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Model Paper 7 — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysicsModel Paper 71 Mark
MCQ
A point source emits sound equally in all directions in a non-absorbing medium. Two points P and Qare at distances of 2 m and 3 m respectively from the source. The ratio of the intensities of the waves at P and Q is:
  • A
    $4: 9$
  • B
    $2: 3$
  • C
    $3: 2$
  • D
    $9: 4$
✓

Answer

(d) $9: 4$
Explanation: Intensity $=\frac{\text { energy }}{\text { time } \times \text { area }}=\frac{\text { power }}{\text { area }}$
From a point source, energy spreads over the surface of a sphere of radius $r$.
$\therefore$ Intensity, $I =\frac{P}{A}=\frac{P}{4 \pi r^2}$
or $I \propto \frac{1}{r^2}$
$
\frac{I_1}{I_2}=\left(\frac{r_2}{r_1}\right)^2=\left(\frac{3}{2}\right)^2=\frac{9}{4}
$

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