Q. The intensity of sound from a radio at a distance of $2\ metres$ from its speaker is $1 \times 10^{- 2}\mu W/m^{2}.$ The intensity at a distance of 10 meters would be
A) $0.2 \times 10^{- 2}\mu W/m^{2}$
B) $1 \times 10^{- 2}\mu W/m^{2}$
C) $4 \times 10^{- 4}\mu W/m^{2}$
D) $5 \times 10^{- 2}\mu W/m^{2}$
Explanation
(c) $I \propto \frac{1}{r^{2}} \Rightarrow \frac{I_{2}}{I_{1}} = \frac{r_{1}^{2}}{r_{2}^{2}} \Rightarrow \frac{I_{2}}{1 \times 10^{- 2}} = \frac{2^{2}}{10^{2}} = \frac{4}{100}$
$\Rightarrow I_{2} = \frac{4 \times 10^{- 2}}{100} = 4 \times 10^{- 4}\ \mu\ W/m^{2}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.