Physics 100 Lecture Notes - Sound and Waves

Chapter 9: Sound and waves Physics 100 Lecture Note 1  Wave motion &light Motivation ▪ A really old question: “What is light?” We experience light all the times Our sign depends on light ▪ To understand it needs an understanding of Electromagnetic fiel...

Chapter 9: Sound and waves Physics 100 Lecture Note 1  Wave motion &light Motivation ▪ A really old question: “What is light?” We experience light all the times Our sign depends on light ▪ To understand it needs an understanding of Electromagnetic fields Wave motion Physics 100 Lecture Note 2  What is a wave What is in a wave? ▪ A wave is an oscillation ( of a physical quantity) that travels through a medium (matter or space) and transfer energy ▪ Examples of a wave include Mexican (or spectator) wave in a stadium Water waves (in the sea or kitchen sink) Earthquake Electromagnetic wave (lights, radio and TV signal, etc.) Physics 100 Lecture Note 3  A wave : is a disturbance that carries energy from place to place. A wave does NOT carry matter with it! It just moves the matter as it goes through it. Physics 100 Lecture Note 4  What is a wave Wave carry motion ▪ We are all familiar with waves ▪ At the beach we see the sea water level changing with high and low regions usually moving towards the beach ▪ There are many other examples of wave motion ▪ For example Every sound we hear travels to our ears is the wave Every light that enters your eye travels in the form of a wave ▪ Wave motion is a different form of motion by objects in a fundamental way When objects move, they go from one place to another In wave motion, the medium oscillates such that the wave passes through it In a Mexican wave, people just stand up and then sit down, and the wave passes through the stadium water molecules move in the same way when the water wave passes through them. Physics 100 Lecture Note 5  Some waves do not need matter (called a “medium”) to be able to move (for example, through space). These are called electromagnetic waves (or EM waves). Some waves MUST have a medium in order to move. These are called mechanical waves. Physics 100 Lecture Note 6 Examples of waves Physics 100 Lecture Note 7 Examples of waves Physics 100 Lecture Note 8  Simple waves Description of simple waves ▪ Most waves are complex: they are a mixture of different wave ▪ To understand waves, we select and characterize the simplest possible motion ▪ A simple wave is characterized by Constant speed Perfect repetition Constant height Constant distance between repeating points ▪ We define some quantities that we use to describe a wave Height is called amplitude Distance between nearby similar points is called wavelength (λ) How many waves pass a point in unit time (like 1 second) is called the frequency The energy of a simple wave is proportional to its amplitude Physics 100 Lecture Note 9 Describing simple waves Physics 100 Lecture Note 10  Medium oscillation and wave motion Longitudinal waves ▪ In a longitudinal wave, the particles oscillate along the direction of motion of the wave ▪ In a sound wave, the density (or the pressure) oscillates along the direction of the wave motion as the wave passes Transverse waves ▪ In a transverse wave, the particles oscillate perpendicular the direction of motion of the wave Physics 100 Lecture Note 11 1. Longitudinal or (Compressional) waves: Waves in which the medium moves back and forth in the same direction as the wave Physics 100 Lecture Note 12 - Parts of longitudinal waves: Compression: where the particles are close together Rarefaction: where the particles are spread apart Physics 100 Lecture Note 13 2.Transverse waves: Waves in which the medium moves at right angles to the direction of the wave (perpendicular). Physics 100 Lecture Note 14 - Parts of transverse waves: Crest: the highest point of the wave Trough: the lowest point of the wave Physics 100 Lecture Note 15 Physics 100 Lecture Note 16  Wave Properties Wave properties depend on what (type of energy) is making the waves: 1. Wavelength: The distance between one point on a wave and the exact same place on the next wave. 2. Physics 100 Lecture Note 17  Wave Properties 2. Frequency: How many waves go past a point in one second; unit of measurement is hertz (Hz). The higher the frequency, the more energy in the wave. 10 waves going past in 1 second = 10 Hz 1,000 waves go past in 1 second = 1,000 Hz 1 million waves going past = 1 million Hz Physics 100 Lecture Note 18  Wave Properties 3. Amplitude: How far the medium moves from rest position (where it is when not moving). Remember that for transverse waves, the highest point is the crest, and the lowest point is the trough. Physics 100 Lecture Note 19  Wave motion Wave speed, wavelength & frequency Wave speed = wavelength × frequency Or symbolically, 𝑣 = λf Where v is the wave speed, λ is the wavelength and f is frequency Some wave speeds ▪ Water waves travel at speeds from 1 to 20 m/s ▪ Sound waves travel at speeds from 340 m/s ▪ Seismic (earthquake) waves travel at speeds from 1000 to 14,000 m/s ▪ Electromagnetic waves travel at speeds from 300,000,000 m/s Physics 100 Lecture Note 20  Examples Example 1 ▪ A tuning fork is a metal that can vibrate to generate a sound wave of a specific tone ▪ What is the wavelength of the sound wave produced by a tuning fork whose frequency is 256 Hz? Note that the speed of sound at sea level at 15 C ̊ is 340.3 m/s 𝑣 340.3 λ= = = 1.33 𝑚 𝑓 256 Example 2 ▪ A jumbo jet travels around 1000 km/hr. is this supersonic 1000𝑚 1000 km/hr = 1000 × = 278 𝑚/𝑠 3600𝑠 ▪ Since 278 m/s is less than 340.3 m/s, this plane is not supersonic Physics 100 Lecture Note 21  Examples Example 3 ▪ Fighter jets usually fly at supersonic speeds. What is the minimum speed for a fighter to be flying at supersonic speed? 1𝑘𝑚/1000𝑚 340.3 m/s = 340.3 × = 1225 𝑘𝑚/ℎ𝑟 1ℎ𝑟/3600𝑠 Physics 100 Lecture Note 22 Sound waves, hearing and the human ear Physics 100 Lecture Note 23 Middle ear Physics 100 Lecture Note 24 Internal ear Physics 100 Lecture Note 25  The elements of human hearing ▪ The outer ear consists of the ear canal, and the outer layer of the eardrum ( also called the tympanic membrane) The eardrum moves if there is an imbalance between the outside pressure and inside pressure due to the presence of sound ▪ The ossicles are three small bones (the malleus (hammer), incus (anvil), and the stapes (stirrup)) that are connected to the eardrum They function together to receive, amplify, and transmit the sound from the eardrum to the inner ear ▪ The oval window receives vibrations from the incus bone of the middle ear Vibrations are transmitted into the inner ear into a fluid called endolymph, which fills the membranous labyrinth and eventually transmits the vibrations to the cochlea Hair cells change motion into changes into electrical signal The nerves carry the electrical signal, which is a copy of the received sound, to the brain Physics 100 Lecture Note 26  Sound wave speed vs altitude ▪ The air pressure and temperature decrease with altitude ▪ The speed of sound also decreases with altitude as the table below shows Altitude (m) Temperature ̊C Speed of sound (m/s) 0 (sea level) 15 340.3 1524 5.1 334.4 3048 -4.8 328.4 4572 -14.7 322.2 6096 -24.6 316.0 7620 -34.5 309.6 9144 -44.4 303.1 10668 -56.0 295.4 12192 -56.6 294.9 13716 -56.6 294.9 15240 -56.6 294.9 16764 -56.6 294.9 18288 -56.6 294.9 Physics 100 Lecture Note 27  Loudspeakers ▪ A loudspeaker (or loud-speaker or speaker) is an electroacoustic transducer Which converts an electrical audio signal into a corresponding sound ▪ The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. ▪ When a change electrical audio signal is applied to its voice electromagnet which is suspended in a circular gap between the poles of a permanent magnet, the electromagnet is forced to move rapidly back and forth. ▪ The rapid motion of the electromagnet causes a diaphragm (usually conically shaped) attached it to move back and forth, pushing on the air to create sound waves Physics 100 Lecture Note 28  Microphones ▪ A microphone is a reverse speaker ▪ It is a transducer that converts sound into an electrical signal ▪ The electrical signal it produces is a replica of the sound that it received ▪ The electrical signal can then be either sent directly to a speaker, sent around or recorded for later use Physics 100 Lecture Note 29  Interference ▪ Unlike material objects, waves can pass through each other ▪ Which means the multiple ( two or more) waves can be at the same place Physics 100 Lecture Note 30 Interference of two waves Physics 100 Lecture Note 31  Introduction to electromagnetic waves ▪ I hope by now you have a clear of what a wave is ▪ Electromagnetic radiation consists of electromagnetic waves Which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum ▪ Electromagnetic waves are produced whenever charged particles are accelerated These waves are sensed by charged particles ▪ Electromagnetic waves traveling in space are not the result of oscillations of material particles as is the case with all other types of waves The carry oscillating electric and magnetic force lines (fields) Physics 100 Lecture Note 32  Electromagnetic waves Figure: Radiation Antenna Figure: Electromagnetic waves Physics 100 Lecture Note 33 Electromagnetic spectrum Physics 100 Lecture Note 34  Visible light Physics 100 Lecture Note 35 The structure of the eye Physics 100 Lecture Note 36  Image formation in the eye ▪ A lense is an image-forming object ▪ The eye has a convex lens at the pupil ▪ The size of the image depends on the power of the lens and the distance between the object and the lens The greater the distance between the object and the lens, the smaller the object This is why far away objects seem small to us ▪ Images formed by convex lenses are inverted and this is the case with the eye The brain fixed it up (flips the image) Physics 100 Lecture Note 37 Color and vision Physics 100 Lecture Note 38  Examples Exercise 1 ▪ What is the frequency of red light whose wavelength is 650 nanometer? 𝑣 3 × 108 𝑓= = = 4.623 × 1014 Hz λ 650 × 10−9 Exercise 2 ▪ What is the wavelength of green light whose frequency is 5.45 × 1014 Hz? 𝑣 3 × 108 λ= = 14 = 5.5 × 10−7 m f 5.45 × 10 Physics 100 Lecture Note 39  Sources and uses of electromagnetic waves ▪ Radio waves: Oscillating charges in macroscopic antennas; Used in radio and wireless communications ▪ Microwaves: Oscillating charge is small cavities; Used for long-distance communication. Heating food ▪ Infrared radiation: molecules and hot objects; ▪ Visible light: atoms and molecules and hot objects (like the sun); plant growth ▪ Ultraviolet radiation (UV): atoms and molecules and really hot objects (like the sun); not good for the human body ▪ X-ray: atoms; used for medical purposes (bone imaging etc.) ▪ Gemma rays: Nuclei of atoms Physics 100 Lecture Note 40 Light Scattering by the Atmosphere: Sky Colours Physics 100 Lecture Note 41 Light Scattering by the Atmosphere: Sky Colours Physics 100 Lecture Note 42  Thermal radiation ▪ Atoms in a solid object containing thermal energy are in an unceasing vibrational motion. ▪ Because of motion, the atoms collide frequently When atoms collide, they accelerate ▪ Since atoms carry charge, the charges accelerate when atoms collide The acceleration of the electrons leads to the emission of electromagnetic waves ▪ The motion of the atoms depends on temperature Atoms move faster in a hotter solid and more slowly in a colder solid And the frequencies of the electromagnetic wave they emit when they accelerate depend on the motion ▪ Stronger collision (because of faster speeds) leads to the emission of electromagnetic radiation with higher frequencies This is why when sufficiently heated, metals start glowing The sun emits light because of heat (the temperature at its surface is about 5250 ̊C) Physics 100 Lecture Note 43 Solar Spectrum Physics 100 Lecture Note 44  UV radiation ▪ UV stands for ultraviolet radiation. ▪ UV is a region of the electromagnetic spectrum with frequency above that of violet light (or wavelength shorter than violet light). ▪ A portion of sun’s radiation is in UV frequencies: The UV radiation from the sun is divided into three bands: UVA, UVB, and UVC. UVA includes ultraviolet radiation in the wavelength range from 320-400 nanometers. The atmosphere does not absorb this band. UVB includes ultraviolet radiation in the wavelength range from 280-320 nanometers. Absorbed by the ozone molecules in the atmosphere. Is particularly effective at damaging DNA. UVC includes ultraviolet radiation with wavelength shorter than 280 nanometers. Absorbed by both ozone and regular oxygen. Is extremely dangerous. ▪ Normal oxygen protects us from UVC but not UVB. ▪ Ozone protects us primarily from UVB Physics 100 Lecture Note 45 The atmosphere and UV radiation ▪ UVC radiation is completely stopped by the oxygen in the atmosphere ▪ UVB radiation is stopped by the ozone in the atmosphere almost completely ▪ UVA radiation is not stopped by neither oxygen nor ozone Physics 100 Lecture Note 46  What is ozone? ▪ Ozone (also known as trioxygen) is a triatomic form of oxygen ▪ Chemical formula:𝑂3 ▪ Ozone is a pale blue gas with distinctively sharp smell ▪ 90% of ozone is found in the stratosphere (10-50 km altitudes) ▪ The ozone layer in the stratosphere lies between 25-30 km above the earth ▪ Ozone in the air is a pollutant harmful to humans (smog in large cities) ▪ But in the stratosphere it is the most efficient gas to stop biologically harmful ultraviolet radiation from the sun ▪ A steady decline of about 4% in the total volume of ozone in Earth’s stratosphere (the ozone layer) was observed in the 1970s ▪ At the same time, a much larger springtime decrease in stratospheric ozone around Earth’s polar regions was also observed ( called the ozone hole) Physics 100 Lecture Note 47 Ozone production ▪ Steps in the production of ozone molecules 𝑂2 + 𝑈𝑉(λ < 240𝑛𝑚) → 𝑂 + 𝑂 𝑂 + 𝑂2 → 𝑂3 𝑂3 + 𝑈𝑉 (240 ≤ λ ≤ 320𝑛𝑚) → 𝑂2 + 𝑂 ▪ Ozone absorbs much wider range of UV frequencies than normal oxygen Physics 100 Lecture Note 48  Ozone Depleting ▪ The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N₂O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). ▪ The concentrations of chlorine and bromine increased markedly in recent decades because of the release of large quantities of chlorofluorocarbons (CFCs) and bromofluorocarbons (BFCs). ▪ These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are liberated by the action of ultraviolet light. ▪ Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. ▪ The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. ▪ Consequently, unabsorbed and dangerous ultraviolet radiation (UVB) is able to reach the Earth’s surface at a higher intensity. ▪ For approximately 5 percent of the Earth’s surface, around the north and south poles, much larger seasonal declines have been seen, and are described as “ozone holes”. Physics 100 Lecture Note 49 Ozone Depleting Figure: Ozone production cycle Physics 100 Lecture Note 50  Ozone Depleting Figure: Production of Ozone depleting substance Physics 100 Lecture Note 51 Ozone Depleting Substances Figure: Decrease of the use of ozone-depleting substances Physics 100 Lecture Note 52  The greenhouse effect ▪ The greenhouse effect is a natural process that helps heating the earth’s surface and atmosphere. ▪ CO₂, H₂O, and methane (CH₄) are called greenhouse gases because they absorb long wave (infrared) radiation from the earth’s surface. ▪ Without these gases, the average temperature of the earth would be 33°C lower than what it is now (15°C). ▪ The problem is that our increased consumption of fossil fuels, the concentration of these gases in the atmosphere is increasing. ▪ With more greenhouse gases in the atmosphere, the temperature of the earth is bound to rise. Physics 100 Lecture Note 53 Physics 100 Lecture Note 54  The greenhouse Process (from EPA) ▪ As energy from the sun passes through the atmosphere is causes several things ▪ A portion of the energy (26% globally) is reflected back to space by clouds and particles ▪ About 19% of the energy available is absorbed by clouds, gases ( like ozone) and particles in the atmosphere ▪ Of the remaining 55% of the solar energy passing through the Earth’s atmosphere, 4% is reflected from the surface back to space ▪ On average about 51% of the sun’s radiation reaches the surface ▪ This energy is then used in a number of processes including the heating of the ground surface ▪ The heating of the ground by sunlight causes the earth’s surface to become a radiator of energy in the infrared ▪ This emission of energy is generally directed to ▪ The majority of the outgoing infrared radiation is absorbed by the greenhouse gases ▪ Absorption of this energy causes additional heat energy to be added to the Earth’s Physics 100 Lecture Note 55  Global warming Figure: Graphic description of global warming Physics 100 Lecture Note 56