Question

A point source of power 5 W is emitting sound waves in a medium. If the medium does not absorb any energy, then the intensity of the sound wave at a distance of 5 m from the source is

• A. $5W{m}^{-2}$
• B. $\frac{1}{5}W{m}^{-2}$
• C. $\frac{1}{25}W{m}^{-2}$
• D. $\frac{1}{20\pi }W{m}^{-2}$

Answer 1

The correct option is D $\frac{1}{20\pi }W{m}^{-2}$

Intensity at a distance $r$,

$I_r=\frac{P_o}{4\pi r^2}$ where $P_o$ is the power of the source

.

Given : $P_o=5Wr=5m$

$I_5=\frac{5}{4\pi \times 5^2}$

$⟹I_5=\frac{1}{20\pi }W{m}^{-2}$