WAVE Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents 1 Mechanical waves Transverse, longitudinal, surface waves Speed of waves in solids, liquids, gases Energy of Mechanical wav[.]
WAVE Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents Mechanical waves Transverse, longitudinal, surface waves Speed of waves in solids, liquids, gases Energy of Mechanical waves Sound Wave Intensity (dB), threshold of hearing , pain threshold Doppler effect The essence of wave motion - the transfer of energy through space without the accompanying transfer of matter Two mechanisms of energy tranfer depend on waves: - Mechanical waves - Electromagnetic radiation All mechanical waves require (1)some source of disturbance, (2)a medium that can be disturbed, (3)and some physical mechanism through which elements of the medium can influence each other Transverse wave – Longitudinal wave – surface wave A longitudinal pulse along a stretched spring The displacement of the coils is parallel to the direction of the propagation Wave that causes the elements of the disturbed medium to move perpendicular to the direction of propagation is called a transverse wave The motion of water elements on the surface of deep water in which a wave is propagating is a combination of transverse and longitudinal displacements, with the result that elements at the surface move in nearly circular paths Each element is displaced both horizontally and vertically from its equilibrium position Characteristics of waves: -Wavelength -Frequency -Period -Amplitude (a) The wavelength of a wave is the distance between adjacent crests or adjacent troughs (b) The period T of a wave is the time interval required for the wave to travel one wavelength Speed of Sound -Depends on the property of the medium: BULK MODULUS B , and DENSITY -Does not depend on the wave characteristics: Amplitude, frequency, period elastic property B v inertial property P Bulk Modulus : B (N / m2 ) V / V V / V Compressibility : B P Bulk modulus B (N/m2) Density (kg/m3) Sound speed (m/s) Steel:160 x 10 ^9 7860 4512 Water: 2.2 x 10^9 1000 1483 The ratio of the change in pressure to the fractional volume compression is called the bulk modulus of the material Solid is difficult to be compressed, V/V small, B is large Sound speed Gases v (m/s) Hydrogen (0°C) 1286 Helium (0°C) 972 Air (20°C) 343 Air (0°C) 331 Liquids at 25°C Glycerol 1904 Sea water 1533 Water 1493 Mercury 1450 Solids Diamond 12000 Pyrex glass 5640 Iron 5130 Aluminum 5100 Copper 3560 Gold 3240 Rubber 1600 v elastic property B inertial property Bsolid Bliquid Bgas solid liquid gas v solid v liquid v gas Sound Speed in an ideal gas v RT Ideal gas law PV nRT m RT (1) The process of sound PV const (2) travelling through an ideal gas can be expected to be adiabatic and therefore the pressure and volume obey the relationship Where: : adiabatic constant Gas constant: R=8.31 J/mol K : molecular mass of gas (g/mol) T: absolute temperature (K) Air : =1.4, T=300K, =29 g/mol, R=8.31 J/mol K Differentiate PV 1dV V dP (3) dP P | dV | / V m P Density: V RT Sound speed B RT v Bulk modulus B V= 347m/s The adiabatic assumption for sound waves means that the compressions associated with the sound wave happen so quickly that there is no opportunity for heat transfer in or out of the volume of air Speed of waves on a string v F F: Tension (N) : the linear mass density (mass per unit length (kg/m) Example 15-1 The tension in a string is provided by hanging an object of mass M kg at one end as shown in Figure 15-4 The length of the string is L 2.5 m and its mass is m 50 g What is the speed of waves on the string? L m 50 10 3 kg 10 (kg / m) L 2.5m Tension F Mg 9.8 29.4( N ) 29.4 v 38.3m / s speed of wave on the 10 Linear mass density string M Frequency of Sound wave For Human being, Sonic frequencies: : 20Hz – 20 000Hz Ultrasonic frequencies: > 20 000Hz Infrasonic frequencies: < 20Hz Ultrasound can be used in medical imaging, detection of defects in objects Similar to light waves, sound waves can have Interference, Diffraction, Reflection phenomena , but different from light waves, sound wave is a longitudinal wave, it can not have polarization ...Contents Mechanical waves Transverse, longitudinal, surface waves Speed of waves in solids, liquids, gases Energy of Mechanical waves Sound Wave Intensity (dB), threshold... transfer of matter Two mechanisms of energy tranfer depend on waves: - Mechanical waves - Electromagnetic radiation All mechanical waves require (1)some source of disturbance, (2)a medium that... detection of defects in objects Similar to light waves, sound waves can have Interference, Diffraction, Reflection phenomena , but different from light waves, sound wave is a longitudinal wave, it