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Sunday, 25 December 2016
Problem set 45
A plane electromagnetic wave is traveling along the positive z-direction. The maximum electric field along the x-direction is 10 V/m. The approximate maximum values of the power per unit area and the magnetic induction B, respectively, are
3.3\times 10^{-7} watts/m^2 and 10 tesla
3.3\times 10^{-7} watts/m^2 and 3.3\times 10^{-8} tesla
0.265 watts/m^2 and 10 tesla
0.265 watts/m^2 and 3.3\times 10^{-8} tesla
The power per unit area transported by the electromagnetic waves is given
\begin{align*}
I&=c\epsilon_0E_0^2\\
&=3\times 10^8\times8.854\times10^{-12}\times10^2\\
&=0.265 watts/m^2
\end{align*}
Magnetic induction
\begin{align*}
B&=\frac{1}{c}E_0\\
&=\frac{1}{3\times10^{-8}}\times10\\
&=3.3\times 10^{-8} \: tesla
\end{align*}
Hence, answer is (D)
A particle moves in one dimension in the potential V=\frac{1}{2}k(t)x^2, where k(t) is a time dependent parameter. Then \frac{d}{dt}\left < V\right >, the rate of change of the expectation value \left < V\right > of the potential energy, is
Consider three inertial frames of reference A, B and C. The frame B moves with a velocity c/2 with respect to A, and C moves with velocity c/10 with respect to B in the same direction. The velocity of C as measured in A is
\frac{3c}{7}
\frac{4c}{7}
\frac{c}{7}
\frac{\sqrt{3}c}{7}
Let V be the velocity of frame B with respect to frame A, u_x be the velocity of frame C with respect to frame A, and u_x' be the velocity of frame C with respect to frame B, then we have
u_x'=\frac{u_x-V}{1-u_xV/c^2}and u_x=\frac{u_x'+V}{1+u_x'V/c^2}\begin{align*}
u_x&=\frac{u_x'+V}{1+u_x'V/c^2}\\
&=\frac{\frac{c}{10}+\frac{c}{2}}{1+\frac{c}{10}\frac{c}{2}\frac{1}{c^2}}=\frac{4}{7}c
\end{align*}
Hence, answer is (B)
Suppose the yz-plane forms a chargeless boundary between two media of permittivities \epsilon_{left} and \epsilon_{right} where \epsilon_{left}:\epsilon_{right}=1:2. If the uniform electric field on the left is \vec E_{left}=c\left(\hat i+\hat j+\hat k\right) (where c is constant), then the electric field on the right \vec E_{right} is
Using relation
E_{left}\epsilon_{left}\cos{\theta_1}=E_{right}\epsilon_{right}\cos{\theta_2}
where, \theta_1 is the angle between \vec E_{left} and the vector normal to the yz-plane and \theta_2 is the angle between \vec E_{right} and the vector normal to the yz-plane. The vector normal to yz-plane is \hat i
Using \epsilon_{left}:\epsilon_{right}=1:2 in above equation
E_{left}\cos{\theta_1}=2E_{right}\cos{\theta_2}\quad--(1)
Magnitude of \vec E_{left} is given by
E_{left}=\sqrt{c^2+c^2+c^2}=\sqrt{3}c
Let us find angle \cos{\theta_1}\begin{align*}
\cos{\theta}&={\textstyle\frac{a_1b_1+a_2b_2+a_3b_3}{\sqrt{a_1^2+a_2^2+a_3^2}\sqrt{b_1^2+b_2^2+b_3^2}}}\\
&=\frac{c}{\sqrt{3c^2}\sqrt{1}}=\frac{1}{\sqrt{3}}
\end{align*}
Hence, equation (1) becomes
\sqrt{3}c\times\frac{1}{\sqrt{3}}=2E_{right}\cos{\theta_2}E_{right}\cos{\theta_2}=\frac{c}{2}
One cane verify that E_{right}=c\left(\hat i+2\hat j+2\hat k\right) satisfies the above condition. For this vector find E_{right} and \cos\theta_2.
Hence, answer is (B)
Which of the following transformations \left(V,\vec A\right)\rightarrow \left(V',\vec A'\right) of electrostatic potential V and the vector potential \vec A is a gauge transformation?
\left(V'=V+ax,\vec A'=\vec A+at\hat k\right)
\left(V'=V+ax,\vec A'=\vec A-at\hat k\right)
\left(V'=V+ax,\vec A'=\vec A+at\hat i\right)
\left(V'=V+ax,\vec A'=\vec A-at\hat i\right)
The general gauge transformation of electrostatic potential V and the vector potential \vec A is given by the equations
V'=V-\frac{\partial f}{\partial t}\vec A'=\vec A+\nabla f
In all options, we have, V'=V+ax. This implies
-\frac{\partial f}{\partial t}=ax
Hence, f=-atx
Hence, \nabla f=-at\hat i
Hence, we have, \left(V'=V+ax,\vec A'=\vec A-at\hat i\right)
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