How Things Work II
How Things Work II PHYS 6060
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This 38 page Class Notes was uploaded by Ubaldo Jacobson on Monday September 21, 2015. The Class Notes belongs to PHYS 6060 at University of Virginia taught by Staff in Fall. Since its upload, it has received 68 views. For similar materials see /class/209757/phys-6060-university-of-virginia in Physics 2 at University of Virginia.
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Date Created: 09/21/15
Lecture 18 1 As a child I was desperate to learn to whistle When a person whistles they purse their lips and force a stream of airthrough the small hole formed The pressure of the air stream outside the lips is A lower than the pressure in the mouth and higher than in the hole formed by the lips B higher than the pressure in the mouth and lower than in the hole formed by the lips C lowerthan the pressure in the mouth and lowerthan in the hole formed by the lips D higherthan the pressure in the mouth and higher than in the hole formed by the lips Air is a fluid and the air in the mouth has a higher pressure since the cheeks are acting to help compress the air much like a pump As the air flows through the constriction of the lips it speeds up and the pressure drops Once outside the mouth the air stream slows and pressure rises to atmospheric pressure Lecture 19 2 The Naval shipyard in my hometown builds a miniature stealth submarine that is being tested in the waters that flow eastward just off the coastline The submarine is coated with a special paint to reduce drag While pointing west the submarine exerts just enough thrust to maintain its position in the slow moving current The pressure on the submarine is A greater at the head and tail but less at the sides B less at the sides and head but greater at the tail C greater at the sides and head but less at the tail D less at the head and tail but greater at the sides In a slow current the shape and coating of the submarine like that of a dolphin encourages laminar flow around the surface so the pressure decreases along the sides of the sub but increases at the front and back Lecture 20 3 Parasailing where a person attached to a parachutetype canopy is pulled by a boat and lifted into the air is a popular sport in the coastal United States where live To maximize the distance one is lifted into the air the design of the parasail should encourage A turbulent wake flow where the air pressure is lower on the top of the sail B laminar flow where the air pressure is greater at the top and bottom of the sail C turbulent wake flow where the air pressure is higher on the top of the sail D laminar flow where the air pressure is lower at the top and bottom of the sail A turbulent wake at the top of the parasail means lower air pressure at the top while the motion of the boat pulling the parasail forward creates high pressure underthe parasail Both actions create the lift needed to send the parasailer skyward Lecture 21 Hurricanes are a fact of life if you live in the southern coastal regions of the United States as I do The wind speed of a hurricane often causes the roofs of houses to be lifted off the frame of the house The primary reason this occurs is because the hurricane s wind speed A creates low pressure at the roof peak while atmospheric pressure remains in the attic B over the roof creates higher than atmospheric pressure in the attic C impacts the sides of the house creating turbulence and high pressure lift D creates low pressure drag along the sides of the house The speed of the wind increases as it flows over the roof of the house creating an area of low pressure above the roof The pressure in the attic remains at ambient atmospheric pressure and the pressure imbalance causes the roof to lift off the house Lecture 22 The Navy Blue Angels jet team put on quite a show at the Pensacola Florida Naval Air Station near my home Smaller propellerdriven airplanes tend to have wings with large surface areas while comparably sized fighter jets like the Navy Blue Angels have greater weights but wings with smaller surface areas than prop planes Which of the following must a jet have as compared to a propeller driven plane A faster takeoff and landing speeds B a more curved asymmetric wing C less kinetic energy transfer between the wing and air D a longer and thicker profile to their wings Because of the smaller wing area but the greater overall weight of a jet there must be a much greater airflow over the wing to induce the necessary lift so the jet accomplishes this by greater speeds in flight while taking off and when landing A prop plane wing actually gets more lift due to the wing curvature and asymmetry while a jet gets lift as a result of speed so its wing is less curved and more symmetric The speed of the jet will impart a greater kinetic energy to the air during flight and since a fighterjet needs maximum speed its wings have a shorter and narrower profile to decrease drag Lecture 23 The de Laval nozzle is the most effective nozzle shape currently known for the exhaust of a rocket and is used on the space shuttle which is tested at Stennis Space Centerjust a few miles from my home The de Laval exhaust has a converging shape that then diverges into a larger cone The exhaust gases in the de Laval nozzle A are pushed backwards by the rocket and give the rocket forward momentum B push backwards on the air and impart a forward acceleration to the rocket C have a higher kinetic energy outside the rocket at liftoff than inside the rocket while in flight D transfer all their momentum but only part of their kinetic energy to the rocket after combustion The rocket s propulsion occurs when the exhaust gases are pushed rearward and from Newton s Third Law the gases push forward which gives the rocket a forward acceleration and forward velocity which results in a forward momentum Lecture 24 A spacecraft that is orbiting the earth has a trajectory that can be elliptical depending on its inertia and the earth s gravity When the satellite is at position B it is two times as far from the earth s center as when it is closest to the earth The force of gravitational attraction between the earth and the satellite at position B is A onefourth as strong as when closest to the earth B onehalf as strong as when closest to the earth C oneeighth as strong as when closest to the earth D onesixteenth as strong as when closest to the earth According to Newton s Law of Universal Gravitation the force of gravitational attraction is inversely proportional to the square of the distance between the objects So if the satellite is twice as far away it has onefourth the attraction If three times as far away it has oneninth the attraction Lecture 25 Some of my relatives live in Michigan where whole home heaters send heated air through ducts to vents located in the house to warm the entire house For houses that are built in colder areas of the country the vents should be located A in the walls close to the floor so air is pushed horizontally into the room B in the ceiling so the air is pushed downward into the room C in the top of the room s corners so the air flows diagonally into the room D in the floor so the warm air is pushed upwards into the room Since warm air rises the best way to warm a room is to direct the air horizontally across the top of the floor so the warm air molecules spread out and then rise warming a larger volume of the room Air from the floor vents simply rises straight up in a stream to the ceiling leaving most of the room still chilled Warm air from above never makes it to the lower part of the room due to the higher density of the cold air there Lecture 26 Camping is a tradition where live Two campfires are lit at a campsite on a cold night One campfire has been burning for a while and has smaller logs but you see a lot of glowing coals underthe wood logs The second campfire is about twice the size of the first and has larger wood and no glowing coals but a fire is licking out from in between the stacked logs To be the warmest you sit next to A the smaller campfire because the glowing coals emit the most infrared radiant heat B the smaller campfire because you can sit closer and get maximum heat radiation C the larger campfire because a larger fire gives off more heat radiation D the larger campfire because the larger wood has the most thermal energy The glowing coals in a fire are the hottest part of the fire and emit the most radiant heat energy The larger fire has no coals and most of the radiant heat of the fire is blocked off by the wood since only some of the fire can be seen between the logs so little heat energy is being radiated Convection is primarily upwards instead of to the sides so it has a limited effect on you Lecture 27 Thomas Edison used carbon filaments in his light bulbs but today incandescent light bulbs use tungsten filaments A new filament is being tried at a factory that needs longer lasting and brighter light from its bulbs For best results the filament in the bulb should A have a higher melting point and sublime less than regularfilaments to extended life and brightness B have a thicker filament to limit subliming which will give it more surface area and more brightness C have a higher melting point but higher surface area so it radiates light better for a longer time D be a metal with few free electrons which increases thermal radiation and longevity At higher temperatures the tungsten filament sublimes at a greater rate and may melt if it gets too hot so a filament that melts at a higher temperature and sublimes less at highertemperatures than tungsten would give a brighter light for a longer time and is the best choice Lecture 28 A halogen bulb emits whiter light than a regular incandescent bulb The halogen bulb does this by A operating at a higher temperature so the spectrum center is shifted toward visible light B increasing the emissivity of the filament in the infrared spectrum which increases its brightness C stimulating the halogen gas to emit photons of light as the gaseous atoms are electrically excited D initiating a chemical reaction between the metal filament and halogen gas with light as a product By operating at a higher temperature the spectrum center is shifted toward visible light since more thermal energy is released in the visible light spectrum and the bulb appears brighterto the human eye Lecture 29 As a representative for a company used to travel a lot and often carried an electric cooler The electric cooler plugs into your car lighter outlet and acts like a small refrigerator On really hot days I would open the cooler door on the side and I could feel the cold airflow out of the cooler Which of the following happened when I left the cooler door open A the interior of the car gets warmer and the entropy inside the car increases B the interior of the car gets cooler and the entropy increase is offset by the decrease in thermal energy of the air C heat expelled by the cooler is offset by the cold air so the interior temperature does not change D The heat expelled is less than the cold air added and total entropy decreases Total energy expelled equals heat removed plus work done to remove it so leaving the cooler door open means more heat has to be removed from the cooler interior To remove heat from the inside of the cooler takes work from ordered electrical energy and the work done to remove a single unit of thermal energy from the cooler results in more than one unit of thermal energy being transferred to the atmosphere as disordered energy The increase in disordered thermal energy means the temperature of the car interior will increase and the entropy inside the car must also increase University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 9 Slides Electronic Air Cleaners Kinetic Energy 0 A moving bumper car has kinetic energy Kinetic energy 2 Mass Speed2 0 A spinning bumper car has kinetic energy Kinetic energy 2 Moment of inertia Angular speed2 0 Overall a bumper can can have both 0 Colliding at high speeds releases lots of energy Physics Concept 0 Acceleration always occurs toward the direction that reduces an obj ect s potential energy as rapidly as possible Electronic Air Cleaners Question A woman rubs her feet on the carpet and gives a shock to her identical twin If the twin also rubs her feet on the carpet before being touched the shock will be Observations About Air Cleaners 0 Dust doesn t settle quickly on its own 0 Mechanical filters gradually plug up 0 Dust clings to things with static electricity 0 Air cleaners could be based on static electricity 1 larger 2 smaller 3 the same size Air Resistance 0 Different from buoyancy Opposes the relative motion of air and object 0 Acts to bring both to one velocity 0 Consists of a matched pair of forces 7 The air pushes on the obj ect 7 The obj ect pushes on the air 7 Forces have equal magnitudes opposite directions 0 Increases with relative speed and cross section University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 35 Slides Knives and Steel 11 and Windows and Glass Steel and Carbon 2 Adding carbon to steel can harden it less slip Steel hardness also depends on heat treatment Heating above 723 C then slow cooling Allows large iron carbide crystallites to form Allows large ferrite crystals to experience slip Softens the steel Heating above 723 C then rapid cooling Produces tiny iron carbide crystallites Strongly impedes slip Hardens the steel Stainless Steel Adding chromium 18 and nickel 8 makes stainless steel resistant to most chemical attacks 188 Stainless steel is austenite Dissolves carbon well Can t be hardened by carbon or heat treatment Nonmagnetic Better stainless steel is hardened by alloying Alloy steels use extraneous elements to distort crystals Distorted crystals can t slip easily Windows and Glass Question Which window of a car can tolerate the larger stress before breaking l The front windshield 2 The side window 3 They re equally strong Observations About Windows Windows are clear but window glass looks greenish Window glass breaks if you heat it too rapidly Some older windows aren t very smooth Some car windows break into tiny pieces Window glass can be bent if you heat it carefully Glasses a Crystalline solid Amorphous solids gooooogp ooooooop 00000000 ooooooop 00000000 No crystal structure No longrange order Resemble frozen liquids 0 Amorphous solid University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 2 Slides Falling Balls Physical Quantities 0 Position 7 an object s location 0 Velocity 7 its change in position with time Newton s First Law Second Version An object that is free of external in uences moves at a constant velocity Physical Quantities 0 Position 7 an object s location 0 Velocity 7 its change in position with time 0 Force 7 a push or a pull Newton s First Law An object that is not subject to any outside forces moves at a constant velocity Question A rotary lawn mower spins its blade rapidly over the lawn and cuts the tops of the grasses off Would the blade still cut the grasses if they weren t attached to the ground Physical Quantities 0 Position 7 an object s location 0 Velocity 7 its change in position with time 0 Force 7 a push or a pull 0 Acceleration 7 its change in velocity with time 0 Mass 7 measure of its inertia University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 10 Slides Electronic Air Cleaners 11 University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 23 Slides Computers 11 and Radios Inverter Takes one input bit provides one output bit Output bit is inverse of input bit Input 4lgt0 Output Input Output 1 0 0 1 NotAnd N AND Takes two input bits provides one output bit Output is inverse of logical and of input bits Input 1 Output Input 2 Input1 Input2 Output 1 O CMOS Logic Bits are represented by charge l is represented by positive charge O is represented by negative or no charge Logic is built from nchannel and pchannel MOSFETS in complementary pairs 1 1 0 1 0 1 1 0 0 1 CMOS Inverter Input charge delivered 3v to two complementary mt MOSFETS Input Output Positive charge on nchannel input delivers negative MOSFET charge to output 0 V Vice Versa CMOS NAND Positive on both inputs delivers negative Input 1 charge to output Output Negative on either input delivers positive charge to output Input 2 1 0V Personal Computers Use CMOS logic for computations Use chargebased memory for fast storage Use magnetization or optical for slow storage Use light radio current or sound for network Speed Limits 0 Bits move no faster than the speed of light 0 Speed of light is 1 foot per nanosecond 0 During one PC cycle bits can move 1 foot 0 Processors can t be bigger than 1 foot Question Today the fastest PCs run at roughly 15 GHZ eday computers may run at l000000 GHZ Compared to present computers those highspeed ones would have to be Radio 1 much larger 2 much smaller 3 about the same size Question If you took an electrically charged ball and shook it up and down rapidly charges in a nearby metal object Would move in response How far away could that metal object be and still respond I1 meter I1 kilometer IThe other side of the universe Observations About Radio Transmit sound long distances without wires lnvolve antennas Seem to involve electricity and magnetism Reception depends on antenna positioning Reception weakens with distance Two styles of radio AM and FM Electromagnets and Energy Electric and magnetic fields contain energy An electromagnet stores magnetic energy Electromagnet consumes energy as it turns on 7Current temporarily experiences a voltage drop Electromagnet releases energy as it turns off 7Current temporarily experiences a voltage rise Electromagnet opposes current charges University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 38 Slides Nuclear Weapons II Structure of Nucleus Nucleus contains two kinds of nucleons Protons are positively charged Neutrons are neutral Two forces are active in a nucleus Electrostatic repulsion between protons 11 Eig frons Nuclear force attraction between touching nucleons At short distances nuclear force is stronger than electric At long distances electric force is stronger than nuclear Nuclear Stability In a nucleus nucleons are in equilibrium To be classically stable equilibrium must be stable To be quantummechanically stable equilibrium must also be the potential energy minimum Quantum mechanics and the Heisenberg uncertainty principle allow the nucleons to try out arrangements outside their equilibrium positions If they nd a path to a new equilibrium they may take it and the nucleus may fall apart Radioactivity Protons repel one another amp neutrons are unstable Large nuclei have two problems Too many protons then too much electrostatic potential Too many neutrons then neutrons are unstable Delicate balance between protons and neutrons Large nuclei tend to fall apart spontaneously Such decay is called ssion Fission is a type of radioactivity Induced Fission Large nuclei can break Fission fragments when struck Collision knocks nucleons out of stable equilibrium Incoming neutron Hard collisions are best at 0 inducing ssion Neutrons make ideal projectiles for inducing ssion Q Proton O Neutron Chain Reaction Neutrons can induce ssion Induced ssion releases Y O neutrons 0235U C o This cycle can repeat I mg lt I o neutron 239 235U 92 Chain reaction Cp BQB L Kr O O E hf 1 ac ISSIOD 1 6 eases energy o 39 Many ssions release 235U o prodigious amounts of energy a Sudden energy release U produces immense explosion Requirement for a Bomb Initial neutron source Fissionable material allowing induced ssion Fissions must release additional neutrons PP NL Material must use ssions ef ciently critical mass University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 3 Slides Falling Balls 11 Weight and Mass 0 An object s weight is proportional to its mass weight oc mass weight constant mass 0 On the Earth s surface that constant is 98 newtonskilogram called acceleration due to gravity Acceleration Due to Gravity Why this strange name force mass acceleration Newton s 2nd law 1 newton E 1 kilogrammetersecond2 de nition 9 8 newtonskilogram 9 8 metersecond2 9 8 metersecond2 is an acceleration Acceleration due to gravity actually is an acceleration On Earth s surface all falling objects accelerate downward at the acceleration due to gravity Why Things Fall Together 0 Increasing an object s mass increases the downward force on it makes it need more force to accelerate 0 These effects balance out perfectly A Falling Ball 0 Falling ball accelerates steadily downward Its acceleration is constant and downward Its velocity increases in the downward direction 0 Falling from rest stationary Velocity starts at zero and increases downward Altitude decreases at an ever faster rate Falling Downward Fall Position time Velocity Acceleration Q 0 m Q 0 s 0 ms 98 m52 49 m Q 1 5 98 ms 98 m52 196 m g 2 s 196 ms 98 ms2 441 m CD 3 51 294 ms 98 m52 A Falling Ball Part 2 0 A falling ball can start by heading upward Velocity starts in the upward direction Velocity becomes less and less upward Altitude increases at an ever slower rate At some point velocity is momentarily zero Velocity becomes more and more downward Altitude decreases at ever faster rate Falling Upward First 441 m i 3 s 0 ms 98 m52 392 m i 2 s 498 ms 98 ms2 5 T196 ms 98 m52 L 245 m Q 0 m CD 0 51294 ms 98 ms2 Fall lime Position Velocity Acceleration Throws and Arcs Gravity only affects vertical motion 0 A ball can coast horizontally while 245 1 5g 553K Question Suppose that I throw a ball upward into the air After the ball leaves my hand is there any force pushing the ball upward falling vertically 5 0 ZVelocily 5 o 10 20 30 4o 50 Distance down field m Ramps Question Can a ball ever push downward on a table with a force greater than the ball s weight Observations About Ramps Lifting an object straight up is often dif cult Pushing the object up a ramp is usually easier 0 The ease depends on the ramp s steepness Shallow ramps require only gentle pushes 0 You seem to get something for nothing 0 How does distance gure in to the picture University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 16 Slides Power Distribution Power 0 Power is energy per unit of time 0 Power is measured in joulessecond or watts 0 Batteries are power sources Loads are power consumers Batter Power 0 Current unis of charge pumped per second 0 Voltage rise energy given per unit of charge current voltage rise power produced Load Power 0 Current is units of charge passed per second 0 Voltage drop energy taken per unit of charge current voltage drop power received Electric Power Distribution Question Electric power reaches the University via high vo tage transmission lines What fraction of the electric charges traveling on those transmission lines pass through this room 1 About 1 2 About 001 3 Exactly 00 Observations About Power Distr 0 Household power is AC alternating current 0 Power comes in voltages like 120V amp 240V 0 Power is transmitted at high voltage 0 Power transformers are visible everywhere 0 Power substations are visible on occasion University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 21 Slides Audio Ampli ers 11 Doped Semiconductors Pure semiconductors are insulating Valence levels are lled and can t conduct Conduction levels are empty and can t conduct Impure semiconductors can be conducting Extra valence levels a valence band conduction Extra electrons conduction band conduction pType Semiconductors Substitute atoms with more empty orbitals Extra empty valence levels Electrons can move through valence levels Conduction Levels Balcony DDDDDDDDDDDDDDDDDDDD engag aa gaa agen aa ermz Level alence evels Ground Floor Energy nType Semiconductors Substitute atoms with more lled orbitals Extra full conduction levels Electrons can move through conduction levels Fermi Level Conduction Levels Balcony DDDQDDDQDDDDDQDDDDQD Qgggggggggg QQ alence evels Ground Floor Energy pnJunction before Before ptype meets ntype Each material can conduct electricity Each material is electrically neutral everywhere 1707196 Hype Conduction Levels Balcony IDDDDDDEDEI gmgmgmggngm QDQDQDQDE39 gaagaaauaaa evels Ground Floor Energy alence pnJunction after After ptype meets ntype Insulating depletion region appears at junction Depletion region is electrically polarized P39U Pe quot39U Pe DDDDDDDD D ngmg g QDQUQDQ QQQQQQQQQQQ Depletion Region Energy Forward Conduction A diode conducts when electrons arrive at the ntype end and leave at the ptype end Depletion region shrinks 1713793 390 83 Ener Q39jlgl lgl39j U D QQQQ39Q39Q39Q39Q39Q39Q D epletion Regio IBISQM A a a University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 11 Slides Xerographic Copiers Xerographic Copiers Question If you were to cover the original document with a red transparent filter would the copier still be be able to produce reasonable copies Observations About Copiers 0 Copies consist of black stuff stuck on paper 0 Afterjams the stuff sometimes wipes off 0 Copiers often run out of toner 0 Copies are often warm after being made 0 Copies are staticy particular transparencies 0 Some copies scan a light some use a ash Electric Fields 1 0 Two views of charge forces ChargeCharge 7 Charge 1 pushes directly on Charge 2 ChargeFieldCharge 7 Charge 1 creates an Electric Field 7 Electric Field pushes on Charge 2 0 Electric Fields are Real Electric Fields 2 0 An electric field is a structure in space that pushes on electric charge 0 The magnitude of the field is proportional to the magnitude of the force on a test charge 0 The direction of the field is the direction of the force on a positive test charge Quantum Physics 1 0 All things travel as waves 0 All things interact as particles Example 1 Light 7 Travels as Waves 7 electromagnetic Waves 7 Emitted and absorbed as particles 7 photons Example 2 Electrons 7 Detected as particles 7 Travel as Waves University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 29 Slides Fluorescent Lights Interference Light from different paths can interfere Constructive fields are in same direction Destructive fields are in opposite directions The two re ections from a lm interfere Different colors may interfere differently Soap film Re ection of Polarized Light Angled re ection varies for polarized light Fluctuating electric field parallel to surface large uctuating surface polarization big re ection Electric field perpendicular to surface small uctuating surface polarization small re ection Polarized Sunlight Most glare is horizontally polarized light Polarizing sunglasses block horizontally polarized light block glare from horizontal surfaces Much of the blue sky is polarized light too Fluorescent Lamps Question A uorescent lamp tube is coated with a white powder on its inside surface If that powder were not there would the lamp appear brighter dimmer or about the same overall brightness but with an unpleasantly bright white line near its center Observations About Fluorescents They often take a few moments to turn on They come in several variations of white They are often whiter than incandescent bulbs They last longer than incandescent bulbs They sometimes hum loudly They icker before they fail completely University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 7 Slides Wheels Wheels Question You are in a tremendous hurry and you want your car to accelerate as quickly as possible when the light turns green Tire damage is not an issue Will you accelerate faster if you burn rubber skid your wheels or if youjust barely avoid skidding your wheels Observations About Wheels 0 Without wheels objects slide to a stop 0 Friction is responsible for the stopping effect 0 Friction seems to make energy disappear 0 Wheels eliminate friction or so it seems 0 Wheels can also propel vehicles but how Friction Opposes the relative motion of two surfaces 0 Acts to bring two surfaces to one velocity Consists of a matched pair of forces 7 Object one pushes on object two 7 Object two pushes on object one 7 Forces have equal magnitudes opposite directions 0 Comes in two types static and sliding Types of Friction 0 Static Friction 7 Acts to prevent objects from starting to slide 7 Forces can vary from zero to an upper limit Sliding Friction 7 Acts to stop objects that are already sliding 7 Forces have xed magnitudes Frictional Forces 0 Increase when you 7 push the surfaces more tightly together 7 roughen the surfaces 0 Peak static force is greater than sliding force 7 Surface features can interpenetrate better 7 Friction force drops When sliding begins University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 24 Slides Radios II Tank Circuit Inductor amp Capacitor share energy Charge ows back and forth through inductor ll 39l39t39t39t39 ll 39l Capacitor Inductor 0 ll ll Ow l Energy shifts back and forth between the two devices Tank Circuit Oscillation Inductor Cltor Capa Tank Circuits in Radio Tanks are resonant devices Tanks build up energy at a speci c frequency Tanks help radios emit radio waves Tanks help radios detect radio waves Emitting Radio Waves 1 A transmitter uses a tank circuit a to slosh charge up and down its antenna A receiver uses a tank circuit to detect charge sloshing on its antenna Transmitter antenna charge affects receiver antenna charge Emitting Radio Waves 2 Accelerating charge emits radio waves Charge produces electric eld Current produces magnetic eld Changing current produces changing magnetic eld produces changing electric eld prod A radio wave consists only of an electric and magnetic eld A radio wave travels through empty space at the speed of light Structure of a Radio Wave Electric eld is l WHY D perpendicular to gt f39l r magnet1c eld was ll ll ll ll V C i M l t39c quot Electr1c eld creates BEETSquot magnetic eld and vice b Velocity D versa Electric eld determines p a I er 1 00 polarization of the wave t Electric University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 22 Slides Computers Computers Question Today the fastest PCs run at roughly 15 GHZ eday computers may run at 1000000 GHZ Compared to present computers those highspeed ones would have to be 1 much larger 2 much smaller 3 about the same size Observations About Computers 0 They respond to inpum with various outputs 0 They handle all kinds of information 0 Information is measured in bits and bytes 0 Some information is lost when power fails 0 Computers work extremely quickly 0 They follow instructions perfectly Analog Representation 0 A number is represented by a physical quantity 0 Number is proportional to the physical quantity 0 Precision is determined by the quantity itself Digital Representation 0 A numbers is represent by physical quantities 0 Physical quantities take on discrete values 0 These values represent pieces of the number 0 Precision is determined by number of quantities Binary Representation 0 Each physical quantity has two values 7 One value is de ned as a 1 7 The other value is de ned as a 0 0 Each quantity represents one information bit 0 A number is represented by several bits 0 The more bis the more precision Bis are relatively immune to noise Example 19 Five bits can represent number from 0 to 31 19 is represented by the bits 10011 Each bit represents a power of 2 124 023 022121120 19 Representing NonNumbers Bits or groups of bits are assigned to objects Characters Colors Days of the week 8 bits a byte can distinguish 256 objects Two bytes can distinguish 65536 objects Quantities Representing Bits Current Magnetization Charge Optical properties Light Radio Waves Sound Computers amp Bits Computation currents Memory charge Disk Drives magnetization CDROMDVDROM optical properties Computing Computers perform logical operations with bits Complicated operations based on simple ones Simplest operations inversion amp notand Any function can be realized from these two Inverter Takes one input bit provides one output bit Output bit is inverse of input bit Input DW Output Input Output 1 0 0 1 University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 26 Slides Microwaves Microwave Ovens Question If you put a CD in a microwave oven it will 1 do nothing 2 burn up the microwave oven 3 burn up the CD Observations About Microwaves Microwave ovens cook food from inside out They can cook foods unevenly They don t defrost foods well You shouldn t put metal inside them Do they make food radioactive or toxic Electromagnetic Spectrum Longwavelength EM waves Radio amp Microwave Mediumwavelength IR Visible UV light Shortwavelength Xrays amp Gammarays Frequency hertz 106 109 1012 1015 1018 1021 l l I I l l l l U HF television Microwaves Far infrared light Infrared light Visible light Ultraviolet light AM radio Shortwave radio VHF television FM radio VHFtelevision X rays Gamma rays a Millimeter waves 0 3 10 12 10 1 5 o o A o lo Wavelength meters Water Molecules Water molecules are unusually polar An electric eld orients water molecules A uctuating electric eld causes water molecules to uctuate in orientation lit 3 if Microwave Heating Microwaves have uctuating electric elds Water molecules orient back and forth Liquid water heats due to molecular friction Ice doesn t heat due to orientational stiffness Steam doesn t heat due to lack of friction Food s liquid water content heats the food University of Virginia Department of Physics Physics 606 How Things Work 11 Lecture 15 Slides Flashlights II A Light Bulb 0 Structure 7 Contains a protected tungsten lament 7 Filament conducts electricity but poorly Filament barely lets charge ow through it 7 Electrostatic potential energy is consumed 7 Thermal energy is produced 0 Current undergoes a drop in voltage 7 Twocell alkaline ashlight 30 volt drop A Simple Circuit 0 A battery 7 the energy source 0 A wire 7 the outgoing current path 0 A light bulb 7 the energy destination the load 0 A wire 7 the return current path Circuits l Steady current requires a circuit path a loop 7 Charge mustn t accumulate anywhere 7 A closed conducting loop avoids accumulation Steady current ow requires energy 7 Currents lose energy and voltage in conductors 7 Missing energy becomes thermal energy 7 Lost energy must be replaced Circuits 2 0 A circuit can transport energy 7 Current obtains energy from a battery 7 Current delivers energy to a light bulb 7 Current starts the trip over again Question If you remove the 2 batteries from a working ashlight and reinstall them backward so that they make good contact inside will the ashlight still work Recharging a Batter 0 Forward discharging current ow 7 Battery pumps charge from 7 end to end 7 Current undergoes voltage rise 7 Battery s chemical potential energy is consumed 0 Reverse recharging current ow 7 Circuit pushes charge from end to 7 end 7 Current undergoes voltage drop 7 Battery s chemical potential energy is replenished
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