Phys141 Principles of Physical Science Chapter 6 Waves Instructor: Li Ma Office: NBC 126 Phone: (713) Webpage: Department of Computer Science & Physics Texas Southern University, Houston Sept. 29, 2004

Waves and Energy Propagation Wave Properties Electromagnetic Waves Sound Waves Skip §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 velocity Ex. 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 oscillation Length 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 position It does not affect the wave velocity The 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 time Unit: 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 length A particle in the medium makes one complete oscillation in a time of one period The frequency and period are inversely proportional: Frequency = 1 / period f = 1 / T v = λ / Torv = 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 (3.00x10 8 m/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 = 3.00x10 8 m/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 = 10 9 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/m 2, joules per second (J/s) through a square meter (m 2 ) The intensity or loudness of sound decreases when the sound waves travel away from the source I ∞ 1/r 2 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 –V sound = 344 m/s The speed of sound increases with increasing temperature V sound = λf, to find the wavelength of a sound wave Sound Waves (cont)