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GATE-2018

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GATE 2018

76) A plane electromagnetic (EM) wave traveling vertically downwards with a frequency of 1000Hz in a homogeneous medium has a skin depth of 100m. The ratio of the amplitude of the EM wave at a depth of 75m with respect to the amplitude at the Earth’s surface is ______________

(Thanks to Neeraja, AU)

Solution:

A(Z) = A_o e^{-\beta Z} -----(1)



A(Z) – amplitude of EM at a depth Z

A_o – amplitude of EM wave at Earth’s surface(Z=0) 

\beta – decay constant

Z – depth

The relation between the skin depth(\delta) and decay constant (\beta) is
 

\delta = \frac{1}{\beta}-----(2)

Skin depth, \delta = 503.8\sqrt{\frac{\rho}{f}}-----(3)

\beta = \frac{1}{\delta}=\frac{1}{100}=0.01


Z=75m

Substitute in eq(1)


\frac{A_{(75)}}{A_o}= e^{-\beta Z}= e^{-0.01 \times 75} = e^{-0.75} 

\frac{A_{(75)}}{A_o}=0.472



\therefore the ratio of the amplitudes= 0.472

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4 comments

  1. How could you find the skin depth without knowing resistivity value

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    Replies
    1. Hi srinu, in the above solution skin depth is given directly, just for idea I placed the formula. Here I'm not using frequency and resistivity to calculate the skin depth.
      Please read question once again, and hopefully you will understand.

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