16: Sound Wave : Cbse-xi-physics

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CBSE-XI-Physics

16: Sound Wave

with Solutions - page 2

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Qstn #5
When sound wave is refracted from air to water, which of the following will remain unchanged?
(a) Wave number
(b) Wavelength
(c) Wave velocity
(d) Frequency
digAnsr: d
Ans : (d) Frequency
When a sound or light wave undergoes refraction, its frequency remains constant because there is no change in its phase.
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Qstn #6
The speed of sound in a medium depends on
(a) the elastic property but not on the inertia property
(b) the inertia property but not on the elastic property
(c) the elastic property as well as the inertia property
(d) neither the elastic property nor the inertia property.
digAnsr: c
Ans : (c) the elastic property as well as the inertia property
Propagation of any wave through a medium depends on whether it is elastic and possesses inertia. A wave needs to oscillate (elastic property) for it to be propagated and if it does not have inertia, the oscillations won't keep on moving to and fro about the mean position.
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Qstn #7
Two sound waves move in the same direction in the same medium. The pressure amplitudes of the waves are equal but the wavelength of the first wave is double the second. Let the average power transmitted across a cross section by the first wave be P₁ and that by the second wave be P₂. Then
(a)
P1=P2
(b)
P1=4P2
(c)
P2=2P1
(d)
P2=4P.
digAnsr: a
Ans : (a) `` {P}_{1}={P}_{2}``
Since the average power transmitted by a wave is independent of the wavelength, we have `` {P}_{1}={P}_{2}``.
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Qstn #8
When two waves with same frequency and constant phase difference interfere,
(a) there is a gain of energy
(b) there is a loss of energy
(c) the energy is redistributed and the distribution changes with time
(d) the energy is redistributed and the distribution remains constant in time.
digAnsr: d
Ans : (d) the energy is redistributed and the distribution remains constant in time.
The energy is redistributed due to the presence of interference. However, as the frequency and phase remain constant , the distribution also remains constant with time.
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Qstn #9
An open organ pipe of length L vibrates in its fundamental mode. The pressure variation is maximum
(a) at the two ends
(b) at the middle of the pipe
(c) at distance L/4 inside the ends
(d) at distances L/8 inside the ends.
digAnsr: b
Ans : (b) at the middle of the pipe

For an open organ pipe in fundamental mode, an anti-node is formed at the middle, where the amplitude of the wave is maximum. Hence, the pressure variation is also maximum at the middle.
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Qstn #10
An organ pipe, open at both ends, contains
(a) longitudinal stationary waves
(b) longitudinal travelling waves
(c) transverse stationary waves
(d) transverse travelling waves.
digAnsr: a
Ans : (a) longitudinal stationary waves
An open organ pipe has sound waves that are longitudinal. These waves undergo repeated reflections till resonance to form standing waves.
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Qstn #11
A cylindrical tube, open at both ends, has a fundamental frequency v. The tube is dipped vertically in water so that half of its length is inside the water. The new fundamental frequency is
(a) v/4
(b) v/2
(c) v
(d) 2v.
digAnsr: c
Ans : c) υ
If v is the velocity of the wave and L is the length of the pipe,
then the fundamental frequency for an open organ pipe is
`` \nu =\frac{v}{2L}``
For a closed organ pipe of length L' = L/2, the fundamental frequency is
`` \nu =\frac{v}{4L\text{'}}=\frac{v\times 2}{4\times L}=\frac{v}{2L}=v``
(When the pipe is dipped in water, it behaves like a closed organ pipe that is half the length)
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Qstn #12
The phenomenon of beats can take place
(a) for longitudinal waves only
(b) transverse waves only
(c) for both longitudinal and transverse waves
(d) for sound waves only.
digAnsr: c
Ans : (c) for both longitudinal and transverse waves
When two or more waves of slightly different frequencies (v₁ - v₂ ≯ 10) travel with the same speed in the same direction, they superimpose to give beats. Thus, the waves may be longitudinal or transverse.
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Qstn #13
A tuning fork of frequency 512 Hz is vibrated with a sonometer wire and 6 beats per second are heard. The beat frequency reduces if the tension in the string is slightly increased. The original frequency of vibration of the string is
(a) 506 Hz
(b) 512 Hz
(c) 518 Hz
(d) 524 Hz.
digAnsr: a
Ans : a) 506 Hz

The frequency of the sonometer may be 512 ± 6Hz, i.e., 506 Hz or 518 Hz.
On increasing the tension in a sonometer wire, the velocity of the wave (v) increases proportionately as the number of beats decreases. Therefore, the frequency of the sonometer wire is 506 Hz.
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Qstn #14
The engine of a train sounds a whistle at frequency v. The frequency heard by a passenger is
(a)
> v
(b)
< v
(c)
=1v
(d)
=v
digAnsr: d
Ans : (d) `` =\,\mathrm{\,v\,}``
For the Doppler effect to occur, there must be relative motion between the source and the observer. However, this is not the case here. Hence, the frequency heard by the passenger is υ.
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Qstn #15
The change in frequency due to Doppler effect does not depend on
(a) the speed of the source
(b) the speed of the observer
(c) the frequency of the source
(d) separation between the source and the observer.
digAnsr: d
Ans : (d) separation between the source and the observer
`` {v}_{0}=\left(\frac{v\pm {u}_{0}}{v\pm {u}_{s}}\right){v}_{s}``
It is clear from the equation that the change in frequency due to Doppler effect depends only on the relative motion and not on the distance between the source and the observer.
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Qstn #16
A small source of sounds moves on a circle as shown in figure (16-Q1) and an observer is sitting at O. Let
v1, v2, v3be the frequencies heard when the source is at A, B and C respectively.
(a)
v1>v2>v3
(b)
v1=v2>v3
(c)
v2>v3>v1
(d)
v1>v3>v2
digAnsr: c
Ans : (c) `` {\,\mathrm{\,\nu \,}}_{2}>{\,\mathrm{\,\nu \,}}_{3}>{\,\mathrm{\,\nu \,}}_{1}``

At B, the velocity of the source is along the line joining the source and the observer. Therefore, at B, the source is approaching with the highest velocity as compared to A and C. Hence, the frequency heard is maximum when the source is at B.
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#
Section : iii

Qstn #1
When you speak to your friend, which of the following parameters have a unique value in the sound produced?
(a) Frequency
(b) Wavelength
(c) Amplitude
(d) Wave velocity
Ans : d) Wave velocity
The frequency, wavelength and amplitude do not have a unique value in the sound produced.
The frequency (and wavelength) changes as the pitch of the sound varies, while the amplitude is different as the loudness varies. However, the speed of sound in the air at a particular temperature is constant, i.e., it has a unique value.
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Qstn #2
An electrically maintained tuning fork vibrates with constant frequency and constant amplitude. If the temperature of the surrounding air increases but pressure remains constant, the produced will have
(a) larger wavelength
(b) larger frequency
(c) larger velocity
(d) larger time period.
digAnsr: a,c
Ans : (a) larger wavelength
(c) larger velocity
The velocity varies with temperature as `` v\propto \sqrt{T}``. Therefore, it increases.
Since the frequency remains constant, the wavelength will increase as `` \lambda \propto v``.
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