Question

In an experiment to determine the velocity of sound in air at room temperature using a resonance tube, the first resonance is observed when the air column has a length of 20.0 cm for a tuning fork of frequency 400 Hz is used. The velocity of the sound at room temperature is 336 ms-1. The third resonance is observed when the air column has a length of ______ cm.

Answer 1

The correct answer is 104
$400 = \frac{ν}{4(L_1+e)} ..... (i)$
$400 = \frac{5v}{4(L_2+e)} ..... (ii)$
By using equation (i) and (ii) , we get
$⇒ L1 + e = \frac{λ}{4} = 21 cm$
$L2 + e = \frac{5λ}{4} = 105 cm$
⇒ e = 1 cm & L 2 = 104 cm