Phys141 Principles of Physical Science

Chapter 6 Waves

Instructor: Li Ma

Office: NBC 126Phone: (713) 313-7028Email: malx@tsu.edu

Webpage: http://itscience.tsu.edu/ma

Department of Computer Science & PhysicsTexas Southern University, Houston

Sept. 29, 2004

Waves and Energy PropagationWave PropertiesElectromagnetic WavesSound WavesSkip §6.5 & §6.6

Topics To Be Discussed

Waves are all around us:– Sound waves for listening– Light waves for seeing– Radio waves for broadcasting– Microwaves– X-rays– Ocean waves– etc

Two main wave-detecting devices: eyes & ears

Waves

What we learned about energy:– Relationship to work– Conservation– Transfer– Forms

Transfer and propagation of energy in matter are not limited to temperature differences

In common cases, the energy is– transferred from disturbance– propagated as waves

Waves and Energy Propagation

Waves have energy Wave motion: transfer of energy as waves When matter is disturbed, energy emanates

from the disturbance. This propagation of energy is in the form of waves.

Wave propagation may need medium:– Energy is transferred from one particle to another– Only energy, not matter, is transferred by waves

Some waves can propagate without medium:– Electromagnetic waves: ex. sunlight on Earth

Waves and Energy Propagation (cont)

Two categories based on particle motion and wave direction:– Longitudinal wave:

The particle motion and wave velocity are parallel to each other

Ex. Stretched or compressed spring

– Transverse wave:The particle motion is perpendicular to the

direction of the wave velocityEx. all electromagnetic radiation

Wave Properties

Wave characteristics to describe periodic wave motion:– Velocity (v):

Speed and direction of the wave motion

– Wavelength (λ):The distance from any point on the wave to the

adjacent point with similar oscillationLength of one complete “wave”It could affect the wave velocity

Wave Properties (cont)

– Amplitude (A):Maximum displacement of any part of wave

(wave particle) from its equilibrium positionIt does not affect the wave velocityThe energy transmitted by a wave is directly

proportional to the square of its amplitude

– Frequency (f):The number of oscillations or cycles that

occurs during a given period of timeUnit: Hertz (Hz), cycles per second, Hz = 1/s

Wave Properties (cont)

– Period (T):The time it takes for the wave to travel a

distance of one wave lengthA particle in the medium makes one complete

oscillation in a time of one periodThe frequency and period are inversely

proportional:

Frequency = 1 / periodf = 1 / T

v = λ / T or v = λf

Wave Properties (cont)

When charged particles such as electrons are accelerated, energy is radiated away from them in the form of electromagnetic waves

Electromagnetic waves consist of– Vibrating electric field– Vibrating magnetic field

Electromagnetic Waves

In electromagnetic waves– Electric and magnetic fields are vector

fields– The field energy radiates outward at the

speed of light (3.00x108m/s in vacuum)– The electric (E) and magnetic (B) field

vectors are at angles of 90° to one another– The velocity vector of the wave is at an

angle of 90° to both of the field vectors

Electromagnetic Waves (cont)

Different ways to accelerate charged particles could produce electromagnetic waves with various frequencies

A specified frequency range corresponds to one kind of electromagnetic waves

Electromagnetic (EM) spectrum– Figure 6.8 on page 122

Electromagnetic Waves (cont)

Electromagnetic (EM) spectrum– Radio waves: relatively low frequencies– X-rays, Gamma rays: relatively high

frequencies– Visible light: between infrared and

ultraviolet, a very small part of the EM spectrum

Radio waves are not sound waves

Electromagnetic Waves (cont)

Electromagnetic radiation consists of transverse waves

Electromagnetic waves can travel through a vacuum

All electromagnetic waves travel at the same speed in a vacuum– Speed of light: c = 3.00x108m/s

We can use c = λf to find wavelength of any electromagnetic waves

Electromagnetic Waves (cont)

Technically, sound is the propagation of longitudinal waves through matter– This matter could be solid, liquid or gas

The wave motion of sound depends on the elasticity of the medium– A longitudinal disturbance produces varying

pressures and stresses in the medium– A series of high- and low-pressure regions travels

outward, forming a longitudinal sound wave

Sound Waves

Sound spectrum has much lower frequencies and is much simpler– Audible region: 20 Hz to 20 kHz– Infrasonic region: below audible region– Ultrasonic region: above audible region

Sound spectrum has an upper limit due to the elastic limitations of materials– about 1 GHz (gigahertz, billion hertz, so

1 GHz = 109 Hz)

Sound Waves (cont)

Intensity (I) of sound– Measurable physical quantity for loudness of

sound– Rate of sound energy transfer through a given

area– Unit: W/m2, joules per second (J/s) through a

square meter (m2)

The intensity or loudness of sound decreases when the sound waves travel away from the source

I ∞ 1/r2

Sound Waves (cont)

Intensity is commonly measured on a logarithmic scale

The sound intensity level is measured on a decibel level (dB) scale– Units: decibel (dB), bel (B), 1 dB = 1/10 B– Comparison of decibel differences and sound

intensity: Table 6.1 on page 127

Ultrasound: sound waves with frequency greater than 20 kHz– Examining parts of body, alternative to X-rays

Sound Waves (cont)

Speed of sound in a particular medium depends on the makeup of the material

In air at 20°C– Vsound = 344 m/s

The speed of sound increases with increasing temperature

Vsound = λf, to find the wavelength of a sound wave

Sound Waves (cont)