G10 Ch7 Waves and Sound PDF

CHAPTER 7 WAVE AND SOUND When we think of the word waves, water wave on the water surface of a pond and sea waves usually come to mind. Besides these waves there are other types of wave such as sound wave, radio waves, etc. Wave is a basic concept of physics. Energy and momentum are transferred thro...

CHAPTER 7 WAVE AND SOUND When we think of the word waves, water wave on the water surface of a pond and sea waves usually come to mind. Besides these waves there are other types of wave such as sound wave, radio waves, etc. Wave is a basic concept of physics. Energy and momentum are transferred through the medium from the wave source. All waves are produced by a vibrating source. Learni ng Outco mes It is expect ed that studen ts will ⚫ examin e wave motion as a form of energy transfe r. compare transverse and longitudinal wave and give suitable examples of each. illustrate displacement-time graph and displacement-position graph. express the concept of wave equation and use it to solve problems. describe the reflection, refraction and diffraction of waves. apply the basic knowledge of generation, propagation and hearing of sound in daily life. 7.1 DESCRIBING WAVE MOTION Wave motion is a method of transferring energy by successiv e disturbanc es through the medium. This movement of energy takes place without transferring matter. For examples, (1) Waves are produced if you drop a stone onto a  quiet surface of a pond. The waves spread out from the point of impact, carrying energy to all parts of the pond (Figure 7.1). But the water in the pond does not move from the centre to the edges. This shows that wave transfer energy without transferring matter. Figure 7.1 Water wave on the surface of a pond [Physics Matter] 66 Textbook Physics Grade 10 (2) Waves can be produced along a rope with one fixed end and moving the other end up and down rapidly shown in Figure 7.2. It can be seen that the rope waves move toward the fixed end, while the rope segments only vibrate up and down about their rest (equilibrium) position. The energy from hand is transferred by the rope waves toward the fixed end. is the medium through which the waves move. The rope direction of wave and sea S Sound re trans rating s rope each. nd in da hrough pond. Figure 71 that wa direction of vibration Figure 7.2 Producing wave on a rope There are two types of waves. They are mechanical waves and electromagnetic waves. The mechanical waves need material medium to propagate and cannot pass through vacuum. Sound waves and seismic waves, which are produced by an earthquake, are mechanical waves. Electromagnetic waves can pass through vacuum and they do not need medium for propagation. Light wave and X-rays are electromagnetic waves. Reviewed Exercise  Give exam ples of mecha nical and electro magne tic waves. Key Words : disturb ance, energy , vibrati on electro magneti c waves 7.2 TRANSVERSE AND LONGITUDINAL WAVES 919-25 ဂုတ်ကျ कृमि र Waves are classified as transverse and longitudinal waves depending on vibration of particles in the medium through which they propagate. 567. If the displacements of particles of the medium are perpendi cular to the directio n of the wave,  such a wave is called a/transv wave. Waves in a vibratin g string are transv erse waves. They can be demonstrated by moving up and down the free end of a rope (or) slinky spring which is fitted at one end as shown in Figure 7.3. Light waves and other electro magneti c waves are also transverse waves. movement of spring  direction of tranfer of energy along spring Figure 7.3 The transverse waves on slinky coiled spring ich a sound nd of a vement Vater Ows the uced by  ves 7.3 CHARACTERISTICS OF WAVES This section will discus s some quantit ies of periodi c waves. Wave Crest and Troug h of Perio dic Wave s The highest and the lowest points (Figure 7.6) which show the maximum displaceme nt of vibrating particle from its rest position (or) equilibrium line are called wave crest and wave trough respectively.  The arrows indicate the direction of displacemen t of the vibrating particle. his: Bonj Wavelen gth (2): The distance between any two consecuti ve wave crests (or) two consecuti ve wave troughs is called wavelengt h. The unit of wavelengt h in SI unit is metre (m). งา Generally the wavelength is the distance between two nearest points of same phase. dis wavelength 入 -78 Jis plament crest trough crest direction of motion of a particle trough distance distance-23060: Figure 7.6 Wave in a vibrating string moona (noz ( တစ Frequency (f): The number of complete waves passing a point per second is called frequency) of waves. The frequency of the wave depends on the vibrating source. The number of oscillation of a vibrating source in one second is also called frequency. The SI unit of frequency is hertz (Hz). One hertz is equal to one complete cycle per second. at /=0 s at/= 1 s AA f = 4 Hz ome second on Figure 7.7 Frequency of a periodic wave (four complete waves pass a point in one second) 69 Grade 10 Physics Textbook If the displacements of particles of medium are parallel to the direction of the waves, such a wave is called a longitudinal wave. Compressional waves in a slinky coiled spring and sound waves are longitudinal waves. A longitudinal wave is demonstrated by rapidly pushing forth and pulling back at one end of a slinky coiled spring while another end is fixed. It can be seen that the back and forth movement of the coil is parallel to the wave direction as shown in Figure 7.4. movement of spring direction of transfer of energy along spring compression extension 00000 Figure 7.4 Longitudinal waves on slinky coiled spring Some waves in nature exhibit a combination of transverse and longitudinal waves. Water waves are good example of combinational waves. The longitudinal slinky spring wave is represented by a graph (Figure 7.5) which shows the compression and extension of spring segments. This graph is similar to the wave produced by the vibrating rope shown in Figure 7.2. movement of spring direction of transfer compression extension of energy along spring Wavelength. Reviewed Exercise Figure 7.5 Graphical presentation of longitudinal wave Describe the similarit ies and differen ces between sound waves and water waves.  Key Word s: transv erse waves , longit udinal wave s, comp ressio n, extens ion 68 3061 Grade 10 Physics Period (T): The time taken by the wave to travel the distance between any two consecutive wave crests (or) the time required for one complete vibration is called period of a wave. The unit of period is the second (s). Thess The period is the reciprocal or inverse of the frequency. Thus, 1 T f at 1=0s ‫للة‬ at 1=Ts Textbook Figure 7.8 Period of the wave 32g 291207 Amplitude: The amplitude of a wave is the maximum value of displacement of vibrating particle. It can be seen on the wave graph shown in Figure 7.6 as the perpendicular distance from the equilibrium line to the wave crest (or) trough. Velocity of Wave (v): Velocity of wave is the speed with which a wave crest travels. The unit of wave velocity is metre per second (m s1). Most of the periodic waves are represented by sine (or) cosine graphs. Therefore they can be called sine waves. The relationship between the frequency, wavelength and velocity of a periodic wave can be obtained by velocity = distance moved >>> INDZD time taken A complete wave travels through the distance equal to its wavelength in time period T. velocity = wavelength period Since, 2 T 1 T= f v = fλ well Sol-ge Reviewed Exercise Write down the relation between period and frequency. Explain it. Key Words: hertz, oscillation 7.4 GRAPHICAL REPRESENTATION OF WAVE Displacement-Distance Graph A displacement-distance graph describes the displacement of all particles at a particular instant of time. Note that displacement of particle is plotted on y-axis and distance moved by the wave is on x-axis. 70 stant wave 아 Textbook Displacement (cm) 0.6- 0 Physics 10 Distance(m) Grade 10 ing nce (0.6 Figure 7.9 Displacement - distance graph The arrows shown on the graph indicate the direction of the displacement of vibrating particle. According to the graph the amplitude is 0.6 cm and wavelength is 5 m respectively. Displacement-Time Graph A displacement-time graph describes the displacement of particle of a certain position as a function of time taken to travel by a wave. Note that displacement of particle is on Y axis and time interval is on X axis. unit Displacement (cm) (0.6 0. 0 0.6+ 10.5 Figure 7.10 Displacement - time graph Time(s) be According to the graph, 1 respectively. Since T the amplitude and the period of the wave are 0.6 cm and 0.5 s the frequency of the wave is 2 Hz. f 7.5 REFLECTION, REFRACTION AND DIFFRACTION OF WAVE Wave can undergo reflection, refraction and diffraction. These phenomena are usually studied by means of water wave in a ripple tank. Ripple Tank: The ripple tank is a convenient piece of apparatus for demonstrating the properties of waves. Wavefront: The surface that joins all the points of same phase is called wavefront. Direction of Movement Waves Wavefronts (a) (b) Figure 7.11 The wavefronts of (a) plane wave and (b) circular waves  71 Grade 10 Reflection When a series of wave strike an obstacle, they are turned back. This turning back of waves is called reflection of waves. When waves strike a straight barrier, the waves are reflected from the barrier. The angle of incidence is equal to the angle of reflection as shown in Figure 7.12. The wavelength and velocity of the wave remain constant in reflection of wave. wavefronts approach direction of travel of the wave before reflection the barrier 45 45 straight barrier wave direction before striking barrier angle of incidence normal angle of reflection reflected wavefronts  Figure 7.12 Reflection of plane wave concave barrier direction of the wave after reflection F Figure 7.13 (a) Reflection of plane wave on concave surface wave direction waves converge after striking concave barrier before striking barrier Figure 7.13 (b) Radio antenna [https://en.m.wikipedia.org/wiki/ yevpatoria-RT-70-radio-telescope] convex barrier F waves diverge after striking convex barrier  Figure 7.14 Reflection of plane wave on convex surface 72 waves cted from ure 712 Textbook Physics Grade 10 antenna wiki lescope transmitter ionosphere receiver Refraction Figure 7.15 Reflection of radio waves The speed of water waves depends on the depth of water. It decreases when the depth of water becomes less deep. By the ripple tank experiment, when water waves pass from deep to shallow water the velocity of wave is lesser and the wavelength is shorter or vice-versa. When waves are incident to the boundary with an angle, the direction of the waves changes. Such a change in direction is called refraction. It can be noticed that in refraction, the wavelength and velocity change but frequency remains the same. Diffraction Deep Water Shallow Water Figure 7.16 Refraction of plane wave Diffraction is the spreading of waves from the straight-on direction through a gap (or) moves around an obstacle. The wave that passes the edges of the gap of the obstacle spread out. Figure 7.17 and 7.18 show water waves in ripple tank spreading out after they pass through the gap. In Figure 7.17 the wider the gap, the less the waves spread out. In Figure 7.18 the narrower the gap, the more the waves spread out. Note that the wavelength does not change after diffraction. wavefront most waves continue on in the same direction  no waves here Figure 7.17 Diffraction of water waves through wide gap most of the waves have changed direction They have been diffracted Figure 7.18 Diffraction of water waves through narrow gap Figure 7.19 Diffraction of radio waves Diffraction around an obstacle includes radio waves that are diffracted as they pass over the hill. (Figure 7.19) Reviewed Exercise 1. What causes refraction of water waves? 2. Does the speed of water waves depend upon the depth of water? Key Words: ripple tank, wavelength 7.6 SOUND WAVE AND SPEED OF SOUND Sound Wave Sound is a form of energy that is transferred from one place to another in a certain medium. Sound wave is produced by a vibrating object placed in a medium. The pressure changes occur alternately in the medium by vibrating object. The medium is usually air, but it can be any gas, liquid (or) solid. Sound wave propagates as a series of compression and rarefaction like longitudinal waves on a vibrating spring. Like other waves, sound wave can be reflected and diffracted. Unlike electromagnetic waves, sound waves need a medium to propagate. Sound wave cannot travel through vacuum. The compression is created in the medium as the vibrating object moves forward, since it pushes molecules together. The compression region has higher pressure. When the object moves back, the molecules are spread out and rarefaction is created and the pressure of that region is low. After the object is vibrated several times, it has created a series of compression and rarefactions travelling away from the vibrating object. The pressure of the medium is changed into  higher and lower alternately. In this way, sound energy propagates through the medium to the ear. When waves enter the ear, they strike the ear drum and make it vibrate. This vibration of ear drum results the hearing of the sound. Sound energy is transferred through the medium by the successive pressure changes among the adjacent parts without moving the medium as a whole. vibrating speaker cone rarefaction ))) compression Figure 7.20 Vibrating loud speaker produces sound wave and travels through air to ear 71 Textbook Physics loudspeaker air pressure is high air pressure is low vibrating diaphragm compression rarefaction high  low distance Grade 10 Occur any like and ace it bject Wavelength Wavelength Figure 7.21 Pressure-distance graph of a propagated sound  waves Audible range: The average person can only hear sound that has a frequency higher than 20 Hz and lower than 20 000 Hz. This interval of frequency is called the audible range (or) hearing range. But the range becomes reduced according to age and health conditions. Sound waves with frequencies greater than 20 000 Hz are called ultrasounds. Some objects vibrating with frequencies under 20 Hz produces sound which cannot be heard by human. This is called infrasound. It is found that, by experiments: dog, bat and dolphin can hear the ultrasound and they communicate with it. On the other hand, elephants can communicate with infrasound. Speed of Sound Since sound propagates from one place to another through a distance in a time interval in a given medium, the speed of sound is speed of sound = distance travelled by sound time taken that SION m is the This ough g the V= t However, sound waves travels at different speeds in different media through which it passes. Generally, the speed of sound depends on the density of the medium. The denser the medium is, the greater the speed because the particles of the medium are tightly bound together. This means that the disturbance can be transferred more quickly from one particle to next. Table 7.1 The speed of sound in some solids, liquids and gases Medium m sl Speed ft s Temperature °C Air 332  1090 0 CO2 259 850 0 Cl, 206 676 0 Water, pure 1 404 4 605 0 Copper 3560 11 680 20 Iron 5130 16.830 20 75