1 \begin{problem*}{24.22} % power <-> EM fields
2 An AM radio station broadcasts isotropically (equally in all
3 directions) with an average power of $4.00\U{kW}$. A dipole recieving
4 antenna $65.0\U{cm}$ long is at a location $4.00\U{miles}$ from the
5 transmitter. Compute the amplitude of the emf that is induced by this
6 signal between the ends of the recieving antenna.
10 To find the signal intensity at our antenna, we note that the power
11 broadcast from the station is spread out over a sphere of radius
12 $R=4.00\U{miles}$. The average intensity is then
14 I = S_\text{avg} = \frac{P}{A} = \frac{P}{4\pi R^2}
15 = \frac{4.00\E{3}\U{W}}{4\pi(4.00\U{miles}\cdot 1.609\E{3}\U{m/mile})^2}
16 = 7.68\U{$\mu$W/m$^2$} \;.
18 From Equation 24.27, we see
20 S_\text{avg} &= \frac{E_\text{max}^2}{2\mu_0 c} \tag{24.27} \\
21 E_\text{max} &= \sqrt{2\mu_0 c S_\text{avg}} = 76.1\U{mV/m}
23 The total voltage difference produced across our length $L=65.0\U{cm}$
26 \Delta V = LE_\text{max} = \ans{49.4\U{mV}}