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In order to determine her speed, a skydiver carries a tone generator. A friend on the ground at the landing site has equipment for receiving and analyzing sound waves. While the skydiver is falling at terminal speed, her tone generator emits a steady tone of $1.80 \mathrm{kHz}$ . (Assume that the air is calm, that the speed bf sound is $343 \mathrm{~m} / \mathrm{s}$ , independent of altitude.) (b) If the skydiver were also carrying sound-receiving equipment sensitive enough to detect waves reflected from the ground, what frequency of waves would she receive?

Use a graphing utility to graph

y=\sin x+\frac{\sin 2 x}{2}+\frac{\sin 3 x}{3}+\frac{\sin 4 x}{4}

in a $\left[-2 \pi, 2 \pi, \frac{\pi}{2}\right]$ by $[-2,2,1]$ viewing rectangle. How do these waves compare to the smooth rolling waves of the basic sine curve?

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Question

What is the Poynting vector at a distance of $12.0 \mathrm{~km}$ from a radio tower that transmits $3.00-10^4 \mathrm{~W}$ of power? Assume that the radio waves that hit the Earth are reflected back into space. a) Are the radio waves coming from this transmitter polarized or not? b) What is the root-mean-square value of the electric force on an electron at this location?

Solution

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We will use equation for magnitude of Poynting vector at distance $r$ from the source of electromagnetic waves to solve first part of the problem. We will discuss whether the radio waves emitted from the radio tower are polarized in part $a)$ of the problem. We will calculate root-mean-square value of electric field from the determined magnitude of Poynting vector (intensity) of the electromagnetic waves at mentioned distance. We will multiply obtained rms value of electric field by charge of the electron to obtain root-mean-square value of force on electron on that distance.

The given values are:

P=3 \cdot 10^{4} \mathrm{~W}

r=12 \mathrm{~km}

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