 42
Find the average value of the temperature function \(T(x, y, z)=100-25 z\) on the cone \(z^{2}=x^{2}+y^{2},\) for \(0 \leq z \leq 2\)
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A beautiful flux integral Consider the potential function \(\varphi(x, y, z)=G(\rho),\) where \(G\) is any twice differentiable function and \(\rho=\sqrt{x^{2}+y^{2}+z^{2}} ;\) therefore, \(G\) depends only on the distance from the origin. a. Show that the gradient vector field associated with \(\varphi\) is \(\mathbf{F}=\nabla \varphi=G^{\prime}(\rho) \frac{\mathbf{r}}{\rho},\) where \(\mathbf{r}=\langle x, y, z\rangle\) and \(\rho=|\mathbf{r}|\)b. Let \(S\) be the sphere of radius \(a\) centered at the origin and let \(D\) be the region enclosed by \(S\). Show that the flux of \(\mathbf{F}\) across \(S\) is \(\iint_{S} \mathbf{F} \cdot \mathbf{n} d S=4 \pi a^{2} G^{\prime}(a)\) c. Show that \(\nabla \cdot \mathbf{F}=\nabla \cdot \nabla \varphi=\frac{2 G^{\prime}(\rho)}{\rho}+G^{\prime \prime}(\rho)\) d. Use part (c) to show that the flux across \(S\) (as given in part (b)) is also obtained by the volume integral \(\iiint_{D} \nabla \cdot \mathbf{F} d V\) (Hint: Use spherical coordinates and integrate by parts.)
Zero flux fields. Consider the vector field \(\mathbf{F}=\langle a x+b y, c x+d y\rangle .\) Show that \(\mathbf{F}\) has zero flux across any oriented circle centered at the origin, for any \(a, b, c,\) and \(d,\) provided \(a=-d\)
The area of a region R in the plane, whose boundary is the curve \(C\), may be computed using line integrals with the formula $$\text { area of } R=\int_{C} x d y=-\int_{C} y d x$$ Let \(R\) be the rectangle with vertices \((0,0),(a, 0),(0, b),\) and \((a, b),\) and let \(C\) be the boundary of \(R\) oriented counterclockwise. Use the formula \(A=\int_{C} x d y\) to verify that the area of the rectangle is \(a b\).
Zero circulation fields. For what values of \(b\) and \(c\) does the vector field \(\mathbf{F}=\langle b y, c x\rangle\) have zero circulation on the unit circle centered at the origin and oriented counterclockwise?
One of Maxwell's equations for electromagnetic waves is \(\nabla \times \mathbf{B}=C \frac{\partial \mathbf{E}}{\partial t},\) where \(\mathbf{E}\) is the electric field, \(\mathbf{B}\) is the magnetic field, and \(C\) is a constant. a. Show that the fields \(\mathbf{E}(z, t)=A \sin (k z-\omega t) \mathbf{i}\) and \(\mathbf{B}(z, t)=A \sin (k z-\omega t) \mathbf{j}\) satisfy the equation for constants \(A, k,\) and \(\omega,\) provided \(\omega=k / C\). b. Make a rough sketch showing the directions of \(\mathbf{E}\) and \(\mathbf{B}\).
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