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Physics Resonance: Problem set 45 -->

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Sunday, 25 December 2016

Problem set 45

  1. 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
    1. 3.3\times 10^{-7} watts/m^2 and 10 tesla
    2. 3.3\times 10^{-7} watts/m^2 and 3.3\times 10^{-8} tesla
    3. 0.265 watts/m^2 and 10 tesla
    4. 0.265 watts/m^2 and 3.3\times 10^{-8} tesla
  2. 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
    1. \frac{1}{2}\frac{dk}{dt}\left < x^2\right > +\frac{k}{2m}\left < xp+px\right >
    2. \frac{1}{2}\frac{dk}{dt}\left < x^2\right > +\frac{1}{2m}\left < p^2\right >
    3. \frac{k}{2m}\left < xp+px\right >
    4. \frac{1}{2}\frac{dk}{dt}\left < x^2\right >
  3. 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
    1. \frac{3c}{7}
    2. \frac{4c}{7}
    3. \frac{c}{7}
    4. \frac{\sqrt{3}c}{7}
  4. 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
    1. c\left(2\hat i+\hat j+\hat k\right)
    2. c\left(\hat i+2\hat j+2\hat k\right)
    3. c\left(\frac{1}{2}\hat i+\hat j+\hat k\right)
    4. c\left(\hat i+\frac{1}{2}\hat j+\frac{1}{2}\hat k\right)
  5. 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?
    1. \left(V'=V+ax,\vec A'=\vec A+at\hat k\right)
    2. \left(V'=V+ax,\vec A'=\vec A-at\hat k\right)
    3. \left(V'=V+ax,\vec A'=\vec A+at\hat i\right)
    4. \left(V'=V+ax,\vec A'=\vec A-at\hat i\right)

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