First slide

Travelling wave

Question

The intensity of sound from a radio at a distance of 2 metres from its speaker is $1 \times 0^{- 2} μ W / m^{2} .$ The intensity at a distance of 10 meters would be

Moderate

A

$0 .2 \times 10^{- 2} μ W / m^{2}$
B

$1 \times 10^{- 2} μ W / m^{2}$
C

$4 \times 10^{- 4} μ W / m^{2}$
D

$5 \times 10^{- 2} μ W / m^{2}$

Solution

$I \propto \frac{1}{r^{2}} \Rightarrow \frac{I_{2}}{I_{1}} = \frac{r_{1}^{2}}{r_{2}^{2}} \Rightarrow \frac{I_{2}}{1 \times 10^{- 2}} = \frac{2^{2}}{10^{2}} = \frac{4}{100}$
$\Rightarrow I_{2} = \frac{4 \times 10^{- 2}}{100} = 4 \times 10^{- 4} μ W / m^{2}$

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