1Physics 1010:The Physics of Everyday LifeTODAY• Microwaves• Sunlight2Admin Stuff• Requested to extend Extra Credit, so it isdue on Tuesday• Today is last new lecture; from Thursdaycomprehensive review• Final is worth 80pts, as much as all midtermstogether (you drop one midterm)• One more EC, published Thursday• Check your grades, TALK TO THE GRADERS3Oscillating fields cause electrons to moveEM wave speed, c = 3 x 108 m/s--Microwaves4Moving electrons in resistive materialheat it upRecall: electrical resistance-piece of metalElectron slows down because ofresistance in the metal.Kinetic energy of electron istransformed into heat.5Food does not (should not?) contain metal,so how does it get heated?A water inside resonatesB the water inside rotatesC magic!D the wave excites the molecules6Food does not (should not?) contain metal,but it does often contain waterH2O--++positive charges at one end,negative charges at the other –H2O is polarized.+--+Not to scale! Wavelength is far bigger thana molecule.7Rotating and bumping causes heating• Water molecules arerotational oscillator at2.45 GHz• Rotate and bump intoeach other• Converts rotation toheat (random motion)--++--++--++++-++--++--++2.45 billion oscillations per sec= 2.45 GHz = νone wavelengthλ8Which form of water do you think gets heatedmore effectively in the microwave oven?a) water vaporb) liquid waterc) solid water (ice)d) vapor and liquid are equally well heatede) all three are equally well heatedb) Free to rotate, but close enough to bump intoeach other9Liquid water most rapidly heating as freeto rotate, but close enough to bumpVapor- molecules too far apart to rub against each other.Solid- molecules are packed in tootightly, cannot rotate effectively.--++--++--++++-++--++--++--++--++--++++-++--++--++--++--++--++++-++--++--++--++--++--++++-++--++--++Water has good absorption10The arrows indicate the strength and direction of the force that would beexerted by the electromagnetic wave on an electron (or a negative charge).Water molecules are polar, so the hydrogens have a positive charge and theoxygen has a negative charge.What will this water molecule do in response to this force?--++--++Upward force on positiveparts of moleculeDownward force onnegative parts of moleculeRESULT IS ROTATION!a)b)c)11The arrows indicate the strength and direction of the force that would beexerted by the electromagnetic wave on an electron (or a negative charge).Water molecules are polar, so the hydrogens have a positive charge and theoxygen has a negative charge.What will this water molecule do in response to this force?--++--++Upward force on positiveparts of moleculeDownward force onnegative parts of moleculeRESULT IS ROTATION!Rotate 90 degrees clockwise because the force on the oxygen is down andthe force on the hydrogens is up.12The arrows indicate the strength and direction of the force that would beexerted by the electromagnetic wave on an electron (or a negative charge).Water molecules are polar, so the hydrogens have a positive charge and theoxygen has a negative charge.What will this water molecule do in response to this force?Rotate 90 degrees clockwise because the force on the oxygen is down and theforce on the hydrogens is up.--++ATTENTION: Net force onwater molecule is zero,because upward force onpositive hydrogens exactlybalances downward force onnegative oxygen. (Overall,molecule is neutral.)13What is the wavelength of the 2.45 GHzwaves in a microwave oven?a) 8.17 mb) 8.17 cmc) 0.122 md) 0.122 cmc) λ = c/ν = 3.e*108/2.45*109 = 0.122 m14The carbon dioxide molecule has charges distributedroughly as shown below. How well would liquid carbondioxide heat in a microwave oven?a) as well as waterb) better than waterc) not as well as water.+--c) Not as well as water because not polar15Would any of these atoms and moleculesnot heat up in microwave?a. none would heat. b. CH3 c. Carbond. e.-- c. and d. would not heathydrogen tends to suck electrons off of carbon, C, just like with oxygen, so extra negatives on hydrogenatom, extra positives on carbon. ans. e. only b is polar, and so will be rotated by microwavefield. All others, will be no twist on them. Must have positiveand negative charges arranged nonsymmetrically. True inwater, many fats. (oil heats nicely).+-16Humans are protected by the metalsurrounding the microwave ovenIncoming microwaveMetal wallElectrons respond,reflect waves backAcross perfect conductor, novoltage. So if perfect, electronmotion causes electric field tobe zero at conductor17But what about those little holes that wecan see through?• Light gets through• Light is EM waves• Microwaves are EM waves• Why don’t microwaves get through?a) Wavelength of MW is longer than lightb) The wavelength of light is LONGER than the MWc) The holes only let some wavelengths throughd)18We cannot “see” objects (or features) that are smaller than the wavelength of the wave we use (sound, EM).Examples:A electron microscopeBC191) We use electron microscopes to see viruses, because viruses are smaller than the wavelength of visible light2) We use blue lasers (RlueRay) for HDTV DVD because in order to pack ahigher density disk (~40GB) the dots on the disk have to be smaller than the wavelength of red light20This Hot PocketTM comes with a cardboard tubecoated on the inside with a thin layer of a weaklyconducting material. This is supposed to brown theHot PocketTM. How does this work?a) Microwaves enter the thin layer and accelerate electrons,which give up energy due to resistance heating. Hot metalthen warms the bread.b) The thin layer helps focus microwaves into the surface of theHot PocketTM.c) The thin material contains excess water, which heats up well inthe microwave.a) Dry bread (just before browning) has littlewater. So heat something near.21With reflections, places where no electricfield occurwallstanding waveoscillating backand forth betweenthese two22What's the distance between maximaof the intensity?a) 2λb) λc) λ/2d) λ/3wall?c) Half wavelength23Microwave ovens have hot and cold spotsbecause of reflections• Field is wavesin eachdirection12:0024Radio waves also have hot and cold spots, because of reflectionfrom buildings and so on. Suppose you're listening to a radiostation with frequency 890 kHz (wavelength = 337 m), but thereception is poor. About how far would you have to move to findthe nearest hot spot?a) 337 mb) 674 mc) 168 md) 84 mAnswer: d) hot