up to SCI. MISCONCEPTION | COMMENTS | GOOD STUFF | NEW STUFF | HELP RECURRING SCIENCE MISCONCEPTIONS IN K-6 TEXTBOOKS William J. Beaty STILL UNDER CONSTRUCTION WARNING: This file is currently being written, edited, corrected, etc. It does still contain some mistakes of its own. I placed it online as a sort of 'trial by fire' in order to hear readers' responses so I could target weak or unclear sections for improvement. (And, as my site points out, NOBODY is perfect so we should always practice critical thinking. Take all information with a grain of salt, including everything here!) Please feel free to send public comments to me with the COMMENT BOOK. If you prefer that nobody else sees your comments, send private comments to me via this form. * MAIN MISCONCEPTIONS PAGE * Jump to ELECTRICITY MISCONCEPTION PAGE * COMMENT BOOK, publicly express your opinions on all this * Suggest Your Own K-6 Textbook Miscon * Try "SCIENCE MYTHS" SPREAD BY K-6 TEXTBOOKS AM I JUST A NITPICKER? _________________________________________________________________ "Errors, like straws, upon the surface flow; He who would search for pearls must dive below." - John Dryden _________________________________________________________________ THE MISCONCEPTIONS: * SCIENTISTS USE THE SCIENTIFIC METHOD * SOUND TRAVELS BETTER THROUGH SOLIDS & LIQUIDS * GRAVITY IN SPACE IS ZERO * FILLED AND EMPTY BALLOONS DEMONSTRATE THE WEIGHT OF AIR * GASES ALWAYS EXPAND TO FILL THEIR CONTAINERS * FRICTION IS CAUSED BY SURFACE ROUGHNESS * ICE SKATES FUNCTION BY MELTING ICE VIA PRESSURE * THE EARTH HAS 92 CHEMICAL ELEMENTS * LIGHT FROM THE SUN IS PARALLEL LIGHT * A WING'S LIFTING FORCE IS CAUSED BY ITS SHAPE * FOR EVERY ACTION, THERE IS AN EQUAL AND OPPOSITE REACTION * BEN FRANKLIN'S KITE WAS STRUCK BY LIGHTNING * THE MAIN LENS OF YOUR EYE IS INSIDE THE EYE * WHEN ONE PRISM SPLITS LIGHT INTO COLORS, A SECOND IDENTICAL PRISM CAN RECOMBINE THEM * CLOUDS, FOG, AND SHOWER-ROOM MIST ARE MADE OF WATER VAPOR * RAINDROPS HAVE POINTS * AIR IS ALMOST ENTIRELY WEIGHTLESS * SHADOWS VANISH ON CLOUDY DAYS BECAUSE THE SUN ISN'T BRIGHT ENOUGH * INFRARED LIGHT IS A FORM OF HEAT * THERE ARE SEVEN COLORS IN THE RAINBOW * THE EARTH'S NORTH AND SOUTH MAGNETIC POLES RESIDE JUST BELOW THE SURFACE * LASER LIGHT IS "IN PHASE" LIGHT * LASER LIGHT IS PARALLEL LIGHT * LASERS ARE COHERENT BECAUSE ATOMS EMIT IN PHASE * IRON AND STEEL ARE THE ONLY STRONGLY MAGNETIC MATERIALS * RE-ENTERING SPACECRAFT ARE HEATED BY AIR FRICTION * CARS AND AIRPLANES ARE SLOWED DOWN BY AIR FRICTION * THE NORTH MAGNETIC POLE OF THE EARTH IS IN THE NORTH * SALT WATER IS FULL OF SODIUM CHLORIDE MOLECULES * LIGHT AND RADIO WAVES ALWAYS TRAVEL AT "THE SPEED OF LIGHT" _________________________________________________________________ That's the way all the books were: They said things that were useless, mixed-up, ambiguous, confusing, and partially incorrect. How anybody can learn science from these books, I don't know, because it's not science. - Dr. Richard Feynman, in "Surely you're Joking, Mr. Feynman" _________________________________________________________________ _________________________________________________________________ CORRECTED: THERE IS NO SINGLE "SCIENTIFIC METHOD." IT IS A MYTH The rules of science fairs typically require that students follow THE SCIENTIFIC METHOD, or in other words, hypothesis-testing. The students must propose a hypothesis, test it by experiment, then reach conclusions. This supposedly is "The Scientific Method" used by all scientists. Unfortunately this is wrong, and there is no single Scientific Method as such. "The Scientific Method" is a myth spread by school books. It is an extremely widespread myth, but this doesn't make it any more real. "The Scientific Method" is part of school and school books, and is not part of real science. Real scientists use a large variety of methods (perhaps call them "The Methods of Science" rather than "The Scientific Method.") Hypothesis-testing is one of these, but it certainly is not the only one, and it would be a mistake to elevate it above all others. We shouldn't force children to memorize it, and we shouldn't use it to exclude certain types of projects from science fairs. For example, if a scientist has a good idea for designing a new kind of measurment instrument, that certainly is "doing science", but where is The Hypothesis? Where is the experiment? The Atomic Force Microscope (STM/AFM) revolutionized science, but wouldn't such a project be rejected from many science fairs because it's not an experiment? There are many parts of science which cannot easily be forced into the "hypothesis/experiment/conclusion" mold. Nobody performs Astronomy experiments, or Paleontology experiments. Forcing kids to follow a caricature of scientific research distorts science, and it really isn't necessary in the first place. Another example: great discoveries often come about when scientists notice anomalies. As Isaac Asimov said: "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...' " This suggests that important science comes NOT from proposing hypotheses or even from performing experiments, but instead from learning to see what nobody else can see. Discovery comes from something resembling "informed messing around," or unguided play. Yet many educators treat science as deadly serious, and "messing around" is sometimes dealt with harshly. ARTICLES: * Ten Science Myths * The Scientific Method * On Scientific Method _________________________________________________________________ CORRECTED: ICE SKATES DO NOT FUNCTION BY MELTING ICE VIA PRESSURE It is commonly stated that ice skates have low friction because ice melts when pressure is applied to it. This is not quite correct. A demonstration using an ice cube, a wire, and two weights is often provided to illustrate the phenomena. However, while pressure does affect the melting point of ice, the pressure provided by the skates is not enough to melt ice except when the temperature is a fraction of a degree below 0C. Also, the icecube and wire demonstration is very misleading because it is always performed in a heated room, and the wire doesn't melt ice entirely by pressure, it melts the ice by thermal conduction of warm room temperature along the wire. (Also, narrow gaps in ice always freeze closed because the simultaneous melt/freeze process at water/ice boundary acts to flatten points and fill crevices) Another point: the weight of small objects is too low to create high pressure, yet small objects do experience low friction when on ice. The low friction of ice is probably caused by a layer of liquid water a few hundred molecules thick which always spontaneously develops on the surface of ice. Also, melting from frictional heating can provide liquid water as lubrication. Here's more on this whole debate, and also a bit from BAD CHEMISTRY _________________________________________________________________ CORRECTED: THERE ARE NOT 92 ELEMENTS ON EARTH Uranium has the highest atomic number of the elements commonly found in the environment, and some books will tell you that there are 92 elements found on earth: atomic numbers 1 through 92 (hydrogen through uranium) 92). This is wrong. Unfortunately there are two elements below Uranium which are radioactive and have extremely short half lives. These are Technetium and Promethium. These two elements do not occur naturally on Earth, and this reduces the total number of elements found in the environment to 90. However, in the 1970s a natural uranium reactor was found in an ancient streambed in Africa, and the mineral deposits at the site contained traces of a long-lived Plutonium isotope (atomic number 94.) This brings the total number of elements on the Earth back up to 91. (Note: Technicium, though not found naturally on Earth, is present in some stars, detected by spectral analysis.) See THE PHYSICS TEACHER, Vol.27 No.4 p282 _________________________________________________________________ LIGHT FROM THE SUN IS PARALLEL? NOPE. Some books state that light from distant objects is nearly parallel. The more distant the object the more parallel the light, and since the sun is so far away, sunlight is perfectly parallel. This is incorrect. While it is true that light from *each tiny point* on the sun's surface is just about perfectly parallel by the time it reaches our eyes, light from the sun as a whole is not. This is because the sun, though very distant, is very large. A similar situation exists with light from the sky. We wouldn't say that the sky gives parallel light. Yet light from the sky comes from many miles away. If sunlight were perfectly parallel, there would be some interesting effects which are usually smeared out by the sun's disklike image. First, to us the sun would look like a very bright point, like an intensely bright star or a welding arc. Also, shadows on the ground would lack penumbras and be almost perfectly sharp. At nightfall the advancing shadows of distant mountains would be seen to race across the ground. During sunset the sun wouldn't gradually sink below the horizon, instead it would wink out. During the day the variations in air density would cause the ground to be covered by moving patterns of light; patterns similar to those seen on the bottom of a swimming pool but in this case made by "waves" in the sky. Solar and lunar eclipses would lack penumbrae. Looking at the sun might burn your retina, since the parallel light would be focused to a tiny point. And if sunlight was perfectly parallel, a large convex lens could concentrate sunlight into an intense pinpoint rather than into a small disk. Also, a if a small concave lens was placed near the focus of a large convex lens, the pair lenses could be used to concentrate sunlight and form it into a thin, dangerously powerful parallel beam. Try to do this with the real sun, and all you get is a large, projected image of the sun's disk. [sun.gif] _________________________________________________________________ IN AN AIRCRAFT WING, THE LIFTING FORCE DOES NOT COME FROM THE DIFFERENCE IN CURVATURE BETWEEN THE TOP AND BOTTOM SURFACES Some books explain that the lifting force on an aircraft wing is created because the upper wing surface is longer and more curved than the lower. They state that air dividing at the leading edge of the wing must rejoin at the trailing edge, therefore the upper air stream must move faster, and so the wing is pulled upwards by the Bernoulli effect. This is not correct: the air divided by the leading edge does NOT rejoin at the trailing edge. The same books often contain a misleading diagram showing a flat-bottomed wing with flow lines of the surrounding air. (see below.) This diagram actually shows a zero-lift condition. [wing2.gif] Both the explanation and the diagram have serious problems. They wrongly imply that inverted flight is impossible and that an aircraft with equal pathlengths of upper and lower wing surfaces will not fly. They also wrongly suggest that aircraft can violate conservation of momentum by remaining aloft without reacting against the air, and without causing a downward motion of the air. Yet upside-down flight is far from impossible; it is a common aerobatic move. And many wings have equal pathlengths, including even the thin cloth wings of the Wright Brothers' flyer! And anyone standing under a slow, low-flying plane or below the thin, fast wings of a helicopter will know that there is a very great downward flow of air below the wings. All of this indicates that there is a serious problem with the "curved top, flat bottom" explanation. Below is an alternative. As a plane flies, its wings cut through the air at an angle. This "angle of attack" causes the wing to apply a downward force to the air. As a result, the air streams downwards at an angle and the wing is pushed upwards and back. Because the flowing air adheres to the top of the wing, the tilt of the wing also causes the upper surface of the wing to pull downwards on the air above it. The air above the wing moves down and the wing moves up. As any plane flies, a stream of air is sent diagonally downwards by its wings, and the wing acts like a 'reaction engine' much like a jet engine or a rocket. It may help to imagine a hovering helicopter: a helicopter can hover because its rotor applies a downward force to the air, and the air applies an upward force to the rotor. As a result, the air flows downwards and the upward force supports the craft. But a helicopter rotor is a moving wing, and it's this small wing which sends the air downwards. Helicopter rotors are reaction engines, they push air downwards, and the air pushes them upwards. They are not "sucked upwards," and neither are airplanes. You may have seen a plane's downwash of air in movies: a "cropduster" plane sends out a trail of fertilizer mist, and the trail of mist does not float, instead it moves immediately down into the crops, driven downward by the moving air. Air from wings can even be dangerous: if a plane flies too low, the downwash from its wings can knock people over. The "Bernoulli effect" need not be entirely excluded from explanations of flight: it explains how the top of the wing is able to pull downwards on the air flowing over it. And it proves extremely useful in calculations of the lifting force during classes in airplane physics and during experimental work in aerodynamics. * WEBSITE: Airfoil misconceptions in K-6 textbooks * SOME EMAIL DEBATE * My improved explanation:DISK BALLOONS _________________________________________________________________ SOUND TRAVELS BETTER THROUGH SOLIDS? NO. Many elementary textbooks say that sound travels better through solids and liquids than through air, but they are incorrect. In fact, air, solids, and liquids are nearly transparent to sound waves. Some authors use an experiment to convince us differently: place a solid ruler so it touches both a ticking watch and your ear, and the sound becomes louder. Doesn't this prove that wood is better than air at conducting sound? Not really, because sound has an interesting property not usually mentioned in the books: waves of sound traveling inside a solid will bounce off the air outside the solid. The experiment with the ruler merely proves that a wooden rod can act as a sort of "tube," and it will guide sounds to your head which would otherwise spread in all directions in the air. A hollow pipe can also be used to guide the ticking sounds to your head, thus illustrating that air is a good conductor after all. Sound in a solid has difficulty getting past a crack in the solid, just as sound in the air has difficulty getting past a wall. Solids, liquids, and air are nearly equal as sound conductors. It's true that the speed of sound differs in each material, but this does not affect how well they conduct. "Faster" doesn't mean "better." It is true that their transparency is not exactly the same, but this only is important when sound travels a relatively great distance through each material. It's also true that complex combinations of materials conduct sound differently and may act as sound absorbers (examples: water with clouds of bubbles, mixtures of various solids, air filled with rain or snow.) And last: when you strike one object with another, the sound created inside the solid object is louder than the sound created in the surrounding air. So, before we try to prove that solids are better conductors, we had better make sure that we aren't accidentally putting louder sound into the solids in the first place. _________________________________________________________________ GRAVITY IN SPACE IS ZERO? WRONG. Everyone knows that the gravity in outer space is zero. Everyone is wrong. Gravity in space is not zero, it can actually be fairly strong. Suppose you climbed to the top of a ladder that was 300 miles tall. You would be up in the vacuum of space, but you would not be weightless at all. You'd only weigh fifteen percent less than you do on the ground. A 200lb person would weigh 170lb. Yet a spacecraft can orbit 'weightlessly' at the height of your ladder! While you're up there, you might see the Space Shuttle zip right by you. The people inside it would seem as weightless as always. Yet on your ladder, you'd feel nearly normal weight. What's going on? The reason that the shuttle astronauts act weightless is that they're inside a container which is FALLING! If the shuttle were to sit unmoving on top of your ladder (it's a strong ladder,) the shuttle would no longer be falling, and its occupants would feel nearly normal weight. And if you were to leap from your ladder, you would feel weightless (at least until you hit the ground!) So, if the orbiting shuttle is really falling, why doesn't it hit the earth? It's because the shuttle is not falling down, it is moving very fast sideways as it falls, so it falls in a curve. It moves so fast that the curved path of its fall is the same as the curve of the earth, so the Shuttle falls and falls and never comes down. Gravity strongly affects the astronauts in a spacecraft: the Earth is strongly pulling on them so they fall towards it. But they are moving sideways so fast that they continually miss the Earth. This process is called "orbiting," and the proper word for the seeming lack of gravity is called "Free Fall." You shouldn't say that astronauts are "weightless," because if you do, then anyone and anything that is falling would also be "weightless." If we drop a book, does gravity stop affecting it, should we say it becomes weightless? If so, then why does it fall? "Weight" is the force which pulls objects towards the Earth, and this force is still there when objects fall. So to experience GENUINE free fall just like the astronauts, simply jump into the air! Better yet, jump off a diving board at the pool, or bounce on a trampoline, or go skydiving. Bungee-jumpers know what the astronauts experience. _________________________________________________________________ FOR EVERY ACTION, THERE IS NOT AN EQUAL AND OPPOSITE REACTION Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means then, is that for every FORCE applied, an equal and opposite FORCE always appears. So while it's true that a skateboard does fly backwards when the rider steps off it, these "action" and "reaction" motions are not what Newton was investigating. Newton was actually referring to the fact that when you push on something, it pushes back upon you equally, EVEN IF IT DOES NOT MOVE. When a bowling ball pushes down on the Earth, the Earth pushes up on the bowling ball by the same amount. That is a good illustration of Newton's third Law. Newton's Third Law can be rewritten to say: You cannot touch without being touched. Or even simpler: Forces always exist in pairs. _________________________________________________________________ BEN FRANKLIN'S KITE WAS NEVER STRUCK BY LIGHTNING Many people believe that Ben Franklin's kite was hit by a lightning bolt, and this is how he proved that lightning was electrical. A number of books and even some encyclopedias say the same thing. They are wrong. When lightning strikes a kite, the electric current in the string is so high that just the spreading electric currents in the ground can kill anyone standing nearby, to say nothing of the person holding the string! What Franklin actually did was to show that a kite would collect a tiny bit of electric charge-imbalance out of the sky during a thunderstorm. Electric leakage through the air caused his kite and string to become charged, and the hairs on the twine stood outwards. The twine was then used to charge a metal key, and tiny sparks could then be drawn from the key. (He used a metal object because sparks cannot be directly drawn from the twine, it's not conductive enough.) This suggested that some stormclouds carry strong electrical charges, and it IMPLIED that lightning was just a large electric spark. The common belief that Franklin easily survived a lightning strike is not just wrong, it is dangerous: it may convince kids that it's OK to duplicate the kite experiment as long as they "protect" themselves by holding a silk ribbon and employing a metal key. Make no mistake, Franklin's experiment was extremely dangerous. Lightning goes through miles of insulating air, and will not be stopped by a piece of ribbon. If lightning had actually hit his kite, he most probably would have died instantly. See LIGHTNING SURVIVOR RESOURCES _________________________________________________________________ THE MAIN LENS OF YOUR EYE IS INSIDE THE EYE? NOT QUITE. Some textbooks assume that the small lens found deep within the eyeball is the eye's main lens, and the cornea of the eye is simply a protective window. The textbook diagrams even depict light rays passing into the eye and only bending as they pass through this internal lens. But in the human eye, the small lens found within the eyeball is not the main imaging lens. The cornea is actually the main lens; it is the strongly curved transparent front surface of the eye. Most of the bending of the light occurs at the place where the light enters the surface of the cornea. When you look at your eye in the mirror, you are looking directly at the eye's main lens. When you want to change the focusing power of your eye, you apply "contact lenses" to the cornea surface, or you undergo surgery which re-sculpts the cornea's curvature. The smaller lens inside the eye acts only to alter the focus of the eye as a whole. Muscles change its shape in order to correct the focus for near and far viewing. Without this small internal lens, human vision would be blurry, and vision would be unable to accommodate for near and far views. But without the cornea lens, [the human eye would be blind] IMPROVED VERSION: witthout the cornea lens, human vision would rely upon the pinhole-camera effect of the eye's pupil, and vision would be incredibly blurry. Open your eyes underwater in dimly-lit conditions to see what vision would be like without a cornea. [eyeball.jpg] _________________________________________________________________ WHEN ONE PRISM SPLITS LIGHT INTO COLORS, A SECOND IDENTICAL PRISM CANNOT RECOMBINE THEM A single prism can split a sunbeam into a rainbow. Many children's science books show how a second similar prism can be used to recombine the colors. This is incorrect, two prisms do not work as shown. Prisms of two DIFFERENT sizes can split and then focus the colors into momentary recombination at a particular distance. With THREE prisms in a special arrangement, the splitting and complete recombining of colors can be accomplished. But books which depict one prism splitting the colors and a second identical prism recombining the colors into a single white beam are in error, and are no doubt the source of endless frustration for those of us who try to duplicate the effect with real prisms. The "rainbows" can also be recombined by placing a screen at just the right place, and by bouncing the colors off many small mirrors so the colored beams converge upon a screen. Recombination can also be done with a convex lens or a concave mirror and a screen. I hope that very few students will attempt to perform the color recombination experiment depicted in their books, for disappointment awaits. (MORE) [prism.jpg] _________________________________________________________________ CLOUDS, FOG, AND SHOWER-ROOM MIST ARE WATER VAPOR? NO. They are made of small droplets of liquid water. When water evaporates, it turns into a transparent gas called "water vapor." When it condenses again, it can take the form of rivers and oceans, but it also can take the form of clouds, fog, mist, etc. Fog can make surfaces wet, but not because of condensation. Instead, the fog droplets collide with the surface. Fog is a liquid, not a vapor. Fly an ultralight aircraft through a dense cloud, and you'll become soaked! To look for water vapor, look at the bubbles in rapidly boiling water. Look at the small empty space at the spout of a boiling teakettle. Look at the far end of the teakettle's plume of mist, where the mist seems to vanish into the air. Look at the empty air above a wet surface. Water vapor seems invisible because it is transparent. Clouds and fog are not transparent. They are composed of liquid droplets. _________________________________________________________________ RAINDROPS DON'T HAVE POINTS!! Nearly every drawing of raindrops depicts them as having a sharp upper point. This is wrong. Surface tension of water acts like a stretched "bag" around the water, and unless some other force is acting, it pulls the water into a spherical shape. Our eyes do see tiny droplets as a blur, but a flash photograph reveals that small raindrops are nearly spherical. The larger ones are distorted by the pressure of moving air, but this doesn't make points, it makes them somewhat flattened. Think of it this way: underwater bubbles are not pointed as they rise, just as falling water drops are not pointed as they fall. And while it's true that the SYMBOL for water is a droplet with a point, REAL water droplets look nothing like the symbol. And when water drips from a faucet, it never actually has a point. Instead it has a narrow neck, and after the neck has snapped, it is yanked back into the falling ball of water. See Dr. Fraser's BAD SCIENCE for lots more about this. _________________________________________________________________ AIR IS WEIGHTLESS? NO. We are not conscious of air's weight because we are immersed within it. In the same way, even a large bag of water seems weightless when it is immersed in a water tank. The bag of water in the tank is supported by buoyancy. In a similar way, buoyancy from the atmosphere makes a bag of air seem weightless when it's surrounded by air. One way to discover the real weight of air would be to take a bag of air into a vacuum chamber. Another way is to weigh a pressurized and unpressurized football. Here's a simple way to detect the mass of air even though the air seems weightless: open an umbrella, wiggle it slightly forwards and back, then close it and wiggle it again. When you wiggle it when open, you can feel its increased mass because of the air it must move. (Ah, but then we must explain the difference between weight and mass!) _________________________________________________________________ NO, FILLED AND EMPTY BALLOONS DO NOT DEMONSTRATE THE WEIGHT OF AIR Many books contain a incorrect experiment which purports to demonstrate that air has weight. A crude beam-balance is constructed using a meter stick. Deflated rubber balloons are attached to the ends, and the balance is adjusted. One balloon is then inflated, and that end of the balance- beam sags downwards. Unfortunately this experiment lies. When immersed in atmosphere, buoyancy causes full and empty balloons to weigh the same. The experiment secretly relies on the fact that the air within a high-pressure balloon is denser than air within a low pressure balloon. To illustrate the problem, try this instead: attach two opened paper bags to the balance, adjust it, then crush one bag so it contains little air. The balance will not move. What does this teach your class, that air is... weightless? Or, do the balance-beam experiment again, but blow one balloon REALLY full so the rubber feels hard and the balloon is about to pop. Blow up the other balloon so it is ALMOST full, but still a bit stretchy. The balance will show that, even though the balloons are nearly equal in size, the "hard" balloon is heavier. DOes this teach misleading things to your class? No, instead it exposes the dishonesty of the original demonstration. In truth, balloons full of air do not weigh more than empty ones, but COMPRESSED air weighs more than uncompressed air. What if we lived underwater, how could we use the balance beam to measure the weight of water directly? The answer is that we cannot. If a full water-balloon and a collapsed water-balloon were compared underwater, the experiment would indicate that water is weightless. When underwater, a bag full of water weighs just the same as a flattened bag which contains nothing. The situation with air is identical: if we are immersed within a sea of air, we cannot use a balance to easily detect the actual weight of the air. (In fact, a bathtub full of air weighs about a half kilogram, but we cannot sense this weight while living in an atmosphere.) And so the "air balloon balance" experiment could only work correctly if it were lifted out of our ocean of air, and then operated in a vacuum environment. To better demonstrate the weight of air, hook a heavy bottle to a vacuum pump, pump all the air out, seal it, then weigh the bottle. Break the seal and let the air in, then weigh it again. The difference in weight is the weight of the air contained in the bottle. Another: use a balance to compare the weight of two vacuum-containing bottles, then open one of them so it becomes filled with air. Or another: build a balance using upside-down paper bags, then place a candle below one of them, then remove the candle again. That bag rises, indicating that a volume of warm air weighs slightly less than a volume of cool air. (Don't set the bag on fire!!) But note that this experiment says nothing simple and direct about the actual weight of a volume of unheated air. _________________________________________________________________ NO, IN THE EVERYDAY WORLD, GASES DO NOT EXPAND TO FILL THEIR CONTAINERS What is the difference between a liquid and a gas? Both are "fluids", both can flow. Gases are USUALLY less dense than liquids, although gases under fiercely high pressure can approach the density of liquids, so that's not a good criterion. The main difference is that gases are a different phase of matter: a gas can be made to condense into a liquid form, and a liquid can be made to evaporate into gas. Another major characteristic: because there are bonds between its particles, when a liquid IS PLACED INTO A VACUUM ENVIRONMENT, it will not expand continuously, while a gas in a vacuum chamber will expand continuously until it hits the walls. This is very different than the oft-quoted rule that "gases always expand to fill their containers." This rule only works correctly if the container is totally empty: the container must "contain" a good vacuum beforehand. However, we all live in a gas-filled environment. All our containers are pre-filled with air. In our environment, any new quantity of gas will not expand, it will just sit there. If you squirt some carbon dioxide out of a CO2 fire extinguisher, it will not instantly expand to fill the room. Instead pour downwards and form a pool on the floor. It behaves similarly to dense sugar-water which was injected into a tank of water: it pours downwards, and only after a very long time it will mix with the rest of the water. "Mixing" is very different than "expanding to fill!" The rule about gases does not involve mixing, instead it involves compressibility and instant expansion into a vacuum. In an air-filled room, dense gases act much like liquids; they can be poured into a cup or bowl, poured out out onto a tabletop, and then they run off the edge onto the floor where they form an invisible mess. :) Less dense gases will stay where they are put, like smoke or like food coloring which has just been injected into a fishtank. Gas of even lesser density rises and forms a pool on the ceiling. Only in the world of the physicist, where "empty container" always implies a vacuum, does the rule about gasses work properly. _________________________________________________________________ SHADOWS DO VANISH ON CLOUDY DAYS, BUT NOT BECAUSE THE SUN ISN'T BRIGHT ENOUGH Shadows appear when an object blocks a light source. The shape of the shadow is created by the shape of the opaque object AND by the shape of the light source. On a cloudy day the whole sky acts as a light source, and a person's shadow spreads out and becomes a dim fuzzy patch which surrounds the person on the ground on all sides. The shadow is so spread-out that it seems absent entirely. When the sun is visible, the same shadow is concentrated in one specific place and becomes easy to see. But even the shadows made by sunlight will have fuzzy borders, since the sun is a small disk rather than a tiny dot. On cloudy days, the fuzzy borders of your body's shadow become much much larger than the shadow itself, so that the shadow seems to vanish. _________________________________________________________________ FRICTION IS NOT CAUSED BY SURFACE ROUGHNESS Some books point to surface roughness as the explanation of sliding friction. Surface roughness merely makes the moving surfaces bounce up and down as they move, and any energy lost in pushing the surfaces apart is regained when they fall together again. Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickyness. Even scientists once believed this misconception, and they explained friction as being caused by "interlocking asperites", the "asperites" being microscopic bumps on surfaces. But the modern sciences of surfaces, of abrasion, and of lubrication explain sliding friction in terms of chemical bonding and "stick & slip" processes. The subject is still full of unknowns, and new discoveries await those who make surface science their profession When thinking about friction, don't think about grains of sand on sandpaper. Instead think about sticky adhesive tape being dragged along a surface. _________________________________________________________________ NO, INFRARED LIGHT IS NOT A KIND OF HEAT Infrared light is bright but invisible light. When any type of light is absorbed by an object, that object will be heated. Just because human eyes cannot see the light which causes the heating does not mean that the light is made of some mysterious entity called "heat radiation." And this is no benign misconception; those who fall under its sway may believe that *visible* light cannot heat surfaces (after all, visible light is not "heat radiation,"). Or they may wrongly believe that hot objects emit no microwaves (since only IR light is "heat radiation"), or they may believe that IR light is a form of "heat" and is therefor very different than any other type of electromagnetic radiation. The main misconception may have been started long ago, when early physicists believed in the existence of three separate types of radiation: heat radiation, light, and actinic radiation. Eventually they discovered that all three were actually the same stuff: light, but light of various frequencies. Yet the term "heat radiation" still lingers. Since human beings can only see certain frequencies of light, it's easy to see how the confusion got started. Invisible light seems bizarre and mysterious when compared to visible light. But "invisibility" is caused by the human eye and is not a property carried by the light. If humans could see all the light in the infrared spectrum, we would say things like, "of COURSE the electric heater makes things hot at a distance, it is intensely BRIGHT, and light can heat things up when they absorb it." _________________________________________________________________ THERE ARE NOT SEVEN COLORS IN THE RAINBOW There is a very large number. And neither are there sharp divisions between the bands of color, yet numerous textbooks depict them. But in reality, between yellow and green we find yellow-green, and between green and yellowgreen is greenish yellowgreen, and on and on. How many colors are in a rainbow? It's not easy to say, for it depends on the particular eye, and the particular rainbow. What of the teachers and students who look in vain for the yellow-green in their textbook's depiction of rainbows? They've crashed into a long-running textbook misconception: the strange idea that rainbows have exactly seven distinct bands of color and no more, and with nothing in between those 'official' colors. _________________________________________________________________ ACTUALLY, THE EARTH'S NORTH AND SOUTH MAGNETIC POLES RESIDE DEEP WITHIN THE EARTH'S CENTER Many textbooks have an erroneous diagram of the earth which shows a bar magnet within it, and the ends of this bar magnet extend to just beneath the earth's surface. These diagrams depict the magnetic field lines as radiating from spots on the earth's surface. This is very misleading. The earth's magnetic poles actually behave as if they're deep within the earth, down inside the core. Earth's magnetic field does not come from a giant bar magnet, but if we IMAGINE it this way, then the imaginary "bar magnet" inside the earth is short, stubby, disk-shaped, and deep within the center of the planet. The typical textbook diagram is incorrect, and there are no intense magnetic fields at the land near the earth's "north pole" and "south pole." The Geomagnetic "poles" on the earth's surface are simply the points where the field lines are perfectly vertical. Proper diagrams should instead show the field lines to be radiating from poles inside the earth's core. They should show the field lines around the northern and southern areas of the earth's surface as being approximately vertical and parallel, not "radial" like a spiderweb and not concentrated into special points on the surface. Another error associated with the above: some books claim that the earth's field at the magnetic poles is much stronger than elsewhere. This is untrue. The field strength at the north magnetic pole is about the same as the field strength in Virginia! And the strongest field in the earth's northern hemisphere does not appear at the north magnetic pole at all, the north pole actually has a weaker field than elsewhere. The strongest fields in the north are not in one but in two places: west of Hudson bay in Canada, and in Siberia. _________________________________________________________________ LASER LIGHT IS "IN PHASE" LIGHT? WRONG. It is not correct to say that "the waves in laser light are all in phase." When two light waves combine, they inextricably add together to form a new wave, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave. When the light wave within a laser causes atoms to emit smaller, in- phase light waves, the result is not "in phase" light. Instead the result is a single, more intense, amplified wave of light. In-phase emission leads to amplification. If the atoms' emissions weren't in phase, then the atoms would absorb light rather than amplifying it. Each atom in a laser contributes a tiny bit of light, but their light vanishes into the main traveling wave. The light from each atom strengthens the main beam, but loses its individuality in the process. 99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is made from 99 plus 1, or 98 plus 2, or 50 plus 50. All the *PHOTONS* in a single wave of light are in phase. This might be one reason that people say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves. Even if all the photons in laser light are in phase, it is still incorrect to say "all the waves are in phase." Photons are not waves. They are quanta, they are particles, and they do not behave as small, individual waves. The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc., is called "extended-source" light. Starlight and the light from arc welders is "pointsource" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the REAL reason that lasers give spatially coherent light! See below) _________________________________________________________________ LASER LIGHT IS NOT PARALLEL LIGHT Light from most lasers is not parallel light. However, if laser light is passed through the correct lenses, it can be formed into a tight, parallel beam. Light from an ordinary light bulb is different. If light from a light bulb was passed through the same lenses, it would form a spreading beam, and an image of the lightbulb would be projected into the distance. Laser light can form beams because a laser is a pointsource, and when you project the image of a pointsource into the distance, you form a narrow parallel beam! However, it is simply wrong to state that laser light is inherently parallel light. Laser light can be FORMED INTO parallel light, and light from ordinary sources cannot. Most types of lasers actually emit spreading, non-parallel light. Lasers in CD players and in "laser pointers" are semiconductor diode lasers. They create cone-shaped light beams, and if a parallel beam is desired, they require a focusing lens. Take apart an old laser-pointer, and you'll find the plastic lens in front of the diode laser inside. Classroom "HeNe" lasers also create spreading light. The laser tube within a typical classroom laser contains at least one curved mirror (called a "confocal" arrangement,) and it creates light in the form of a spreading cone. It's a little-known fact that manufacturers of classroom lasers traditionally place a convex lens on the end of their laser tubes in order to shape the spreading light into a parallel beam. While it's true that a narrow beam is convenient, I suspect that part of their reason is to force the device to fit our stereotype that lasers produce thin, narrow light beams. The manufacturers could save money by selling "real" lensless laser tubes having spreading beams. But customers would complain, wouldn't they? We have been brought up to believe that laser light is parallel light. _________________________________________________________________ LASERS EMIT COHERENT LIGHT, BUT NOT BECAUSE THE ATOMS EMIT IN-PHASE LIGHT WAVES Laser light has two main characteristics: it is "monochromatic" or very pure in frequency (this also is called "temporally coherent.") Laser light is also "coherent," (called "spatially coherent.") Even fairly advanced textbooks fail to give the real reason why laser light is spatially coherent. They usually point out that the laser's atoms all emit their light in phase. It is true that the fluorescing atoms in a laser all emit light that's in-phase with the waves already traveling between the mirrors. But in-phase emission only creates amplification of the traveling waves, it does not create spatially coherent light. For example, if you were to feed incoherent light into a laser, the atoms would emit in-phase waves, and the laser would amplify the light. But the brighter light would still be incoherent! Lasers can amplify the COHERENT wave which is trapped inside them between the mirrors. But how did the light within the laser get to be coherent in the first place? Lasers create coherent light because of their mirrors. The mirrors form a resonant cavity which preserves coherent light. How does it work? Imagine a simplified laser having flat, parallel mirrors. As light bounces between the mirrors, the light "thinks" that it's traveling down an infinitely long "virtual tunnel". (Have you ever held up two mirrors facing each other? Then you've seen the infinite tunnel.) When a laser is first turned on, it fluoresces; it emits light which is not coherent. Different random light waves start out from different parts of the laser. After a few thousand mirror bounces, all the waves have added and subtracted to form just one single wave. A single wave is coherent (to be incoherent, you have to have at least two different waves.) A similar thing happens with starlight: starlight travels far from its original source and all the waves add up to form a wave with a single wavefront. Starlight is spatially coherent, even though sunlight is not. Laser light is spatially coherent because, among other things, the bouncing light has traveled millions of miles between mirrors, and all the various competing waves have melded together to form a single pure wave. P.S. The monochrome characteristic of laser light is ALSO created by the mirrors. Huh? Yes, but the reason for this is not totally straightforward (and quite a bit beyond the K-6 level of these webpages!) The two mirrors of a laser can trap a standing wave of light. The space between the mirrors is like the string of a guitar: there can be a fundamental wave, or overtone waves, or complicated waves which are a mixture of these. Since the distance between the crests of a lightwave is very small, LOTS of different overtones can fit between the mirrors, and each overtone is a slightly-different color of light. Now for the weird part: when a HeNe laser first operates, many different overtones of red light are amplified and the beam contains many slightly-different colors of red at the same time. As time goes on, some of these colors are amplified a bit more than others, and this uses up the available energy coming from the power supply. In other words, the different waves start competing for limited resources! Just one wave "wins" in the end, and all of the other overtones drop out of the running. The laser light is not just red light. It is a single pure overtone-wave of red light, one where the string of waves just perfectly fit in the space between the two mirrors. _________________________________________________________________ IRON AND STEEL ARE NOT THE ONLY STRONGLY MAGNETIC MATERIALS There are numerous others. Nickel and Cobalt metals are very magnetic. (U.S. "nickel" coins contain copper which spoils the effect, so try Canadian nickels made before 1985.) Most other materials are "diamagnetic," and are repelled visibly by very strong magnets, although some materials are "paramagnetic" and are attracted. Supercold liquid oxygen is attracted by magnets. Some but not all types of stainless steel are nonmagnetic. There are even some metals which are individually nonmagnetic, but which become strongly magnetic when mixed together, chromium and platinum for example, and compounds of manganese and bismuth. _________________________________________________________________ RE-ENTERING SPACE CAPSULES ARE NOT HEATED BY AIR FRICTION They are heated as they plow into the atmosphere and compress the air ahead of them. Ever pump up a bicycle tire and discover that the pump and the tire have become hot? The same effect causes spacecraft and supersonic aircraft to heat up as they compress the air at their leading edges. The heat doesn't come from *rubbing* upon the air, it comes from *squeezing* the air. This applies mostly to blunt objects such as Apollo reentry vehicles. It does not apply as much to the Space Shuttle: with wings oriented mostly edge-on to the moving air, the surfaces of the Shuttle ARE heated by friction. _________________________________________________________________ CARS AND AIRPLANES ARE NOT SLOWED DOWN BY AIR FRICTION They are slowed because it takes energy to stir the air. While direct friction between the air and the car's surface does play a part, the work done in stirring the air far exceeds the work done in direct frictional heating. If vehicles did not send air swirls and vortices spinning off as they moved, they would barely be slowed by the air at all. Eventually the swirling air is slowed by friction and ends up warmer, but this occurs long after the vehicle has passed. _________________________________________________________________ THE NORTH MAGNETIC POLE OF THE EARTH IS NOT IN THE NORTH Opposite poles attract. If we hold two bar magnets near each other, the "N" pole of one magnet is attracted by the "S" pole of another. If we suspend a bar magnet by a thread, the "N" pole of that magnet will point... toward's Earth's north! Something is wrong here. Alike poles should not attract. Either the "N" and "S" printed on all bar magnets is reversed, or the "N" and "S" on the Earth is backwards. Which is it? Physics defines "N-type" magnetic poles as being the north-pointing ends of compasses or magnets. Wind an electromagnet, see which end points to the north, and that end is the N pole of the electromagnet. Therefor, the magnetic pole inside the northern hemisphere of the Earth is a south-type magnetic pole. The Earth's northern magnetic pole is an S! It has to be this way, otherwise it would not attract the N-pole of a compass. This is a long-standing but arbitrary physical standard, much the same as defining electrons as being negative. Like it or not, we are stuck with negative electrons, seconds which last about 1/100,000 of a day, backwards Earth poles, centimeters which are about as wide as a small finger, etc. Interesting email msgs on magnetic polarity Also see Dexter Magnetics for more on this. _________________________________________________________________ ACTUALLY THERE ARE NO SODIUM CHLORIDE MOLECULES IN SALT WATER Salt is not made of NaCl molecules. Salt is made of a three-dimensional checkerboard of oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNaClNa. When salt dissolves, it turns into independent atoms. Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine! The atoms are not poisonous and reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron. Because of this, the remaining electrons behave as a filled electron shell, so they cannot easily react and form chemical bonds with other atoms except by electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so it to cannot bond with other atoms. These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from each other as the water molecules surround them, and they float through the water separately. _________________________________________________________________ LIGHT AND RADIO WAVES DO NOT ALWAYS TRAVEL AT "THE SPEED OF LIGHT" They only travel at the "speed of light" (186,000 miles per second) while moving through a perfect vacuum. Light waves travel a bit slower in the air, and they travel LOTS slower when moving through glass. Why does light bend when it enters glass at an angle? Because the waves SLOW DOWN. Why can a prism split white light into a spectrum? Because within the glass THE SPEED OF LIGHT WAVES IS DIFFERENT FOR DIFFERENT WAVELENGTHS. And while the numerical value for the speed of light in a vacuum, "c," is very important in all facets of physics, as far as light waves are concerned there is no single unique speed called "The Speed Of Light." [note for advanced students: ok ok, I'll add this: light *waves* within a transparent medium are slow, even though the wave's photons are thought to jump from atom to atom at a speed of c.] Created and maintained by Bill Beaty. Mail me at: billbeskimo.com. If you are using Lynx, type C to email.