How Things Work Physics of Everyday Life by Lou Bloomfield Excerpt

8/8/2019 How Things Work Physics of Everyday Life by Lou Bloomfield Excerpt

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[[If you enjoy reading these questions and answers, please look for my new book How Everything Works:Making Physics out of the Ordinary at your favorite bookstore (and encourage them to stock it if they

haven't already). Thanks Lou Bloomfield]]

1551. Upon removing a cup of coffee I'd heated for one minute in a microwave oven, I noticed a small antrunning about, apparently unharmed. Curious, I gave it another one minute ride and when the door wasopen, it was still running about. How come an ant is apparently unharmed after two minutes in amicrowave? -- KMBMost likely, the ant never left the floor or walls of the microwave oven, where it was as close as possible tothose metal surfaces. The six sides of the cooking chamber in a microwave oven are made from metal (orpainted metal) because metal reflects microwaves and keeps them bouncing around inside the chamber.

Metals are good conductors of electricity and effectively "short out" any electric fields that are parallel totheir surfaces. Microwaves reflect from the metal walls because those walls force the electric fields of the

microwaves to cancel parallel to their surfaces and that necessitates a reflected wave to cancel the incidentwave. Because of that cancellation at the conducting surfaces, the intensity of the microwaves at the wallsis zero or very close to zero.

The ant survived by staying within a tiny fraction of the microwave wavelength (about 12.4 cm) of themetal surfaces, where there is almost zero microwave intensity. Had the ant ventured out onto your cup, itwould have walked into real trouble. Once exposed to the full intensity of the microwaves, it would nothave fared so well.

1550. My wife makes blueberry pancakes for my daughter daily. Twice recently she noticed and brought tomy attention a curious event in the Microwave oven. Frozen Blueberries placed inside a microwave oven tothaw, caused a popping sound and a small flame to appear amidst the blueberries. The flame selfextinguishes. There is no apparent damage to the blueberries or the bowl they were contained in. -- HA,New JerseyI think that you've rediscovered an experiment in which people cut a grape almost in half, open the twohalves like a book and lay it flat on a plate. In the microwave, the thin bridge between the halves carbonizesand than emits flames. Basically, the fruit pieces or berries are acting as antennas for the microwaves,which drive electric currents through the narrow bridges between parts. The berries aren't great conductors,but they're not true insulators either. Those bridges overheat (like an overloaded extension cord) and burnup. The flames come from the burning bridges.If you let the flames go on long enough and enough carbon develops, you'll probably start getting plasmaballs in the oven (lots of fun, but not great for the oven... you can scorch its top surface because thoseplasma balls rise and skittle around the ceiling of the oven). Anyway, you can probably find the carbonareas if you look closely enough, but they're no worse than a little burnt toast.

1549. My boyfriend and I are having this debate on whether or not to squeeze the air out of a 2 liter bottle

of Coke after opening it. He thinks it will keep the Coke carbonated longer and I disagree. Who is right? TN, Ft. Collins, COYours is actually a complicated question. After you open the soda, the CO2 dissolved in the soda is nolonger in equilibrium with the gas above soda. When you cap the bottle, CO2 will gradually escape fromthe liquid until it forms a dense gas so that CO2 molecules from that gas return to the liquid solution asoften as they leave the solution for the gas. In other words, the equilibrium between dissolved CO2 andgaseous CO2 has to be reestablished.


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By shrinking the volume of gas over the soda, your boyfriend reduces the number of CO2 molecules thatmust enter the gas phase in order to reestablish that equilibrium. BUT, when dense gas develops in thesqueezed bottle, the high pressure of that gas will reinflate the bottle to its original size. The benefits ofshrinking the gas volume will thus be lost.

To succeed in keeping more of the CO2 molecules in solution, you have to make sure that the squeezedbottle stays squeeze. That's hard to do. You're probably better off pouring the soda gently into a smallerbottle, one that just barely holds all of the liquid. That smaller bottle won't expand as a dense gas of CO2forms above the liquid soda and the soda will reestablish its equilibrium without losing too many of itsdissolved CO2 molecules.

1548. When you travelling in a jet plane, why do objects on the ground look as though they are still ormoving slowly? K, IndiaWhen you watch something move, what you really notice is the change in the angle at which see you it.Nearby objects don't have to be traveling fast to make you turn your head quickly to watch them go by soyou perceive them as moving rapidly. An object that is heading directly toward you or away from youdoesn't appear to be moving nearly as quickly because its change in angle is much smaller.

When you watch a distant object move, you don't see it change angles quickly so you perceive it as moving

relatively slowly. Take the moon for example: it is moving thousands of miles an hour yet you can't see itmove at all. It's just so far away that you see no angular change. And when you look down from a high-flying jet, the distant ground is changing angles slowly and therefore looks like it's not moving fast.1547. If I were to heat up a brownie and a white piece of cake, would the brownie heat up faster byradiation transfer because of its darker color? BIn principle, the brownie would heat up faster by radiation in a hot environment and cool off faster byradiation in a cold environment. A black object is better at both absorbing thermal radiation and emittingthermal radiation, so the brownie would soak up more thermal radiation in the hot environment and give offmore thermal radiation in the cold environment.

In practice, however, most of the radiation involved in baking these desserts and letting them cool on akitchen counter is in the infrared and it's hard to tell just what color a brownie or cake is in the infrared. It'slikely that both are pretty dark when viewed in infrared light. Basically, even things that look white to your

eye are often gray or black in the infrared. Thus I suspect that both the brownie and cake absorb most of thethermal radiation they receive while being baked and emit thermal radiation efficienty while they're coolingon the counter.

1546. How can light "travel" through a vacuum when there were no "particles" in the vacuum on which itcould "transmit" its charge? DCLight has no charge at all. It consists only of electric and magnetic field, each endlessly recreating the otheras the pair zip off through empty space at the speed of light.

The fact that light waves can travel in vacuum, and don't need any material to carry them, was disturbing tothe physicists who first studied light in detail. They expected to find a fluid-like aether, a substance that wasthe carrier of electromagnetic waves. Instead, they found that those waves travel through truly empty space.

One thing led to another, and soon Einstein proposed that the speed of light was profoundly special and thatspace and time were interrelated by way of that speed of light.

1545. For my industrial design project, I am redesigning the microwave oven and adding some extrafunctions. Is it possible for microwaves to somehow measure food properties such as calories, sugar, salt,vitamins, and fat content? How can I translate those readings onto an LCD display so that the user can seethem, and can they also be transferred to a computer via Bluetooth? IBWhat you propose to do is far more difficult than you imagine. Determining the chemical contents of foodis hard, even with a well-equipped laboratory and permission to destroy the food in order to study it. The


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idea of analyzing a casserole in detail simply by beaming microwaves at it is science fiction. Think howmuch easier airport security would be if they could chemically analyze everything that came in the frontdoor just by beaming microwaves at it.

That said, however, let me make two comments. First, the question quickly turns to computer interfaceissues, as though the chemical analysis part is trivial in comparison to computer presentation part. Physicalscience and computer science are truly different fields and not everything in the scientific domain can bereduced to a software package. Physics and chemistry haven't disappeared with the advent of computersand there will never be a firmware upgrade for your microwave oven that will turn it into a nutritionalanalysis laboratory. As a society, we've gone a bit too far in replacing science education with technologyeducation, particularly computer software.

Second, while remote chemical analysis isn't easy, it can be done in certain cases with the clever use ofphysics and chemistry. One of my friends here at Virginia, Gaby Laufer, has developed an instrument thatstudies the infrared light transmitted by the air and can determine whether that air contains any of a broadvariety of toxic or dangerous gases in a matter of seconds. Air's relative transparency makes it easier toanalyze than an opaque casserole, but even when you can see through something it's not trivial to see whatit contains. Gaby's instrument does a phenomenal job of fingerprinting the gas's absorption features andidentifying trouble.

Note added: a reader informed me that there are now microwave ovens that can read bar codes and adjusttheir cooking to match the associated food. A scale in the base of the oven can determine the food's weightand cook it properly. Another reader suggested that a microwave oven might be able to measure the food'smicrowave absorption and weight in order to adjust cooking power and time. While that's also a goodpossibility, ovens that sense food temperature or the humidity inside the oven can achieve roughly the sameresult by turning themselves off at the appropriate time.

1544. If something is coasting or moving at a steady pace, is it experiencing a net force of zero? NPThat's exactly right! Coasting and zero net force go hand-in-hand: when an object is experiencing zero netforce, it doesn't accelerate and thus it coasts. A coasting object is an inertial object, meaning that it moves ata steady pace along a straightline path. And if the coasting object is at rest, it stays at rest.

To clarify the term "net force," note that when an object is experiencing several separate forces, it doesn'taccelerate in response to each one individually. Instead, it accelerates in response to the sum of all theforces acting on it: the net force. Remember that forces have directions associated with them (forces arevector quantities), so when you sum them you must consider their directions carefully. The proper force toconsider in Newton's second law is actually the net force on the object. If you know both the net force onthe object and the object's mass, you can predict the object's acceleration. And if the net force is zero, thenthe object doesn't accelerate at all it coasts.

1543. Can/should a microwave be disposed with the normal trash, what if any are the environmentalimpacts of the magnetron or other parts sitting in a landfill? DNRI figure that some day, we'll turn to our landfills as resources for precious elements like copper and gold.That assumes, of course, that we survive global warming. In the meantime, we'll just keep throwing stuff

out.

Despite the scary title "microwave radiation," a microwave oven is basically just another householdelectronic device. It is an extremely close relative of a convention cathode-ray-tube television set. If you'reOK with putting CRT televisions and computer monitors in the landfill, you should have no problems withputting microwave ovens there, too. Even when the microwave oven is on, all it has inside it is microwaveradiation and that's just not a big deal. The instant you turn it off, it doesn't even have those microwaves init. It's just boring inert electronic parts and they'll sit in the landfill for generations, rusting and decayinglike every other abandoned electronic gadget. I'd rather see it go to a recycling center and have its preciousmaterials returned to the resource bin, but as landfill junk goes, it's not all that bad. Given that toxic


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chemicals are the primary concern with landfills, microwave ovens are probably rather innocuous. Theyhave no radioactive contents and although the high-voltage capacitor might have oil in it, that oil can nolonger be the toxic PCBs that were common a few decades ago. Even when that oil leaks into theenvironment, it's probably not going to do much.

So there you have it, microwave ovens go to their graves no more loudly or dangerously than oldtelevisions or computers or cell phones.

In fact, I might start calling cell phones "microwave phones" because that's exactly what they are. Theycommunicate with the base unit by way of microwave radiation. Given the number of people who have cellphones semi-permanently installed in their ears, concerns about microwave radiation should probably beredirect from microwave ovens to "microwave phones." Think about it next time your six-year-old talks foran hour with her best friend on that "microwave phone."1542. Why do deep water wells need a pump at the bottom rather than one at the top? LG, VancouverWhile it's easy to push on water, it's hard to pull on water. When you drink soda through a straw, you mayfeel like you're pulling on the water, but you're not. What you are actually doing is removing some air fromthe space inside the straw and above the water, so that the air pressure in that space drops belowatmospheric pressure. The water column near the bottom of the straw then experiences a pressureimbalance: the usual atmospheric pressure below it and less-than-atmospheric pressure above it. Thatimbalance provides a modest upward force on the water column and pushes it up into your mouth.

So far, so good. But if you make that straw longer, you'll need to suck harder. That's because as the columnof water gets taller, it gets heavier. It needs a more severe pressure imbalance to push it upward and supportit. By the time the straw and water column get to be about 40 feet tall, you'll need to suck every bit of airout from inside the straw because the pressure imbalance needed to support a 40-foot column of water isapproximately one atmosphere of pressure. If the straw is taller than 40 feet, you're simply out of luck.Even if you remove all the air from within the straw, the atmospheric pressure of the water below the strawwon't be able to push the water up the straw higher than about 40 feet.

To get the water to rise higher in the straw, you'll need to install a pump at the bottom. The pump increasesthe water pressure there to more than 1 atmosphere, so that there is a bigger pressure imbalance availableand therefore the possibility of supporting a taller column of water.

OK, so returning to your question: once a well is more than about 40 feet deep, getting the water to thesurface requires a pump at the bottom. That pump can boost the water pressure well above atmospheric andthereby push the water to the surface despite the great height and weight of the water column. Suctionsurface pumps are really only practical for water that's a few feet below the surface; after that, deeppressure pumps are a much better idea.

1541. My eight year old daughter asked me, "If light is the fastest thing in the universe what is the secondfastest thing in the universe?" JPW, Lancaster, PAYour daughter's question is a cute one. I like it because it highlights the distinction between the speed oflight and all other speeds. The speed of light is unimaginably special in our universe. Strange though it maysound, even if light didn't exist there would still be the speed of light and it would still have the same value.

The speed of light is part of the geometry of space-time and the fact that light travels at "the speed of light"is almost a cosmic afterthought. Gravity and the so-called "strong force" also travel at that speed.

OK, so there is actually a multi-way tie for first place in the speed rankings. Your daughter's question iswhat comes next? The actual answer is that its a many-way tie between everything else. With enoughenergy, you can get anything moving at just under the speed of light, at least in principle. For example,subatomic particles such as electrons, protons, and even atomic nuclei are routinely accelerated to justunder the speed of light in sophisticated machines around the world. The universe itself has naturalaccelerators that whip subatomic particles up until they are traveling so close to the speed of light that it'shard to tell that they aren't quite at the speed of light. Nonetheless, I assure you that they're not. The speed


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of light is so special that nothing that has any mass at all can possibly travel at the speed of light. Only theephemeral non-massive particles such as light particles (photons), gravity particles (gravitons), and strongforce particles (gluons) can actually travel at the speed of light. In fact, once photons, gravitons, and gluonsbegin to interact with matter, they don't travel at the speed of light either. It's sort of a guilt-by-association:as soon as these massless particles leave the essential emptiness of the vacuum and begin to interact withmatter, even they can't travel at the speed of light anymore.

That said, I can still offer the likely second place finisher on the speed list. I'm going to skip over light,gravity, and the strong force traveling in extremely dilute matter because that's sort of cheating &mdash ifyou take something that naturally travels at the speed of light and slow it down the very, very slightest bit,of course it will come ridiculously close to the speed of light. In real second place are almost certainlycosmic ray particles. These cosmic rays are actually subatomic particles that are accelerated to fantasticenergies by natural processes in the cosmos. How such accelerators work is still largely a mystery but someof the cosmic ray particles that reach our atmosphere have truly astonishing energies once in a while asingle cosmic ray particle that is smaller than an atom will carry enough energy with it that it is capable ofmoving small ordinary objects around. Even if it carries the energy of a fly, that's a stupendous amount ofenergy for an atomic fragment. Those cosmic ray particles are traveling so close to the speed of light that itwould be a photo-finish with light itself.

1540. I have a large commercial superconducting magnet and am looking for a high-value-added product ormanufacturing process to pursue with it. Is there anything you have learned in your research that would beworth producing? PTAs a general observation, the bottleneck in scientific research and technological innovation is almostalways the ideas, not the equipment. Occasionally, a revolutionary piece of equipment comes on the sceneand makes a whole raft of developments possible overnight. But a commercial superconducting magnetisn't revolutionary; you can buy one off the shelf. As a result, all the innovations that were waiting formagnets like that to become available were mopped up long ago and any new innovations will take newideas.Coming up with good ideas is hard work and if I had them, I'd have gotten hold of such a magnet myself.Although science is often taught as formulas and factoids, its really about thinking and observing, andgood ideas are nearly always more important than good equipment. Good ideas don't linger unstudied forlong when commercial equipment is all it takes to pursue them.

1539. How do glasses work and the physics behind them? SDM, MissouriLike a camera, your eye collects light from the scene youre viewing and tries to form a real image of thatscene on your retina. The eyes front surface (its cornea) and its internal lens act together to bend all thelight rays from some distant feature toward one another so that they illuminate one spot on your retina.Since each feature in the scene youre viewing forms its own spot, your eyes cornea and lens are forming areal image of the scene in front of you. If that image forms as intended, you see a sharp, clear rendition ofthe objects in front of you. But if your eye isnt quite up to the task, the image may form either before orafter your retina so that you see a blurred version of the scene.

The optical elements in your eye that are responsible for this image formation are the cornea and the lens.

The cornea does most of the work of converging the light so that it focuses, while the lens provides the fineadjustment that allows that focus to occur on your retina.

If youre farsighted, the two optical elements arent strong enough to form an image of nearby objects onyour retina so you have trouble getting a clear view while reading. Your eye needs help, so you wearconverging eyeglasses. Those eyeglasses boost the converging power of your eye itself and allow your eyeto form sharp images of nearby objects on your retina.

If youre nearsighted, the two optical elements are too strong and need to be weakened in order to formsharp images of distant objects on your retina. Thats why you wear diverging eyeglasses.


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People are surprised when I tell them that theyre nearsighted or farsighted. They wonder how I know. Mytrick is simple: I look through their eyeglasses at distant objects. If those objects appear enlarged, theeyeglasses are converging (like magnifying glasses) and the wearer must be farsighted. If those objectsappear shrunken, the eyeglasses are diverging (like the security peepholes in doors) and the wearer isnearsighted. Try it, youll find that its easy to figure out how other people see by looking through theirglasses as they wear them.

1538. The new soft drink dispenser at a nearby store has touch pads that release soda as long as you arepressing on them. I noticed that if I press a pad with something other than my fingers (like a straw or carkey) nothing happens, no matter how hard I press. Yet with my fingers, I sometimes don't even have tomake actual contact just very close proximity. What is happening here? RLBThose touch pads are sensing your presence electronically, not mechanically. More specifically, electriccharge on the pad pushes or pulls on electric charge on your finger and the pads electronics can tell thatyou are there by how charge on the pad reacts to charge on your finger.

Because your finger and your body conduct electricity, the pads electric charge is actually interacting withthe electric charge on your entire body. In contrast, a straw is insulating, so the pad can only interact with

charge at its tip, and while your car keys are conducting, they are too small to have the effect that yourbody has on that pad.

There are at least two ways for a pad and its electronics to sense your body and its electric charges. Thefirst way is for the electronics to apply a rapidly alternating electric charge to the pad and to watch for thepads charge to interact with charge outside the pad (i.e., on your body). When the pad is by itself, theelectronics can easily reverse the pads electric charge because that charge doesnt interact with anything.But when your hand is near the pad or touching it, its much harder for the electronics to reverse the padselectric charge. If youre touch the pad, the electronics has to reverse your charge, too, so the electronicssense a new sluggishness in the pads response to charge changes. Even when youre not quite touching thepad, the electronics has some add difficulty reversing the pads charge. Thats because the pads chargecauses your finger and body to become electrically polarized: charges opposite to those on the pad areattracted onto your finger from your body so that your finger becomes electrically charged opposite to the

charge of the pad. When the electronics then tries to withdraw the charge from the pad in order to reversethe pads charge, your fingers charge acts to make that withdrawal difficult. The electronics finds that itmust struggle to reverse the pads charge even though youre not in direct contact with the pad. Overall,your finger complicates the charge reversals whenever its near or touching the pad.

The second way for the pads electronics to sense your presence is to let your body act as an antenna forelectromagnetic influences in the environment. We are awash in electric and magnetic fields of all sorts andthe electric charge on your body is in ceaseless motion as a result. Youve probably noticed that touchingcertain input wires of a stereo amplifier produces lots of noise in the speakers; thats partly a result of theelectromagnetic noise in our environment showing up as moving charge on your body. The little pad on thesoda dispenser picks up a little of this electromagnetic noise all by itself. When you approach or touch thepad, however, you dramatically increase the amount of electromagnetic noise in the pad. The padselectronics easily detect that new noise.

In short, soda dispenser pads are really detecting large electrically conducting objects. Their ability to senseyour finger even before it makes contact is important because they need to work when people are wearinggloves. I first encountered electrical touch sensors in elevators when I was a child and I loved to experimentwith them. Conveniently, theyd light up when they detected something and there was no need to clean upspilled soda. Wed try triggering them with elbows and noses, and a whole variety of inanimate objects.They were already pretty good, but modern electronics has made touch pads even better. The touchswitches used by some lamps and other appliances function in essentially the same way.


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1537. Why do washed clothes dry faster in open air than in a closed room? A, Aizawl, IndiaWhat thrills me about your question is that while we've all noticed this effect, we're never taught why ithappens. Let me ask your question in another way: we know that opening a window makes the clothes dryfaster, but how do the clothes know that the window is open? Who tells them?

The explanation is both simple and interesting: the rate at which water molecules leave the cloths doesn'tdepend on whether the window is open or closed, but the rate at which water molecules return to the clothscertainly does. That return rate depends on the air's moisture content and can range from zero in dry air toextremely fast in damp air. Air's moisture content is usually characterized by its relative humidity, with100% relative humidity meaning that air's water molecules land on surfaces exactly as fast as watermolecules in liquid water leave its surface. When you expose a glass of water to air at 100% relativehumidity, the glass will neither lose nor gain water molecules because the rates at which water moleculesleave the water and land on the water are equal. Below 100% relative humidity, the glass will graduallyempty due to evaporation because leaving will outpace landing. Above 100% relative humidity, the glasswill gradually fill due to condensation because landing will outpace leaving.

The same story holds true for wet clothes. The higher the air's relative humidity, the harder it becomes forwater to evaporate from the cloths. Landing is just too frequent in the humid air. At 100% relative humiditythe clothes won't dry at all, and above 100% relative humidity they'll actually become damper with time.

When you dry clothes in a room with the window open and the relative humidity of the outdoor air is lessthan 100%, water molecules will leave the clothes more often than they'll return, so the clothes will dry. Butwhen the window is closed, the leaving water molecules will remain trapped in the room and will graduallyincrease the room air's relative humidity. The drying process will slow down as the water-molecule returnrate increases. When the room air's relative humidity reaches 100%, drying will cease altogether.

1536. Why does steam make ironing cotton pants so much easier? AB, VirginiaWater "plasticizes" the cotton. A plasticizer is a chemical that dissolves into a plastic and lubricates itsmolecules so that they can move across one another more easily. Cotton is almost pure cellulose, a polymerconsisting of sugar molecules linked together in long chains. Since sugar dissolves easily in water, waterdissolves easily in cellulose. Even though cellulose scorches before it melts, it can be softened by heat andwater. When you iron cotton pants, the steam dissolves into the cellulose molecules and allows the fabric to

smooth out beautifully.1535. A co-worker who is an intelligent electrical engineer said an ungrounded microwave is dangerousbecause microwaves can then escape through the holes in the door. Aside from the electrical dangers, Idisagreed because I think it is just the size of the holes vs. the wavelength of the microwaves. Does lack ofa ground allow some microwaves to escape through the holes in the microwave door? LG, MaineYoure right. Whether the microwave oven is grounded or not makes no difference on its screens ability toprevent microwave leakage. In fact, the whole idea of grounding something is nearly meaningless at suchhigh frequencies. Since electrical influences can't travel faster than the speed of light and light only travels12.4 cm during one cycle of the ovens microwaves, the oven can't tell if it's grounded at microwavefrequencies; its power cord is just too long and there just isnt time for charge to flow all the way throughthat cord during a microwave cycle.

When you ground an appliance, youre are making it possible for electric charge to equilibrate between that

appliance and the earth. The earth is approximately neutral, so a grounded appliance cant retain largeamounts of either positive or negative charge. Thats a nice safety feature because it means that you wontget a shock when you touch the appliance, even if one of its power wires comes loose and touches the case.Any charge that the power wire tries to deposit on the case will quickly flow to the earth as the applianceand earth equilibrate.

But charge cant escape from the appliance through the grounding wire instantly. Light takes about 1nanosecond to travel 1 foot and electricity takes a little longer than that. For charge to leave your appliancefor the earth might well require 50 nanoseconds or more. Thats not a problem for ordinary powerdistribution, so grounding is generally a great idea. Each cycle of the 60-Hz AC power in the U.S. takes 18


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milliseconds to complete, so the appliance and earth have plenty of time to equilibrate with one another.But a cycle of the microwave power in the oven takes less about 0.4 nanoseconds to complete and theresjust no time for the appliance and earth to equilibrate. At microwave frequencies, the electric currentflowing through a long wire is wavelike, meaning that at one instant in time the wire has both positive andnegative patches, spaced half a wavelength apart along its length. Its carrying an electromagnetic ripple.

The metal screen on the ovens door has to reflect the microwaves all by itself. It does this without aproblem because the holes are so much smaller than 12.4 centimeters that currents easily flow around themduring a cycle of the microwaves. Those currents are able to compensate for the holes in the screens andcause the microwaves to reflect perfectly.

1534. A bird lands on an uninsulated 10,000 volt power line. Will it become extra crispy? RKS, TexasNo. Birds do this all the time. What protects the bird is the fact that it doesnt complete a circuit. It touchesonly one wire and nothing else. Although there is a substantial charge on the power line and some of thatcharge flows onto the bird when it lands, the charge movement is self-limiting. Once the bird has enoughcharge on it to have the same voltage as the power line, charge stops flowing. And even though the powerlines voltage rises and falls 60 times a second (or 50 times a second in some parts of the world), the overallcharge movement at 10,000 volts just isnt enough to bother the bird much. At 100,000 volts or more, thecharge movement is uncomfortable enough to keep birds away, so you dont see them landing on the

extremely high-voltage transmission lines that travel across vast stretches of countryside.

The story wouldnt be the same if the bird made the mistake of spanning the gap from one wire to another.In that case, current could flow through the bird from one wire to the other and the bird would run theserious risk of becoming a flashbulb. Squirrels occasionally do this trick when they accidentally bridge apair of wires. Some of the unexpected power flickers that occur in places where the power lines runoverhead are caused by squirrels and occasionally birds vaporizing when they let current flow betweenpower lines.1533. Why do I sometimes shock myself when I kiss Uncle Al? BSIf both of you were electrically neutral before the kiss, nothing would happen. Evidently, one of you hasdeveloped a net charge and that charge is suddenly spreading itself out onto the other person during thekiss. That charge flow is an electric current and you feel currents flowing through your body as a shock.

Most likely, one of you has been in contact with a insulating surface that has exchanged charge with you.For example, if you walked across wool carpeting in rubber-soled shoes, that carpeting has probablytransferred some of its electrons to your shoes and your shoes have then spread those electrons out ontoyou. Rubber binds electrons more tightly than wool and so your shoes tend to steal a few of electrons fromwool whenever it gets a chance. If you walk around a bit or scuff your feet, you'll typically end up withquite a large number of stolen electrons on your body. When you then go and kiss Uncle Al, about half ofthose electrons spread suddenly onto him and that current flow is shocking!1532. There is a video circulating on the internet which purports to show an "inventor" who has a machinethat burns water. Water is broken down into hydrogen and oxygen which is then burned to produce....morewater! I maintain that the net energy produced would be about zero since energy must be expended toseparate water into hydrogen and oxygen. Your comments please. ST, ArizonaYou have it exactly right. Water itself is burned hydrogen, and the energy required to separate water into

hydrogen and oxygen is equal to the energy released when the hydrogen subsequently burns back intowater. Energy in and energy out. Just as in bicycling, if you want to roll downhill, you have to pedal uphillfirst.

Anyone who claims to be able to extract useful energy through a process that starts with water and endswith water is a charlatan. Either they aren't producing any useful energy or it's coming from some othersource. In these sorts of frauds, there is usually some electrical component that is supposedly needed tokeep a minor part of the apparatus functioning. That component isn't insignificant at all; it's what actuallykeeps the entire apparatus functioning!


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Hydrogen has such a mythical aura to it, but in the context of energy, it's just another fuel. Actually, it'smore of any energy storage medium than a basic fuel. That's because hydrogen doesn't occur naturally onearth and can only be produced by consuming another form of energy. There is so much talk about "thehydrogen economy" and the notion that hydrogen will rescue us from our dependence on petroleum.Sadly, politicians who promote hydrogen as the energy panacea neither understand science nor respectthose who do. Since it takes just as much energy to produce hydrogen from water as is released when thathydrogen burns back into water, hydrogen alone won't save us.

As we grow progressively more desperate for useable energy, the amount of fraud and misinformation willonly increase. There are only a few true sources for useable energy: solar energy (which includes windpower, hydropower, and biomass), fossil fuels (which include petroleum and coal), geothermal energy, andnuclear fuels. Hydrogen is not among them; it can be produced only at the expense of one of the others.Even ethanol, which is touted as an environmentally sound replacement for petroleum, has its problems;producing a gallon of ethanol can all too easily consume a gallon of petroleum.

Where energy is concerned, watch out for fraud, hype, PR, and politics. If we survive the coming energyand climate crises, it will be because we've learned to conserve energy and to obtain it primarily from solarand perhaps nuclear sources. It will also be because we've learned to set politics and self-interest aside longenough to make accurate analyses and sound decisions.

1531. What does it mean if a light bulb uses 60 watts? B, Los AngelesThe watt is a unit of power, equivalent to the joule-per-second. One joule is about the amount of energy ittakes to raise a 12 ounce can of soda 1 foot. A 60 watt lightbulb uses 60 joules-per-second, so the power itconsumes could raise a 24-can case of soda 2.5 feet each second. Most tables are about 2.5 feet above thefloor. Next time you leave a 60-watt lightbulb burning while you're not in the room, imagine how tiredyou'd get lifting one case of soda onto a table every second for an hour or two. That's the mechanical effortrequired at the generating plant to provide the 60-watts of power you're wasting. If don't need the light, turnoff lightbulb!

1530. Does space dust settle on orbiting space shuttles? A, Troy, MTWhat a great question! I love it. The answer is no, but there's much more to the story.

I'll begin to looking at how dust settles in calm air near the ground. That dust experiences its weight due togravity, so it tends to descend. Each particle would fall like a rock except that it's so tiny that it experiencesoverwhelming air resistance. Instead of falling, it descends at an incredibly slow terminal velocity, typicallyonly millimeters per second. It eventually lands on whatever is beneath it, so a room's floor graduallyaccumulates dust. But dust also accumulates on vertical walls and even on ceilings. That dust is held inplace not by its weight but by electrostatic or chemical forces. When you go into an abandoned attic, mostof the dust is on the floor, but there's a little on the walls and on the ceiling.

OK, now to the space shuttle. The shuttle is orbiting the earth, which means that although it has weight andis falling freely, it never actually reaches the earth because it's heading sideways so fast. Without gravity, itsinertia would carry it horizontally out into space along a straight line path. Gravity, however, bends thatstraight line path into an elliptical arc that loops around the earth as an orbit.

So far no real surprises: dust near ground level settles in calm air and the shuttle orbits the earth. Thesurprise is that particles of space dust particles also orbit the earth! The shuttle orbits above the atmosphere,where there is virtual no air. Without air to produce air resistance, the dust particles also fall freely. Thosewith little horizontal speed simply drop into the atmosphere and are lost. But many dust particles havetremendous horizontal speeds and orbit the earth like tiny space shuttles or satellites.

Whether they are dropping toward atmosphere or orbiting the earth, these space dust particles are typicallytraveling at velocities that are quite different in speed or direction from the velocity of the space shuttle.The relative speed between a dust particle and the shuttle can easily exceed 10,000 mph. When such a fast-


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moving dust particle hits the space shuttle, it doesn't "settle." Rather, it collides violently with the shuttle'ssurface. These dust-shuttle collisions erode the surfaces of the shuttle and necessitate occasional repairs orreplacements of damaged windows and sensors. Astronauts on spacewalks also experience these fastcollisions with space dust and rely on their suits to handle all the impacts.

Without any air to slow the relative speeds and cushion the impacts, its rare that a particle of space dustlands gracefully on the shuttle's surface. In any case, gravity won't hold a dust particle in place on theshuttle because both the shuttle and dust are falling freely and gravity doesn't press one against the other.But electrostatic and chemical attractions can hold some dust particles in place once they do land. So theshuttle probably does accumulate a very small amount of accumulated space dust during its travels.1529. Why do scantron-type tests only read #2 pencils? Can other pencils work? MW, Montgomery, ALThe #2-pencil requirement is mostly historical. Because modern scantron systems can use all thesophistication of image sensors and computer image analysis, they can recognize marks made with avariety of materials and they can even pick out the strongest of several marks. If they choose to ignoremarks made with materials other than pencil, it's because they're trying to be certain that they'rerecognizing only marks made intentionally by the user. Basically, these systems can "see" most of thedetails that you can see with your eyes and they judge the markings almost as well as a human would.

The first scantron systems, however, were far less capable. They read the pencil marks by shining light

through the paper and into Lucite light guides that conveyed the transmitted light to phototubes. Wheneversomething blocked the light, the scantron system recorded a mark. The marks therefore had to be opaque inthe range of light wavelengths that the phototubes sensed, which is mostly blue. Pencil marks were theobvious choice because the graphite in pencil lead is highly opaque across the visible light spectrum.Graphite molecules are tiny carbon sheets that are electrically conducting along the sheets. When you writeon paper with a pencil, you deposit these tiny conducting sheets in layers onto the paper and the paperdevelops a black sheen. It's shiny because the conducting graphite reflects some of the light waves from itssurface and it's black because it absorbs whatever light waves do manage to enter it.

A thick layer of graphite on paper is not only shiny black to reflected light, it's also opaque to transmittedlight. That's just what the early scantron systems needed. Blue inks don't absorb blue light (that's why theyappear blue!), so those early scantron systems couldn't sense the presence of marks made with blue ink.Even black inks weren't necessarily opaque enough in the visible for the scantron system to be confident

that it "saw" a mark.

In contrast, modern scantron systems used reflected light to "see" marks, a change that allows scantronforms to be double-sided. They generally do recognize marks made with black ink or black toner fromcopiers and laser printers. I've pre-printed scantron forms with a laser printer and it works beautifully. Butmodern scantron systems ignore marks made in the color of the scantron form itself so as not to confuseimperfections in the form with marks by the user. For example, a blue scantron form marked with blue inkprobably won't be read properly by a scantron system.

As for why only #2 pencils, that's a mechanical issue. Harder pencil leads generally don't produce opaquemarks unless you press very hard. Since the early scantron machines needed opacity, they missed too manymarks made with #3 or #4 pencils. And softer pencils tend to smudge. A scantron sheet filled out using a #1pencil on a hot, humid day under stressful circumstances will be covered with spurious blotches and the

early scantron machines confused those extra blotches with real marks.

Modern scantron machines can easily recognize the faint marks made by #3 or #4 pencils and they canusually tell a deliberate mark from a #1 pencil smudge or even an imperfectly erased mark. They can alsodetect black ink and, when appropriate, blue ink. So the days of "be sure to use a #2 pencil" are pretty muchover. The instruction lingers on nonetheless.

One final note: I had long suspected that the first scanning systems were electrical rather than optical, but Icouldn't locate references. To my delight, Martin Brown informed me that there were scanning systems thatidentified pencil marks by looking for their electrical conductivity. Electrical feelers at each end of the


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markable area made contact with that area and could detect pencil via its ability to conduct electric current.To ensure enough conductivity, those forms had to be filled out with special pencils having highconductivity leads. Mr. Brown has such an IBM Electrographic pencil in his collection. This electrographicand mark sense technology was apparently developed in the 1930s and was in wide use through the 1960s.1528. If a home looses some of its power during a power outage and the lights shine dim, will it burn up themotor in the refrigerator? Will it damage other appliances (TV, VCR. stereo. etc)? Should the maindisconnect be shut off? J, OhioPower outages come in a variety of types, one of which involves a substantial decrease in the voltagesupplied to your home. The most obvious effect of this voltage decrease is the dimming of the incandescentlights, which is why it's called a "brownout." The filament of a lightbulb is poor conductor of electricity, sokeeping an electric charge moving through it steadily requires a forward force. That forward force isprovided by the voltage difference between the two wires: the one that delivers charges to the filament andthe one that collects them back from the filament. As the household voltage decreases, so does the force oneach charge in the filament. The current passing through the filament decreases and the filament receivesless electric power. It glows dimly.

At the risk of telling you more than you ever want to know, I'll point out that the filament behavesapproximately according to Ohm's law: the current that flows through it is proportional to the voltagedifference between its two ends. The larger that voltage difference, the bigger the forces and the morecurrent that flows. This ohmic behavior allows incandescent lightbulbs to survive decreases in voltage

unscathed. They don't, however, do well with increases in voltage, since they'll then carry too much currentand receive so much power that they'll overheat and break. Voltage surges, not voltage decreases, are whatkill lightbulbs.

The other appliances you mention are not ohmic devices and the currents that flow through them are notsimply proportional to the voltage supplied to your home. Motors are a particularly interesting case; theaverage current a motor carries is related in a complicated way to how fast and how easily it's spinning. Amotor that's turning effortlessly carries little average current and receives little electric power. But a motorthat is struggling to turn, either because it has a heavy burden or because it can't obtain enough electricpower to overcome starting effects, will carry a great deal of average current. An overburdened or non-starting motor can become very hot because it's wiring deals inefficiently with the large average current,and it can burn out. While I've never heard of a refrigerator motor dying during a brownout, it wouldn'tsurprise me. I suspect that most appliance motors are protected by thermal sensors that turn them off

temporarily whenever they overheat.

Modern electronic devices are also interesting with respect to voltage supply issues. Electronic devicesoperate on specific internal voltage differences, all of which are DC direct current. Your home issupplied with AC alternating current. The power adapters that transfer electric power from the home'sAC power to the device's DC circuitry have evolved over the years. During a brownout, the older types ofpower adapters simply provide less voltage to the electronic devices, which misbehave in various ways,most of which are benign. You just want to turn them off because they're not working properly. It's just as iftheir batteries are worn out.

But the most modern and sophisticated adapters are nearly oblivious to the supply voltage. Many of themcan tolerate brownouts without a hitch and they'll keep the electronics working anyway. The power unitsfor laptops are a case in point: they can take a whole range of input AC voltages because they prepare their

DC output voltages using switching circuitry that adjusts for input voltage. They make few assumptionsabout what they'll be plugged into and do their best to produce the DC power required by the laptop.

In short, the motors in your home won't like the brownout, but they're probably protected against thepotential overheating problem. The electronic appliances will either misbehave benignly or ride out thebrownout unperturbed. Once in a while, something will fail during a brownout. But I think that most of thedamage is down during the return to normal after the brownout. The voltages bounce around wildly for asecond or so as power is restored and those fluctuations can be pretty hard some devices. It's probablyworth turning off sensitive electronics once the brownout is underway because you don't know what willhappen on the way back to normal.


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1527. My husband put a large metal bowl in our new microwave oven and tore a small hole in the oven'smetal screen while trying to close the door. My husband isn't concerned, but the oven is mounted over thestove at face level and it certainly concerns me. Can we use it? E, Ontario, CanadaThat tear in the window screen presents three potential problems: microwave leakage, evanescent waves,and arcing. As long as the hole is small, less than a centimeter or so, it's not likely to allow muchmicrowave leakage. The oven's microwaves have a wavelength of 12.4 centimeters and they'll reflect fromconducting surfaces with holes much smaller than that wavelength. A foot from your oven, there probablywon't be any significant microwave intensity, although the only way to be sure is with a microwave leakagemeter.

The evanescent wave problem is more likely. When any electromagnetic wave reflects from a conductingsurface that has small holes in it, there is what is known as an evanescent wave extending into andsomewhat beyond each hole. It's as though the wave is trying to figure out whether or not it can passthrough the opening and so it tries. Even when it discovers that the hole is far too small for it pass through(i.e., much smaller than its wavelength), it still offers electromagnetic intensity in the region just beyond thehole. The extent of the evanescent wave increases with the size of the hole. The microwave oven's screenhas very small holes and it is located inside the glass window. The evanescent waves associated with thoseholes cut off so quickly that you can hold your hand against the glass and not expose your skin to

significant microwaves. But once you've torn a larger hole in the screen, the evanescent waves can extendfarther through that screen and perhaps out beyond the surface of the glass window. If you press your handagainst the window just in front of the tear while the microwave oven is on, you may burn your hand.

Finally, there is the issue of arcing. To reflect the microwaves, the conducting screen must carry electriccurrents. The microwaves' electric fields push electric charge back and forth in the conducting screen and itis that moving charge (i.e., electric current) that ultimately redirects the microwaves back into the cookingchamber as a reflection. Those electric currents in the screen are real and they're not going to take kindly tothat tear. It's a weak spot in the conducting surface through which they flow. Weak electrical paths can heatup like lightbulb filaments when they carry currents. Moreover, charge that should flow across the tornregion can accumulate on sharp edges and leap through the air as an arc. If either of these processeshappens, it may scorch the window and the screen, and cause increasing trouble.

You could be lucky: the leakage could be zero, the evanescent waves could remain far enough inside thewindow to never cause injury, and the tear could never heat up or arc. But the risk of operating thisdamaged microwave oven is not insignificant. Since it's an installed unit, I'd suggest replacing the screen orthe door (assuming that such replacements are available).1526. Your answer to question #1393 is fine for the hypothetical case of the earth orbiting around the moon,but I don't see how it works for the real case where the moon orbits the earth. What is the real reason for thetides? DMThere is nothing hypothetical about the earth orbiting the moon; it's as real as the moon orbiting the earth.The earth and the moon are simply two huge balls in otherwise empty space and though the mass of one is81 times the mass of the other, they're both in motion. More specifically, they're in orbit around theircombined center of mass the effective location of the earth-moon system.

Since the earth is so much more massive than the moon, their combined center of mass is 81 times closer to

the middle of the earth than it is to the middle of the moon. In fact, it's inside the earth, though not at themiddle of the earth. As a result, the earth's orbital motion takes the form of a wobble rather than a moreobvious looping path. Nonetheless, the earth is orbiting.

I hope that you can see that there is no reason why the earth should be fixed in space while the moon orbitsabout it. You've been sold a bill of goods. The mistaken notion that the moon orbits a fixed earth is awonderful example of the "factoid science" that often passes for real science in our society.

Because thinking and understanding involve hard work, people are more comfortable when the thought andunderstanding have been distilled out of scientific issues and they've been turned into memorizable sound


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bites. Those sound bites are easy to teach and easy to test, but they're mostly mental junk food. A goodteacher, like a good scientist, will urge you to question such factoids until you understand the sciencebehind them and why they might or might not be true.

When my children were young, I often visited their schools to help teach science. In third grade, therequired curriculum had them categorizing things into solutions or mixtures. Naturally, I showed them avariety of things that are neither solutions nor mixtures. It was a blast. Science is so much more interestingthan a collection of 15-second sound bites.1525. Is it true that the bigger the lens on a camera, the more light goes through it and the better the photoor video? My film teacher says that while this idea is logically correct, he didn't know if it was true. Yourlecture slides say the answer is yes, but my teacher still doesn't believe it. We were wondering about yoursource for this material. PJI'll assume that by "bigger lens" you mean one that is larger in diameter and that therefore collects all thelight passing through a larger surface area. While a larger-diameter lens can project a brighter image ontothe image sensor or film than a smaller-diameter lens, that's not the whole story. Producing a better photo orvideo involves more than just brightness.

Lenses are often characterized by their f-numbers, where f-number is the ratio of effective focal length toeffective lens diameter. Focal length is the distance between the lens and the real image it forms of a distantobject. For example, if a particular converging lens projects a real image of the moon onto a piece of paper

placed 200 millimeters (200 mm) from the lens, then that lens has a focal length of 200 mm. And if the lensis 50 mm in diameter, it has an f-number of 4 because 200 mm divided by 50 mm is 4.

Based on purely geometrical arguments, it's easy to show that lenses with equal f-numbers project imagesof equal brightness onto their image sensors and the smaller the f-number, the brighter the image. Whethera lens is a wide-angle or telephoto, if it has an f-number of 4, then its effective focal length is four times theeffective diameter of its light gathering lens. Since telephoto lenses have long focal lengths, they need largeeffective diameters to obtain small f-numbers.

But notice that I referred always to "effective diameter" and "effective focal length" when defining f-number. That's because there are many modern lenses that are so complicated internally that simplydividing the lens diameter by the distance between the lens and image sensor won't tell you much. Many ofthese lenses have zoom features that allow them to vary their effective focal lengths over wide ranges and

these lenses often discard light in order to improve image quality and avoid dramatic changes in imagebrightness while zooming.

You might wonder why a lens would ever choose to discard light. There are at least two reasons for doingso. First, there is the issue of image quality. The smaller the f-number of a lens, the more precise its opticsmust be in order to form a sharp image. Low f-number lenses are bringing together light rays from a widerange of angles and getting all of those rays to overlap perfectly on the image sensor is no small feat.Making a high-performance lens with an f-number less than 2 is a challenge and making one with an f-number of less than 1.2 is extremely difficult. There are specialized lenses with f-numbers below 1 andCanon sold a remarkable f0.95 lens in the early 1960's. The lowest f-number camera lens I have everowned is an f1.4.

Secondly, there is the issue of depth-of-focus. The smaller the f-number, the smaller the depth of focus.

Again, this is a geometry issue: a low-f-number lens is bringing together light rays from a wide range ofangles and those rays only meet at one point before separating again. Since objects at different distances infront of the lens form images at different distances behind the lens, it's impossible to capture sharp imagesof both objects at once on a single image sensor. With a high-f-number lens, this fact isn't a problembecause the light rays from a particular object are rather close together even when the object's image formsbefore or after the image sensor. But with a low-f-number lens, the light rays from a particular object cometogether acceptably only at one particular distance from the lens. If the image sensor isn't at that distance,then the object will appear all blurry. If a zoom lens didn't work to keep its f-number relatively constantwhile zooming from telephoto to wide angle, its f-number would decrease during that zoom and its depth-


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of-focus would shrink. To avoid that phenomenon, the lens strategically discards light so as to keep its f-number essentially constant during zooming.

In summary, larger diameter lenses tend to be better at producing photographic and video images, but thatassumes that they are high-quality and that they can shrink their effective diameters in ways that allowthem to imitate high-quality lenses of smaller diameters when necessary. But flexible characteristics alwayscome at some cost of image quality and the very best lenses are specialized to their tasks. Zoom lenses can'tbe quite as good as fixed focal length lenses and a large-diameter lens imitating a small-diameter lens bythrowing away some light can't be quite as good as a true small-diameter lens.

As for my sources, one of the most satisfying aspects of physics is that you don't always need sources.Most of the imaging issues I've just discussed are associated with simple geometric optics, a subject that ispart of the basic toolbox of an optical physicist (which I am). You can, however, look this stuff up in anybook on geometrical optics.1524. Can I warm plates in my microwave oven? ACYes, but it's not a good idea. Depending on the type of plate, you can either damage your microwave ovenor damage the plate.

If a plate is "microwave safe," it will barely absorb the microwaves and heat extremely slowly. In effect,the microwave oven will be operating empty and the electromagnetic fields inside it will build up to

extremely high levels. Since the walls of the oven are mirrorlike and the plate is almost perfectlytransparent to microwaves, the electromagnetic waves streaming out of the oven's magnetron tube bouncearound endlessly inside the oven's cooking chamber. The resulting intense fields can produce various typesof electric breakdown along the walls of the cooking chamber and thereby damage the surface with burnsor arcs. Furthermore, the intense microwaves in the cooking chamber will reflect back into the magnetronand can upset its internal oscillations so that it doesn't function properly. Although magnetrons areastonishingly robust and long-lived, they don't appreciate having to reabsorb their own emittedmicrowaves. In short, your plates will heat up slowly and you'll be aging your microwave oven in theprocess. You could wet the plates before putting them in the microwave oven to speed the heating anddecrease the wear-and-tear on the magnetron, but then you'd have to dry the plates before use.

If a plate isn't "microwave safe," then it will absorb microwaves and heat relatively quickly. If it absorbsthe microwaves uniformly and well, then you can probably warm it to the desired temperature without any

problems as long as you know exactly how many seconds it takes and adjust for the total number of platesyou're warming. If you heat a plate too long, bad things will happen. It may only amount to burning yourfingers, but some plates can't take high temperatures without melting, cracking, or popping. Unglazedceramics that have soaked up lots of water will heat rapidly because water absorbs microwaves strongly.Water trapped in pores in such ceramics can transform into high-pressure steam, a result that doesn't seemsafe to me. And if a plate absorbs microwaves nonuniformly, then you'll get hotspots or burned spots on theplate. Metalized decorations on a plate will simply burn up and blacken the plate. Cracks that contain waterwill overheat and the resulting thermal stresses will extend the cracks further. So this type of heating can bestressful to the plates.

1523. How deep under water can I go while breathing from a hose that rises above the surface of the water? DF, Downers Grove, IL

You can only go a few feet under water before you'll no longer be able to draw air into your lungs throughthat hose. It's a pressure problem. The water pressure outside your chest increases rapidly as you go deeper,but the air pressure inside the hose and your mouth barely changes at all. Pretty soon, you'll have so muchmore pressure outside your lungs than inside them that you won't be able to draw in any more air. Yourmuscles just won't be strong enough.

The water pressure increases quickly with depth because each layer of water must support the weight of allthe water layers above it. Since water is dense, heavy stuff, the weight piles on quickly and it takes only 10meters (34 feet) of descent to increase the water pressure from atmospheric to twice atmospheric. Incontrast, the air in the hose is light, fluffy stuff, so its pressure increases rather slowly with depth. Even


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though each layer of air has to support the weight of all the layers of air above it, the rise in pressure isextremely gradual. It takes miles of atmosphere above the earth for the air pressure to build up toatmospheric pressure near the ground. The air pressure in your hose is therefore approximately unchangedby your descent into the water.

With the water pressure outside rising quickly as you go deeper and the air pressure in your mouth risingincredibly slowly as you go deeper, you quickly find it hard to breathe. Your muscles can push your chestoutward against a modest pressure imbalance between outside and inside. But by the time you're a few feetbelow the surface, you just can't draw air into your lungs through that hose anymore. You need pressurizedair, such as that provided by a scuba outfit or a deep-sea diver's compressor system.1522. Would ice in the freezer absorb the smell in the freezer? ML, Auckland NZDespite the freezer's low temperature and the motionlessness of all the frozen foods inside it, there is stillplenty of microscopic motion going on. Every surface inside the freezer is active, with individual moleculeslanding and leaving all the time. Whenever a molecule on the surface of a piece of food manages to gatherenough thermal energy from its neighbors, it will break free of the surface and zip off into the air as a vapormolecule. And whenever a vapor molecule in the air collides with the surface of another piece of food, itmay stick to that surface and remain there indefinitely.

Since the freezer has a nearly airtight seal, the air it contains remains inside it for a long time. That meansthat the odor molecules that occasionally break free of a pungent casserole at one end of the freezer have

every opportunity to land on and stick to an ice cube at the other end. With time, the ice cube acquires thescent of the casserole and becomes unappealing.

To stop this migration of molecules, you should seal each item in the freezer in its own container. That way,any molecules that leave the food's surface will eventually return to it. Since ice cubes are normallyexposed to the air in the freezer, keeping the odor molecules trapped in their own sealed containers keepsthe freezer air fresh and the ice cubes odor-free.1521. I was told the holes in the front door of a microwave oven were shaped round because the microwavebeam is shaped as a square. Thus, this means that a square shape object cannot pass through a round shapedobject. Is this a true statement or not? -- BH, TexasNo, there is no square-peg in round-hole effect going on in microwave ovens. Microwaves reflect fromconducting surfaces, just as light waves reflect from shiny metals, and they can't pass through holes inconducting surfaces if those holes are substantially smaller than their wavelengths. The holes in the

conducting mesh covering the microwave oven's window are simply too small for the microwaves and themicrowaves are reflected by that mesh.

Microwaves themselves have no well-defined shape but they do have firm rules governing their overallstructures. Books usually draw microwaves (and all other electromagnetic waves) as wavy lines, as thoughsomething was truly going up and down in space. From that misleading representation, it's easy for peopleto suppose that electromagnetic waves can't get through certain openings.

In reality, electromagnetic waves consist of electric and magnetic fields (influences that push on electriccharge and magnetic pole, respectively) that point up and down in a rippling fashion, but nothing actuallytravels up and down per say. The spatial structures of these fields are governed by Maxwell's equations, aset of four famous relationships that bind electricity and magnetism into a single, unified classical theory.Maxwell's equations dictate the structures of electromagnetic waves and predict that electromagnetic waves

on one side of a conducting surface can't propagate through to the other side of that surface. Even if thereare small holes in the conducting surface, holes that are much smaller that the wavelength of the waves,those waves can't propagate through the surface. More specifically, the fields die off exponentially as theytry to penetrate through the holes and the waves don't propagate on the far side.

The choice of round holes in the oven mesh is simply a practical one. You can pack round holes prettytightly in a surface while leaving their conducting boundaries relatively robust. And round holes treat allelectromagnetic waves equally because they have no wide or narrow directions.


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1520. What happens when sheets of paper, long rolled up into a tube, are unrolled but simply won't ever lieflat again? -- PDPaper consists mostly of cellulose, a natural polymer (i.e. plastic) built by stringing together thousands ofindividual sugar molecules into vast chains. Like the sugars from which it's constructed, cellulose'smolecular pieces cling tightly to one another at room temperature and make it rather stiff and brittle.Moreover, cellulose's chains are so entangled with one another that it couldn't pull apart even if itsmolecular pieces didn't cling so tightly. These effects are why it's so hard to reshape cellulose and whywood or paper don't melt; they burn or decompose instead. In contrast, chicle -- the polymer in chewinggum -- can be reshaped easily at room temperature.

Even though pure cellulose can't be reshaped by melting, it can be softened with water and/or heat. Likeordinary sugar, cellulose is attracted to water and water molecules easily enter its chains. This waterlubricates the chains so that the cellulose becomes somewhat pliable and heat increases that pliability.When you iron a damped cotton or linen shirt, both of which consist of cellulose fibers, you're takingadvantage of that enhanced pliability to reshape the fabric.

But even when dry, fibrous materials such as paper, cotton, or linen have some pliability because thin fibersof even brittle materials can bend significantly without breaking. If you bend paper gently, its fibers willbend elastically and when you let the paper relax, it will return to its original shape.

However, if you bend the paper and keep it bent for a long time, the cellulose chains within the fibers willbegin to move relative to one another and the fibers themselves will begin to move relative to other fibers.Although both of these motions can be facilitated by moisture and heat, time along can get the job done atroom temperature. Over months or years in a tightly rolled shape, a sheet of paper will rearrange itscellulose fibers until it adopts the rolled shape as its own. When you then remove the paper from itsconstraints, it won't spontaneously flatten out. You'll have to reshape it again with time, moisture, and/orheat. If you press it in a heavy book for another long period, it'll adopt a flat shape again.

1519. Why is a car's rear window put and kept under stress, and what has this to do with polarization? --BD, Leuven, BelgiumThe rear window of a car is made of tempered glass -- the glass is heated approximately to its softeningtemperature and then cooled abruptly to put its surface under compression, leaving its inside material under

tension. That tempering process makes the glass extremely strong because its compressed surface is hard totear. But once a tear does manage to propagate through the compressed surface layer into the tense heart ofthe glass, the entire window shreds itself in a process called dicing fracture -- it tears itself into countlesslittle cubes.

The stresses frozen into the tempered glass affect its polarizability and give it strange characteristics whenexposed to the electromagnetic fields in light. This stressed glass tends to rotate polarizations of the lightpassing through it. As a result, you see odd reflections of the sky (skylight is polarized to some extent).Those polarization effects become immediately apparent when you wear polarizing sunglasses.1518. Why must you "shake down" a mercury fever thermometer? I was told by one manufacturer thatmercury expands but does not contract. Also, is it true that the rounded glass acts as a magnifier because thebore is so small? -- JBMercury does expand with temperature; moreover, it expands more rapidly with temperature than glass

goes. That's why the column of mercury rises inside its glass container. While both materials expand asthey get hotter, the mercury experiences a larger increase in volume and must flow up the narrow channelor "capillary" inside the glass to find room for itself. Mercury is essentially incompressible so that, as itexpands, it pushes as hard as necessary on whatever contains it in order to obtain the space it needs. That'swhy a typical thermometer has an extra chamber at the top of its capillary. That chamber will receive theexpanding mercury if it rises completely up the capillary so that the mercury won't pop the thermometer ifit is overheated. In short, the force pushing mercury up the column can be enormous.


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The force pushing mercury back down the column as it cools is tiny in comparison. Mercury certainly doescontract when cooled, so that the manufacturer is telling you nonsense. But just because the mercurycontracts as it cools doesn't mean that it will all flow back down the column. The mercury needs a push topropel it through its narrow channel.

Mercury is attracted only weakly to glass, so it doesn't really adhere to the walls of its channel. However,like all liquids, mercury has a viscosity, a syrupiness, and this viscosity slows its motion through any pipe.The narrower the pipe, the harder one has to push on a liquid to keep it flowing through that pipe. In fact,flow through a pipe typically scales as the 4th power of that pipe's radius, which is why even modestnarrowing of arteries can dramatically impair blood flow in people. The capillaries used in feverthermometers are so narrow that mercury has tremendous trouble flowing through them. It takes big forcesto push the mercury quickly through such a capillary.

During expansion, there is easily enough force to push the mercury up through the capillary. However,during contraction, the forces pushing the mercury back down through the capillary are too weak to keepthe column together. That's because the only thing above the column of liquid mercury is a thin vapor ofmercury gas and that vapor pushes on the liquid much too feebly to have a significant effect. And whilegravity may also push down on the liquid if the thermometer is oriented properly, it doesn't push hardenough to help much.

The contracting column of mercury takes hours to drift downward, if it drifts downward at all. It oftenbreaks up into sections, each of which drifts downward at its own rate. And, as two readers (Michael HughKnowles and Miodrag Darko Matovic) have both pointed out to me in recent days, there is a narrowconstriction in the capillary near its base and the mercury column always breaks at that constriction duringcontraction. Since the top portion of the mercury column is left almost undisturbed when the column breaksat the constriction, it's easy to read the highest temperature reached by the thermometer.

Shaking the thermometer hard is what gets the mercury down and ultimately drives it through theconstriction so that it rejoins into a single column. In effect, you are making the glass accelerate so fast thatit leaves the mercury behind. The mercury isn't being pushed down to the bottom of the thermometer;instead, the glass is leaping upward and the mercury is lagging behind. The mercury drifts to the bottom ofthe thermometer because of its own inertia.

You're right that the glass tube acts as a magnifier for that thin column of mercury. Like a tall glass ofwater, it acts as a cylindrical lens that magnifies the narrow sliver of metal into a wide image.

1517. I recently bought a used microwave oven. The enamel coating under the glass turntable tray is rustedin a ring around the track that the turntable rotates on. Should I repair this or is it ok to just use it as is? --AA, Kettering, OhioAs long as the oven's metal bottom is sound underneath the rust, there isn't a problem. The cookingchamber walls are so thick and highly conducting that they reflect the microwaves extremely well evenwhen they have a little rust on them. However, if the metal is so rusted that it loses most of its conductivityin the rust sites, you'll get local heating across the rusty patches and eventually leakage of microwaves. Ifyou're really concerned that there may be trouble, run the microwave oven empty for about 20 seconds and

then (carefully!) touch the rusty spots. If they aren't hot, then the metal underneath is doing its job just fine.1516. While shopping for a new microwave I was asking the salesperson at a local store some questionsregarding microwaves. He proceeded to tell me how dangerous they were and that they used to sell somesort of testers to see if the new microwaves they were selling "leaked radiation". He told me that they alldid and that microwaves give off "harmful" radiation. He said that it affects the food that we cook in it andcan cause cancer. He said "Think about it, when you get an x-ray the tech covers himself with a lead shieldand here we are putting our food into this and there is no lead shield. Needless to say I did not purchase amicrowave yesterday, and was wondering if you could please give me some insight on this and tell me iswhat this salesperson told me is true. Are microwave ovens really harmful? Do they cause cancer? Whatabout the food, does it become toxic. A friend of mine is totally into all organic food and she "unplugged"


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her microwave years ago and never used it since. She swears it is harmful. Please help. Heating food in apot is so inconvenient!! -- KOThe salesperson you spoke to was simply wrong. If you'll allow me to stand on my soapbox for a minute,I'll tell you that this is a perfect example of how important it is for everyone to truly learn basic sciencewhile they're in school and not to simply suffer through the classes as a way to obtain a degree. Thesalesperson is apparently oblivious to the differences between types of "radiation," to the short- and long-term effects of those radiations, and to the importance of intensity in radiation.

Let's start with the differences in types of radiation. Basically, anything that moves is radiation, from visiblelight, to ultraviolet, to X-rays, to microwaves, to alpha particles, to neutrons, and even to flying pigeons.These different radiations do different things when they hit you, particularly the pigeons. While "ionizingradiations" such as X-rays, ultraviolet, alpha particles, and neutrons usually have enough localized energyto do chemical damage to the molecules they hit, "non-ionizing radiation" such as microwaves and pigeonsdo not damage molecules. When you and your organic friend worry about toxic changes in food orprecancerous changes in your tissue, what really worry you are molecular changes. Microwaves andpigeons don't cause those sorts of changes. Microwaves effectively heat food or tissue thermally, whilepigeons bruise food or tissue on impact.

Wearing a lead apron while working around ionizing radiation makes sense, although a simple layer offabric or sunscreen is enough to protect you from most ultraviolet. To protect yourself against pigeons,

wear a helmet. And to protect yourself against microwaves, use metal. The cooking chamber of themicrowave oven is a metal box (including the screened front window). So little microwave "radiation"escapes from this metal box that it's usually hard to detect, let alone cause a safety problem. There just isn'tmuch microwave intensity coming from the oven and intensity matters. A little microwaves do nothing atall to you; in fact you emit them yourself!

If you want to detect some serious microwaves, put that microwave detector near your cellphone! Thecellphone's job is to emit microwaves, right next to your ear! Before you give up on microwave ovens, youshould probably give up on cellphones. That said, I think the worst danger about cellphones is driving intoa pedestrian or a tree while you're under the influence of the conversation. Basically, non-ionizing radiationsuch as microwaves is only dangerous if it cooks you. At the intensities emitted by a cellphone next to yourear, it's possible that some minor cooking is taking place. However, the cancer risk is almost certainly nil.

Despite all this physics reality, salespeople and con artists are still more than happy to sell you protectionagainst the dangers of modern life. I chuckle at the shields people sell to install on your cellphones toreduce their emissions of harmful radiation. The whole point of the cellphone is to emit microwave signalsto the receiving tower, so if you shield it you spoil its operation! It would be like wrapping an X-raymachine in a lead box to protect the patient. Sure, the patient would be safe but the X-ray machine wouldbarely work any more.

Returning to the microwave cooking issue, once the food comes out of the microwave oven, there are nolingering effects of its having been cooked with microwaves. There is no convincing evidence of anychemical changes in the food and certain no residual cooking microwaves around in the food. If you'reworried about toxic changes to your food, avoid broiling or grilling. Those high-surface-temperaturecooking techniques definitely do chemical damage to the food, making it both tasty and potentially a tinybit toxic. One of the reasons why food cooked in the microwave oven is so bland is because those chemical

changes don't happen. As a result, microwave ovens are better for reheating than for cooking.

1515. Is it possible to capture and keep ionized gases or air in a container of some sort? That way theycould be sprayed out at any time just like room deodorant. -- CWNo, you cannot store charged gases in any simple container. If you try to store a mixture of positively andnegatively charge gas particles in a single container, those opposite charges will attract and neutralize oneanother. And if you try to store only one type of charge in a container, those like charges will repel and pushone another to the walls of the container. If the container itself conducts electricity, the charges will escapeto the outside of the container and from there into the outside world. And if the container is insulating, the


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charges will stick to its inside surface and you'll have trouble getting them to leave. Moreover, you'll havetrouble putting large numbers of those like-charged gas particles into the container in the first place becausethe ones that enter first will repel any like charges that follow.

1514. What packing material protects best? When we drop an egg wrapped in various packaging materials,we know the force that gravity exerts on the egg but how do we know the force of the impact? -- DL,Springboro, OhioI like to view problems like this one in terms of momentum: when it reaches the pavement, a falling egghas a large amount of downward momentum and it must get rid of that downward momentum gracefullyenough that it doesn't break. The whole issue in protecting the egg is in extracting that momentumgracefully.

Momentum is a conserved physical quantity, meaning that it cannot be created or destroyed. It can only bepassed from one object to the other. When you let go of the packaged egg and it begins to fall, thedownward momentum that gravity transfers into the egg begins to accumulate in the egg. Before you let go,your hand was removing the egg's downward momentum as fast as gravity was adding it, but now the eggis on its own!

Because momentum is equal to an object's mass times its velocity, the accumulating downward momentum

in the egg is reflected in its increasing downward speed. With each passing second, the egg receives anotherdose of downward momentum from the earth. By the time the egg reaches the pavement, it's movingdownward fast and has a substantial amount of downward momentum to get rid of. Incidentally, the earth,which has given up this downward momentum, experiences an opposite response--it has acquired an equalamount of upward momentum. However, the earth has such a huge mass that there is no noticeable increasein its upward speed.

To stop, the egg must transfer all of its downward momentum into something else, such as the earth. It cantransfer its momentum into the earth by exerting a force on the ground for a certain amount of time. Atransfer of momentum, known as an impulse, is the product of a force times a time. To get rid of itsmomentum, the egg can exert a large force on the ground for a short time or a small force for a long time,or anything in between. If you let it hit the pavement unprotected, the egg will employ a large force for ashort time and that will be bad for the egg. After all, the pavement will push back on the egg with an

equally strong but oppositely directed force and punch a hole in the egg.

To make the transfer of momentum graceful enough to leave the egg intact, the protective package mustprolong the momentum transfer. The longer it takes for the egg to get rid of its downward momentum, thesmaller the forces between the egg and the slowing materials. That's why landing on a soft surface is a goodstart: it prolongs the momentum transfer and thereby reduces the peak force on the egg.

But there is also the issue of distributing the slowing forces uniformly on the egg. Even a small force canbreak the egg if it's exerted only on one tiny spot of the egg. So spreading out the force is important.Probably the best way of distributing the slowing force would be to float the egg in the middle of a fluidthat has the same average density as the egg. But various foamy or springy materials will distribute theforces nearly as well.

In summary, (1) you want to bring the egg to a stop over as long as period of time as possible so as toprolong the transfer of momentum and reduce the slowing forces and (2) you want to involve the wholebottom surface of the egg in this transfer of momentum so that the slowing forces are exerted uniformly onthe egg's bottom surface. As for the actual impact force on the egg, you can determine this by dividing theegg's momentum just before impact (its downward speed times its mass) by the time over which the egggets rid of its momentum.


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1513. Can infrared lasers, thermal cameras, digital cameras, or optical fiber cameras be used to see throughwalls of homes or to monitor people's conversations? -- CB, ConnecticutI'm beginning to think that movies and television do a huge disservice to modern society by blurring thedistinction between science and fiction. So much of what appears on the big and little screen is just fantasy.

The walls of your home are simply hard to look through. They block visible, infrared, and ultraviolet lightnearly perfectly and that doesn't leave snoopers many good options. A person sitting outside your homewith a thermal camera--a device that "sees" the infrared light associated with body-temperature objects--ora digital camera is going to have a nice view of your wall, not you inside. There are materials that, whileopaque to visible light, are relatively transparent to infrared light, such as some plastics and fabrics.However, typical wall materials are too thick and too

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How Things Work Physics of Everyday Life by Lou Bloomfield Excerpt