11/18/20101HW12 due Saturday noonToday: finish microwavesstanding waveswhat makes the sparks flyTo-day: Tara’s office hours are 4-5 (instead of 2-3)Recall: microwavesIn an oven, wavelength = 12.2 cmAct on water molecules:+--+-+-+Question:The carbon dioxide molecule has charges distributed roughly as shown below. How well would liquid carbon dioxide heat in a microwave oven?a) as well as waterb) better than waterc) not as well as water.+--Answer: c)This Hot PocketTMcomes with a cardboard tube coated on the inside with a thin layer of a weakly conducting material. This is supposed to brown the Hot PocketTM. How does this work? a) Microwaves enter the thin layer and accelerate electrons, which give up energy due to Ohmic heating.b) The thin layer helps focus microwaves into the surface of the Hot PocketTM.c) The thin material contains excess water, which heats up well in the microwave.Answer: a)Consider the front of the oven.There are lots of little holes – how does this help?Ripple tank demoMicrowaves (big) do not "fit" through the small holes in the front of the oven.To prevent microwaves from leaking, the holes must be much smaller than the wavelength.11/18/20102Detail: reflection of microwaves from a metal surface incoming waveMetal wallReflected waveRope demoInterference: Imagine two charges oscillating in opposite directions, generating microwaves. What force will a faraway charge experience??a) Force is twice as largeb) Force is half as largec) Force is unchangedd) Force is essentially zeroAnswer: d)Interference: Imagine two charges oscillating in the samedirection. What force will a faraway charge experience??a) Force is twice as largeb) Force is half as largec) Force is unchangedd) Force is essentially zeroAnswer: a) : constructive interferenceJust to make sure we all agree on notation: the microwave is determined by the forces it exerts on charges.Half a cycle later, what will the forces be like?a) The sameb) Up and down forces reversec) The electron in the middle feels a force upd) The electron in the middle feels a force downJust to make sure we all agree on notation: the microwave is determined by the forces it exerts on charges.Half a cycle later, what will the forces be like?a) The sameb) Up and down forces reversec) The electron in the middle feels a force upd) The electron in the middle feels a force downAnswer: bSchematic of reflectionSurface of metal wall11/18/20103Schematic of reflectionSurface of metal wallSchematic of reflectionSurface of metal wallSchematic of reflectionSurface of metal wallSchematic of reflectionSurface of metal wallwallstanding waveoscillating backand forth between these twoRope demowallQ: what's the distance between maximaof the intensity?a) 2lb) lc) l/2d) l/3?A: c)11/18/20104The same thing is true in the oven12:00Marshmallow demoRemark:We have just made a measurement of the wavelength. If the microwave frequency is 2.45 GHz, what, therefore, is our measurement of the speed of light?Let me emphasize this: we have just measured the speed of lightusing marshmallows!Speed of light = ln12:00Three-dimensional structure of the microwave pattern.Now I'm going to raise the marshmallows by about 3cm.What will the pattern of big and small marshmallows look like?The take-home lesson: microwave ovens have "hot spots"and "cold spots" – cooking can be uneven. 12:00Question:Radio waves also have hot and cold spots, because of reflection from buildings and so on. Suppose you're listening to a radio station with frequency 890 kHz (wavelength = 337 m), but the reception is poor. About how far would you have to move to find the nearest hot spot?a) 337 mb) 674 mc) 168 md) 84 mAnswer: d) hot spots are spaced by 1/2 wavelength, so fromcold to hot is 1/4 wavelength.Return to rope – if the wavelength is not well matched to the size of the box, the amplitude is weak.Conversely, if the match is good, the amplitude grows.This matters when you’re designing a microwave oven.11/18/20105And another thing: you're not supposed to run the microwave oven when it's empty. Intensity builds and builds.Leak detector demo with water present--++--++--++--++--++--++Absorption of microwaves by water molecules.Question:We talked about heating water; we saw the CD spark; what if we put both water and a CD in the microwave?a) the water heats; the CD sparksb) the water heats; the CD does not sparkc) the water does not heat; the CD sparksd) the water does not heat; the CD does not sparkDo the experimentWhat makes the sparks fly?A: sharp pointy parts.Here’s a piece of metal (a fork, say) with a pointy endin the big part of the fork,Electrons have room to spread out- - - - - - - -- - - - - - - - ---------on the pointy part,they get crammed together-- ------ ------ ----Red charges are “nailed down” negative charge.Green charge is puck (free to move)For which of these choices is puck most likely not to move (e.g. net force on puck is zero)? ABCAnswer is A. Look at forces from each charge and add them up. -- ----AColor-code force from each charge.Explains WHY charges pile up on pointse-e-e-e-e-e-e-++++++++++++++++++++++++e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-e-ABFor these shapes: Which of the “green” electrons is will experience higher max force on it?Remember that electrons on objects aren’t totally stationary … they shake. VA. When electron next to “green” gets a little bit closer, large increase in net force on green electron.Why get sparks off (or on to) sharp pointy wires?in big object, room to spread out- - - - - - - -- - - - - - - - -----on point, get crammed togetherCompare putting same voltage on big round object to putting it on sharp pointy object.- - - - - - - -- - - - - - - - -----The microwave sloshed the electrons around and send a bunch of them all at onceTo the sharp end.When there’s a lot on the point, they push each other off,resulting a spark!----11/18/20106Electrical discharge – like lightning.Energetic electrons blow air atoms apart!!This converts electrical energy into kinetic energy of electrons.e-e-e-e-e-e-e-Electrical discharge – like lightning.Energetic electrons blow air atoms apart!!This converts electrical energy into kinetic energy of electrons.Eventually these electrons re-combine with the atoms. Where doesTheir kinetic energy go?a) Heating the air atomsb) Giving off lightc) back into electrostatic energyd) More than one of thesee-e-e-e-e-e-e-Ans: e. Part of the energy is light, but part is the