Unformatted text preview: Microwaves and Faraday Cages Shielding Communications, Cooking Microwave Oven Mysteries UCSD: Physics 8; 2007 Shielding and Faraday Cages What keeps microwaves in the microwave? Why is cell reception poor in some elevators? Why is inside a car safer than outside in a lightning storm? How can satellite dishes work with just a mesh you can see through? All of these relate to the behavior of metal in the presence of an electric field electromagnetic radiation consists (in part) of an oscillating electric field 2 UCSD: Physics 8; 2007 Electrons on the move Metals are good conductors of electricity This means that their electrons are free to move They move when an electric field is present A conducting sphere placed in an electric field: E-field
+ + ++ + _ result is an internal _ _ _ electric field that is exactly equal and opposite external field, so the Electrons in the metal feel zero total force _ E-field shows direction of motion of +ve charge. Positive charges are the nuclei of atoms that do not move. Instead electrons flow the other way, leaving net +ve on top, and - on bottom ve
3 UCSD: Physics 8; 2007 Faraday Cage: Electric Field is Zero A conducting sphere or metal box zero electric field inside Same thing happens if the sphere is hollow the electrons are all pushed to the surface + ++ + + - - - - - Wrap transistor radio or mobile phone in aluminum foil or put in a can 4 UCSD: Physics 8; 2007 Faraday Cages in Practice Your car is a partial Faraday cage: lightning will flow on the outer skin, leaving the inside relatively quiet Elevators are poor bad for reception metal walls shield electromagnetic radiation: no E-field inside The microwave oven is an inside-out Faraday cage generate strong electric fields inside, but outside is zero electric field Bottom line is that a metal sheet shields electric fields in the context of electromagnetic waves, we can say that metal surfaces reflect incident EM waves can either confine E-fields within box, or keep them out 5 UCSD: Physics 8; 2007 Metallic Reflection: Wiggling Electrons Why does metal reflect EM waves? because the surface electrons are made to vibrate with the oscillating E-field this acceleration of the electron itself produces electromagnetic radiation phase is such that transmitted wave is perfectly canceled So the microwave has all these metal (reflective) walls, which keeps the microwaves inside. But how does the mesh on the front door do the job? after all, light can get through, and light is also EM radiation Key issue is: can electrons redistribute themselves quickly enough? for a certain frequency of EM radiation, need charge redistribution on timescale shorter than time for one wave to pass = 1/frequency
6 UCSD: Physics 8; 2007 Need to traverse around hole much faster than period of EM wave The signal to move, from electric field, travels near light speed individual electrons do not move this fast, but each moves a bit if electrons have time, they will "patch up" holes with appropriate electric field across the void: as if hole isn't there Getting around the holes EM radiation can not get through holes if hole size is much smaller than wavelength distance = speed time is equivalent to = c/f = cT (T is wave period) meshes work provided hole size << can still see through, since for light is much smaller than microwaves Electrons must redistribute around hole, but this does not require a single electron to make the journey. Just like in the case of electrical current, electrons push each other.. Radio inside a wire box
7 UCSD: Physics 8; 2007 Microwaves: Just Another EM Wave Microwaves are like any other electromagnetic wave occupying region between radio and infrared Wavelengths from 1 mm to 1 m are microwaves this definition is not strict think of a meter stick: every set of marks: from 1 mm to 1 m are all in the microwave category Microwaves used for lots of things trans-continental communications cell phones microwave ovens weather radar astronomy (confirmed Big Bang) 8 UCSD: Physics 8; 2007 Microwave Communications You've seen microwave towers these are relay stations forming a communication network across the country much of our telephone, internet, etc. connections run this way Principle advantage over radio: BANDWIDTH TV station, for instance, requires 6 MHz of bandwidth At 60 MHz (like channel 2, 3), this is 10% of the frequency Over one octave of frequency, from 60 MHz to 120 MHz, you would only fit 10 TV stations At 10 GHz (3 cm), one octave (from 714 GHz, e.g.) could fit over 1000 TV stations (or LOTS of phone activity) Also penetrates haze, fog, smoke, light rain, snow, clouds makes this a reliable means for communication
9 UCSD: Physics 8; 2007 Communications Demo: We can amplitude-modulate the signal strength of a microwave transmitter Receiver gets varying signal strength, and can relate the signal strength to a speaker Important parameters of communication: polarization must match pointing/beaming must be okay no opaque matter in the way 10 UCSD: Physics 8; 2007 Water in Microwave Ovens Water is a polar molecule + - + This means it will try to orient itself a particular way in an electric field Once oriented, a net force exists on molecule one side is a tiny bit closer to source, so kQq/r2 is larger net effect is attraction 11 UCSD: Physics 8; 2007 Microwave Oven, continued Microwaves have time-varying electric field As electric field changes direction, water flips back and forth: they can't help it
oscillating electric field time
12 UCSD: Physics 8; 2007 Microwave Frequency Microwave ovens use a frequency of 2.45 GHz 12 cm wavelength This is ideally suited for the time it takes to flip a water molecule half-cycle is 200 picoseconds Imagine microwaving steam molecules are far apart they flip back and forth, but who cares: they don't heat up turn off microwaves, and nobody has moved anywhere thermal energy is, after all, kinetic motion Now crowd molecules into liquid water no "elbow" room to do their exercise bump into each other and heat up
13 UCSD: Physics 8; 2007 Microwave made by Magnetron Electrons emitter by central cathode at thousands of volts DC (Danger!) Exterior magnet makes electrons spiral on way to outer (red) cathode Electrons make microwave electric field at resonant frequency depends on size of cavity Resonant field makes electrons bunch up enhancing microwave generation 65% efficient, but frequency is not constant (it drifts in time) Beryllium oxide: poisonous if crushed/ inhaled 14 UCSD: Physics 8; 2007 Steam, Water, and Ice In ice, molecules are locked into bonded arrangements, and can't break loose to flip-flop So of steam, water, and ice, only liquid water is heated by microwave More on the bumping: how does this make heat? imagine hydrogen atoms as being like boxing gloves when they smack a neighboring molecule, it gets set into motion (kinetic energyheat) one bump leads to another, and pretty soon, the whole pile of molecules (a.k.a., hot dog) gets hot 15 UCSD: Physics 8; 2007 Thawing Food in Microwave If ice is unaffected, how can a microwave defrost food? this is actually hard for the microwave to do some few molecules will be loose and can be wiggled, these will quickly heat up their surroundings, making more liquid runaway process in little pockets: ice still unaffected Defrost cycling: allowing time for diffusion = heat spread by motion of molecules rather than let a few pockets run away with all the heat, turn off magnetron and allow time for thermal diffusion Thermal diffusion is natural time it takes heat to propagate through a medium relates to thermal conductivity: the ease with which heat is transported 16 UCSD: Physics 8; 2007 Thermal Conductivity Different materials have different efficiencies for distributing heat
Material Silver Copper Aluminum Stainless Steel Ice Glass, Concrete, Wood Many Plastics Air (stagnant) Styrofoam Therm. Cond. (W/m/K) 406 385 205 14 1.6 0.8 0.4 0.02 0.01 our buildings plastics feel warm to touch but usually in motion better than air!
17 Comments why room-T metals feel cold why cooking pots have this why cooking spoons are S.S. UCSD: Physics 8; 2007 Conventional ovens rely on conduction Heating food from the outside, one relies entirely on thermal conduction/diffusion to carry heat in Relevant parameters are: thermal conductivity, (how fast does heat move) (W/m/K) heat capacity, cp (how much heat does it hold) (J/kg/K) mass, m (how much stuff is there) (kg) size, R--like a radius (how far does heat have to travel) (m) (cp/)(m/R) 4(cp/) R2 where is density, in kg/m3: m/((4/3)R3) m/4R3 faster if: cp is small, is large, R is small (these make sense) for typical food values, 6 minutes (R/1 cm)2 egg takes ten minutes, turkey takes 5 hours
18 Just working off units, derive a timescale: UCSD: Physics 8; 2007 The microwave shortcut At 2.45 GHz, microwaves penetrate into food (looks partially transparent) and excite water molecules internally 2.45 GHz is a good compromise: lower frequency would not be readily absorbed (food too transparent); higher frequency would not penetrate well, heating the outside (food too opaque) Ideally, food cooks uniformly throughout eliminating restriction of thermal diffusion time except for ice, which isn't warmed by microwaves Parts of microwave over can lack waves microwaves are reflected by walls, and set up standingwave interference patterns leaving hot spots and cold spots helps to rotate food through this stationary radiation pattern 19 UCSD: Physics 8; 2007 Metal in the Microwave Electrons are free to move in metal charges are forced to flow in response to the electric field if the metal is thin (foil, twist-tie, decorative trim), it can't carry much current, and gets very hot risk of fire Also, sharp points concentrate the electric field and promote sparks foil edges, twist-ties, decorative trim (same culprits) present sharp, thin edges where sparks are likely to form Bulky metallic objects with smooth edges present NO PROBLEM to microwaves the walls are, after all, metal spoons, juice concentrate lids, metal plates okay forks, ragged-edged can-opened lids not okay
20 UCSD: Physics 8; 2007 Are microwaves harmful? Microwaves vibrate and heat water molecules As long as the flux is low (e.g., outside microwave, or from cell phone antenna), no harm is done nowhere is there a high-enough concentration to develop significant heat/boiling But if the microwave door is open (or poorly closing) you're asking for trouble Also standing in front of microwave transmission antenna could cook you mildly, but potentially lethally 21 UCSD: Physics 8; 2007 Microwave Experiments Never run microwave for more than 10 seconds without some form of water inside to absorb energy The microwaves will go back into the magnetron, overheating and possibly destroying it Boiling water in cup of ice described fully in book Marshmallow stop microwave before marshmallow explodes: a heck of a mess CD do this only for a few seconds to see sparky light-show only works on metallic-layer CDs (not organic CD-Rs) CD will be destroyed abort before CD turns into a pile of goo (awful mess) Do these only in a microwave that you take full responsibility for in case you break it or make an un-cleanable mess 22 UCSD: Physics 8; 2007 References Check out: http://rabi.phys.virginia.edu/HTW/microwave_ovens.html for an excellent question/answer forum from a guy who has his head screwed on straight. You'll get little misinformation here 23 ...
View Full Document
This note was uploaded on 04/10/2008 for the course PHYS 8 taught by Professor Tytler during the Spring '08 term at UCSD.
- Spring '08