Question

A radio transmitter sends out $60\, W$ of radiation. Assuming that the radiation is uniform on a sphere with the transmitter at its centre, the intensity (in $W/m^2$) of the wave at a distance $12\, km$ is

• A. $5.33 \times 10^{-8}$
• B. $3.33 \times 10^{-6}$
• C. $2.12 \times 10^{-8}$
• D. $6.66 \times 10^{-8}$
• E. $3.33 \times 10^{-8}$

Answer 1

Answer: (E)

Power of transmitted radiation, $P_{T}=60\, W$.
$\therefore $ Intensity of wave, $I=\frac{P_{T}}{A}$
$\because A=4 \pi R^{2}$
$4 \pi(12 Km )^{2} (\because R=12\, km$ given $)$
$\Rightarrow A=4 \pi \times 144 \times 10^{6}$
$\therefore I=\frac{60}{4 \pi \times 144 \times 10^{6}}$
$=3.31 \times 10^{-8}\, W / M ^{2}$