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Chapter 9: Problem 13

Consider the region defined by \(|x|,|y|\), and \(|z|<1\). Let \(\epsilon_{r}=5, \mu_{r}=4\), and \(\sigma=0 .\) If \(J_{d}=20 \cos \left(1.5 \times 10^{8} t-b x\right) \mathbf{a}_{y} \mu \mathrm{A} / \mathrm{m}^{2}(a)\) find \(\mathbf{D}\) and \(\mathbf{E} ;(b)\) use the point form of Faraday's law and an integration with respect to time to find \(\mathbf{B}\) and \(\mathbf{H} ;(c)\) use \(\nabla \times \mathbf{H}=\mathbf{J}_{d}+\mathbf{J}\) to find \(\mathbf{J}_{d} \cdot(d)\) What is the numerical value of \(b\) ?

Short Answer

Expert verified
In summary, use the given values for \(\epsilon_r\), \(\mu_r\), and \(\sigma\) to calculate the absolute permittivity \(\epsilon\) and permeability \(\mu\). Then, find the electric and displacement fields using the given conduction current density and the relationships between the fields. Calculate the magnetic fields using Faraday's law and the relationship between \(\mathbf{B}\) and \(\mathbf{H}\). Find the displacement current density using Ampère's circuital law and the computed values of \(\mathbf{H}\) and \(\mathbf{J}_d\). Finally, compare the \(x\) and \(y\) components of \(\mathbf{J}_d\) and \(\mathbf{H}\) to determine the numerical value of \(b\).

Step by step solution

01

Calculate \(\epsilon\) and \(\mu\)

Since we are given the relative permittivity \(\epsilon_r\) and the relative permeability \(\mu_r\), we can calculate the absolute permittivity \(\epsilon\) and permeability \(\mu\) using the following formulas: $$ \epsilon = \epsilon_{r} \epsilon_{0} $$ $$ \mu = \mu_{r} \mu_{0} $$ Where \(\epsilon_{0}\) and \(\mu_{0}\) are the vacuum permittivity and permeability, respectively. \(\epsilon_{0} = 8.854 \times 10^{-12} \, \mathrm{F/m}\) and \(\mu_{0} = 4 \pi \times 10^{-7} \, \mathrm{T \cdot m/A}\). Plug in the given values to find \(\epsilon\) and \(\mu\).
02

Calculate the displacement and electric fields

Using the given conduction current density, \(J_d = 20 \cos(1.5 \times 10^8 t - b x) \mathbf{a}_y \mu A/m^2\), we can derive the displacement field \(\mathbf{D}\) and electric field \(\mathbf{E}\) using the following relationships: $$ \mathbf{D} = \epsilon \mathbf{E} $$ $$ \mathbf{E} = \frac{1}{\sigma} \mathbf{J}_d $$ Substitute the given values for \(J_d\) and \(\sigma\), and then calculate \(\mathbf{E}\) and \(\mathbf{D}\).
03

Find the magnetic fields

Use the point form of Faraday's law and integrate with respect to time to find the magnetic fields \(\mathbf{B}\) and \(\mathbf{H}\): $$ \nabla \times \mathbf{E} = -\frac{\partial\mathbf{B}}{\partial t} $$ $$ \mathbf{B} = \int[\nabla \times \mathbf{E}] \, dt $$ Compute \(\nabla \times \mathbf{E}\), then integrate with respect to time to find \(\mathbf{B}\). Next, use the relationship between \(\mathbf{B}\) and \(\mathbf{H}\) to find \(\mathbf{H}\): $$ \mathbf{B} = \mu \mathbf{H} $$
04

Determine the displacement current density

Use Ampère's circuital law to find the displacement current density \(\mathbf{J}\) $$ \nabla \times \mathbf{H} = \mathbf{J}_d + \mathbf{J} $$ Substitute the given values and computed \(\mathbf{H}\) to find \(\mathbf{J}\).
05

Find the numerical value of \(b\)

Now that we have all of the fields, we can find the numerical value of \(b\) in the expression for \(J_d\). To do this, observe the relationship between the fields and use the given values of the coefficients. Comparing the \(x\) and \(y\) components of \(\mathbf{J}_d\) and \(\mathbf{H}\) will reveal the value of \(b\).

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Most popular questions from this chapter

In a region where \(\mu_{r}=\epsilon_{r}=1\) and \(\sigma=0\), the retarded potentials are given by \(V=x(z-c t) \mathrm{V}\) and \(\mathbf{A}=x\left(\frac{z}{c}-t\right) \mathbf{a}_{z} \mathrm{~Wb} / \mathrm{m}\), where \(c=1 \sqrt{\mu_{0} \epsilon_{0}}\) (a) Show that \(\nabla \cdot \mathbf{A}=-\mu \epsilon \frac{\partial V}{\partial t} .(b)\) Find \(\mathbf{B}, \mathbf{H}, \mathbf{E}\), and \(\mathbf{D} .(c)\) Show that these results satisfy Maxwell's equations if \(\mathbf{J}\) and \(\rho_{v}\) are zero.

Find the displacement current density associated with the magnetic field \(\mathbf{H}=A_{1} \sin (4 x) \cos (\omega t-\beta z) \mathbf{a}_{x}+A_{2} \cos (4 x) \sin (\omega t-\beta z) \mathbf{a}_{z}\)

Let the internal dimensions of a coaxial capacitor be \(a=1.2 \mathrm{~cm}, b=4 \mathrm{~cm}\), and \(l=40 \mathrm{~cm}\). The homogeneous material inside the capacitor has the parameters \(\epsilon=10^{-11} \mathrm{~F} / \mathrm{m}, \mu=10^{-5} \mathrm{H} / \mathrm{m}\), and \(\sigma=10^{-5} \mathrm{~S} / \mathrm{m}\). If the electric field intensity is \(\mathbf{E}=\left(10^{6} / \rho\right) \cos 10^{5} t \mathbf{a}_{\rho} \mathrm{V} / \mathrm{m}\), find \((a) \mathbf{J} ;(b)\) the total conduction current \(I_{c}\) through the capacitor; \((c)\) the total displacement current \(I_{d}\) through the capacitor; \((d)\) the ratio of the amplitude of \(I_{d}\) to that of \(I_{c}\), the quality factor of the capacitor.

Given \(\mathbf{H}=300 \mathbf{a}_{z} \cos \left(3 \times 10^{8} t-y\right) \mathrm{A} / \mathrm{m}\) in free space, find the emf developed in the general \(\mathbf{a}_{\phi}\) direction about the closed path having corners at \((a)(0,0,0),(1,0,0),(1,1,0)\), and \((0,1,0) ;(b)(0,0,0)(2 \pi, 0,0)\), \((2 \pi, 2 \pi, 0)\), and \((0,2 \pi, 0)\)

Given Maxwell's equations in point form, assume that all fields vary as \(e^{s t}\) and write the equations without explicitly involving time.
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