Question Answers
Waves
1. When sound waves travel through a medium, does the temperature at various points remain constant? Explain.
When sound waves travel through a medium, the density of the medium in the space changes and compression and rarefaction are formed. The formation of compression and rarefaction during the propagation of sound is an adiabatic process. Hence, the temperature at various points does not remain constant.
Why does sound travel faster in metals than in air?
The velocity of sound in a medium is given by $$v = - \sqrt{\frac{E}{\rho}}$$
Where E and ρ are the elasticity and density of the solid respectively.
For a metal, the value of E is relatively higher than the density ρ in comparison to air i.e. the ratio E/ρ is more for metal than that for air. Hence, in metal, the velocity of sound is greater than air.
3. The speed of sound in humid air is more than that in dry air, why?
The velocity of sound in air is given by $$v = - \sqrt{\frac{\gamma P}{\rho}}$$
Humid air contains a lot of water vapour in them. Since the density of water vapour is very less compared to pure air, the density of moist air will also be less. Then according to the above relation, the velocity of humid air (damp air) is greater than the dry air.
4. Although the density of solid is high, the velocity of sound is greater in solid, explain.
The velocity of sound in a medium is given by $$v = - \sqrt{\frac{E}{\rho}}$$
Where E and ρ are the elasticity and density of the solid respectively.
For a solid, the value of E is relatively higher than the density ρ i.e. the ratio E/ρ will be more. Hence, in solid, the velocity of sound is greater although its density is high.
5. Do sound waves need a medium to travel from one point to other point in space? What properties of the medium are relevant?
Yes, sound waves need a medium to travel from one point to another point in space. The relevant properties required for the medium are the elastic and inertial properties. The medium should have elastic property so that the particle return to their mean position after being displaced. The medium must also have inertial property because of which the vibrating particles tend to maintain their state of motion (when the particles moves towards the mean position).
6. Why echo cannot be heard in small room?
The human ear can hear echo if the reflected sound appears to be distinct from the original sound. For this the reflected sound wave should be reflected from at least 17m from the source. If the distance is smaller than 17m, the reflected sound cannot be distinguished from the original sound by human ear. A small room is generally less than 17m and thus echo is not heard in it.
7. Distinguish between light waves and sound waves
Light waves are electromagnetic waves. It does not require material medium to propagate from one region of space to another region. Light can travel in vacuum. Sound waves are mechanical waves. It require material medium for the propagation. The particle in the medium oscillate to transfer energy from one region of space to another region.
8. Can two person on the moon hear the sound of each other? Explain
No, the two person on the moon cannot hear the sound of each other. It is because there is no atmosphere on the moon. Hence sound waves cannot propagate as there is no medium. Thus, we cannot hear sound on the moon surface.
9. Which type of wave propagates in liquid? Explain
Liquid molecules are mobile and also can slide (shear) over one another. Besides they are highly incompressible. So, inside the liquid, the waves are longitudinal, the velocity being higher that in air. At the surface however, the waves are transverse.
10. Do sound waves undergo reflection, refraction and polarization phenomenon? Explain.
Sound waves can undergo reflection and refraction but not polarization. Sound wave is a longitudinal wave and thus the particle oscillates parallel to the direction of propagation of wave. Such type of oscillation cannot be controlled using barriers and hence cannot be polarized.
11. A diver under water is unable to hear the sound produced in air, why?
When the sound produced in air is incident on water surface, most of the incident intensity is reflected back into the air and very less intensity is refracted into the water. So, the diver under water is unable to hear the sound.
12. What causes the rolling sound of thunder?
The rolling sound of thunder is caused by the multiple reflections of the sound of lightning between the clouds and the earth's surface.
13. When a stone is thrown on the surface of water, a wave travel out. Where does the energy come from?
As the stone hit the surface of water, it losses most of its KE as it slows down. This KE is then carried by waves in water. Thus, energy in these waves come from the KE of falling stone.
14. What is meant by a non-dispersive medium?
A medium in which speed of a wave is independent of frequency of wave is called a non-dispersive medium. For example, air is a non-dispersive medium for sound waves.
15. Is an oscillation a wave? Give a reason.
No, an oscillation is not a wave. The term wave implies the transfer of energy through successive vibrations of the particles of the medium. Hence the oscillations of the body do not constitute a wave.
16. How is energy transmitted in wave motion?
The neighboring oscillating parts of the medium are coupled together through elastic forces or intermolecular forces. During wave motion, a part of the medium is set into oscillation. This part hands over its motion to the next part of the medium due to the forces and so on. This results in transmission of energy.