MCQ
A closed organ pipe of radius $r_1$ and an open organ pipe of radius $r_2$ and having same length $'L'$ resonate when excited with a given tunning fork. Closed organ pipe resonates in its fundamental mode where as open organ pipe resonates in its first overtone, then
  • A
    $r_2- r_1 = L$
  • B
    $r_2- r_1 = L/2$
  • $r_2- 2r_1 = 2.5 L$
  • D
    $2r_2- r_1 = 2.5 L$

Answer

Correct option: C.
$r_2- 2r_1 = 2.5 L$
c
For closed organ pipe with end correction: Frequency of different modes,

$\nu_{n}^{\prime}=\frac{(2 n-1) v}{4\left(L+0.6 r_{1}\right)} \quad$ where $0.6 r_{1}$ is the end correction

Now for fundamental mode i.e $n=1, \quad \nu_{1}^{\prime}=\frac{v}{4\left(L+0.6 r_{1}\right)}$

For open organ pipe with end correction: Frequency of $m^{t h}$ mode,

$\nu_{m}=\frac{m v}{2\left(L+2 \times 0.6 r_{2}\right)} \quad$ where $0.6 r_{2}$ is the end correction on one side.

Now for first overtonei.e $m=2, \quad \nu_{2}=\frac{2 v}{2\left(L+1.2 r_{2}\right)}$

But $\quad \nu_{1}^{\prime}=\nu_{2}$

$\frac{v}{4\left(L+0.6 r_{1}\right)}=\frac{2 v}{2\left(L+1.2 r_{2}\right)}$

$L+1.2 r_{2}=4 L+2.4 r_{1} \Longrightarrow r_{2}-2 r_{1}=2.5 L$

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