Lecture physics a2 waves

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

1 Mechanical waves

Transverse, longitudinal, surface waves Speed of waves in solids, liquids, gases Energy of Mechanical waves

2 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

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

Wave that causes the

elements of the disturbed

medium to move

perpendicular to the direction

of propagation is called a

transverse wave

A longitudinal pulse along a stretched spring The displacement of the coils is parallel to the direction of the propagation

(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

Characteristics of waves:

-Wavelength

-Frequency

-Period

-Amplitude

-Depends on the property of the medium: BULK MODULUS B , and DENSITY  -Does not depend on the wave characteristics: Amplitude, frequency, period





property inertial

property

elastic

v

The ratio of the change in pressure to the fractional volume compression is called the bulk modulus of the

material

P

V / V B

1

: ility Compressib

) m / N

( V / V

P B

: Modulus

















Bulk modulus B

(N/m 2 )

Density  (kg/m3)

Sound speed (m/s) Steel:160 x 10 ^9 7860 4512

Water: 2.2 x 10^9 1000 1483

Solid is difficult to be compressed, V/V small,

B is large

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





property inertial

property

elastic v

Sound speed

gas liquid

solid

gas liquid

solid

gas liquid

solid

v v

v

B B

B



















Air :  =1.4, T=300K,

 =29 g/mol, R=8.31 J/mol K

V= 347m/s

Sound Speed in

an ideal gas





v

Where:

: adiabatic constant

Gas constant: R=8.31 J/mol K

: molecular mass of gas (g/mol)

T: absolute temperature (K)















































RT B

v

RT

P V

m

P V /

| dV

|

dP B

) 3 ( 0 dP V dV V

P

(2) const PV

(1) RT

m nRT

PV

1

Ideal gas law The process of sound travelling through an ideal gas can be expected to be

adiabatic and therefore the pressure and volume obey

the relationship Differentiate Bulk modulus

Density:

Sound speed

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.

L

M

s / m 3

38 10

2

4 29 v

) N ( 4 29 8

9 3 Mg F

) m / kg ( 10

2 m

5 2

kg 10

50 L

m

2

2 3































 Linear mass density

Tension

speed of wave on the

string

Speed of waves on a string

F: Tension (N)

: the linear mass density (mass per unit length (kg/m)



v

Example 15-1

The tension in a string is provided by hanging an object of mass M 3 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?

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