Author: Lou Bloomfield

What becomes of a log’s gravitational energy when you burn it?

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If one takes firewood to the top of a hill and burns it there, does the firewood’s gravitational energy disappear? — V

When you carry the firewood up the hill, you transfer energy to it and increase its gravitational potential energy. When you then burn the wood, you seem to make this energy disappear. After all, there doesn’t appear to be any difference between burning wood in the valley and burning wood on the top of the hill. The wood is gone either way.

But appearances can be deceiving. Since energy is a conserved quantity, the energy that you invest in the firewood can’t disappear. It simply becomes difficult to find because it is dispersed in the burned gases that were once the wood.

To find that energy, imagine compressing the burned gases into a small container to make their weight more noticeable and reduces buoyant effects due to the atmosphere. You could then carry those burned gases, which include all of the firewood’s atoms, back down the hill. As you descended, the container of burned gases would transfer its gravitational potential energy to you.

I’ve swept a number of details under the rug, such as the fact that many of the oxygen atoms in your container were originally part of the atmosphere rather than the log. But even when all those details are taken into account, the answer is the same: the firewood’s gravitational energy doesn’t disappear, it just gets more difficult to find.

Can microwaves be used to heat air?

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Could a magnetron be tuned to heat air, oxygen, or nitrogen? Is it a specific frequency or a range? What is the frequency? — VM, Martinsville, Indiana

No, those gases don’t absorb microwaves significantly regardless of frequency. Diatomic molecules are nearly oblivious to long wavelength electromagnetic waves. In fact, that’s why they don’t contribute to the “greenhouse effect.” Oxygen does have an unusual absorption band in the near infrared, but that’s about it.

What should you do about uneven cooking in a microwave oven?

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My microwave oven seems to mostly heat things on the periphery of the plate
and the part in the center is significantly cooler. Is this considered
faulty operation and should I get something replaced? — MD

It’s quite possible that the pattern of microwaves inside your oven is more intense at some places than in others — that’s why most microwaves have carousels in them to move the food around. I don’t think that the pattern will change much with age, but it’s possible that your oven isn’t producing as much microwave power as it once did and you notice the low-intensity regions more than before. It’s not a true “fault”, but it is a nuisance. If you get tired of putting up with it, you should probably replace the oven. It used to be that you could purchase carousel inserts for the ovens, but I don’t see them for sale anymore.

How can you make a laser beam fade with distance?

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Is it possible to make a visible laser beam fade after 2 or 3 feet for safety reasons? — RB, Arvada, Colorado

Since light carries energy, a laser beam can’t simply disappear after a couple of feet — something would have to absorb it and its energy. Since the atmosphere is extremely transparent to visible light, it won’t do the trick.

Since eye safety requires limiting the amount of laser power that can enter a person’s eye, you can make a laser more eye-safe by enlarging its beam. Even a powerful laser can be eye-safe if only a small fraction of the laser light can enter a person’s iris and focus on their retina.

Although it’s natural to think of a laser beam as a narrow pencil of light that stays narrow forever, that’s not really the case. The diameter of a laser beam changes with distance from its source. The beams from typical lasers, including laser pointers, start relatively narrow and widen as gradually as the physics of light propagation will allow. But with the help lenses, you can change that widening process dramatically. For example, if you send a typical laser beam through a converging lens that has a focal length of 1 foot, the laser beam will converge to a very narrow “beam waist” 1 foot beyond the lens and will then spread relatively quickly with distance. It will return to its original diameter 1 foot beyond its waist and to 10 times its original diameter 10 feet beyond its waist. With its light spread out by a factor of 10 in both height and width, it will have only 1/100th the intensity (power per unit area) of the original beam. Because of its large size, only a fraction of the beam and its light power will now enter a person’s iris and focus on their retina.

Using this scheme, you can have a beam that is extremely intense for the first 2 feet, including a super-intense waist at the 1-foot mark. But beyond that point, the beam spreads quickly and soon becomes so wide that it is no longer a eye hazard.

Frost forms only when the relative humidity reaches 100%

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We flew from SeaTac to Maui last week. Because of snow on the ground and not enough deicer, many planes were unable to take off. On the return trip, the flight had a realtime listing on their t.v. screen of where we were (showing the progress we were making) and also showed altitude, flight speed and outdoor temperature. I noted that the outdoor temperature at 36,000 feet was 60 degrees below zero! So then I wondered….if planes can’t take off without deicer at 32 degrees Fahrenheit, how can they “fly” at even colder temperatures? — VN, Anacortes, Washington

The problem for planes isn’t the temperature, it’s the humidity. When the air reaches 100% relative humidity, moisture in that air begins to condense on objects such as plane wings. The moisture can also condense into rain, snow, or sleet and then fall onto those plane wings.

If the temperature of overly moist air is 32 F or below, planes preparing for takeoff can accumulate heavy burdens of ice. When water vapor condenses as ice directly onto the wings themselves, that condensation process is called deposition and is familiar to you as frost. Deposition is a relatively slow process, so most of the trouble for planes occurs when it is actually snowing or sleeting. Removing the ice then requires either heat or chemicals.

When the plane is flying at high altitudes, however, the air is extremely dry. Even though the air temperature is far below the freezing temperature of water, the fraction of water molecules in the air is nearly zero and the relative humidity is much less than 100%. That means that an ice cube suspended in that dry air would actually evaporate away to nothing. Technically, that “evaporation” of ice directly into water vapor is call sublimation and you’ve seen it before. Think of all the foods that have experienced freezer burn in your frost-free (i.e., extremely dry air) refrigerator or the snow that has mysteriously disappeared from the ground during a dry spell even though the temperature has never risen above freezing. Both are cases of sublimation — where water molecules left the ice to become moisture in the air.

Can special eyeglasses let you see invisible radiation?

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I’ve read reference to “Smart” eyeglasses or contact lenses that can present more than just the visible portion of the electromagnetic spectrum. I’m wondering if you have any sources for these type of devices that are available to we civilians. — GJ, Wells, Nevada

Since our eyes are only sensitive to light that’s in the visible range, any “smart” optical system would have to present whatever it detects as visible light. That means it has to either shift the frequencies/wavelengths of non-visible electromagnetic radiation into the visible range or image that non-visible radiation and present a false-color reproduction to the viewer. Let’s consider both of these schemes.

The first approach, shifting the frequencies/wavelengths, is seriously difficult. There are optical techniques for adding and subtracting optical waves from one another and thereby shifting their frequencies/wavelengths, but those techniques work best with the intense waves available with lasers. For example, the green light produced by some laser pointers actually originated as invisible infrared light and was doubled in frequency via a non-linear optical process in a special crystal. The intensity and pure frequency of the original infrared laser beam makes this doubling process relatively efficient. Trying to double infrared light coming naturally from the objects around you would be extraordinarily inefficient. In general, trying to shift the frequencies/wavelengths of the various electromagnetic waves in your environment so that you can see them is pretty unlikely to ever work as a way of seeing the invisible portions of the electromagnetic spectrum.

The second approach, imaging invisible portions of the electromagnetic spectrum and then presenting a false-color reproduction to the viewer, is relatively straightforward. If it’s possible to image the radiation and detect it, it’s possible to present it as a false-color reproduction. I’m talking about a camera that images and detects invisible electromagnetic radiation and a computer that presents a false-color picture on a monitor. Imaging and detecting ultraviolet and x-ray radiation is quite possible, though materials issues sometimes makes the imaging tricky. Imaging and detecting infrared light is easy in some parts of the infrared spectrum, but detection becomes problematic at long wavelengths, where the detectors typically need to be cooled to extremely low temperatures. Also, the resolution becomes poor at long wavelengths.

Camera systems that image ultraviolet, x-ray, and infrared radiation exist and you can buy them from existing companies. They’re typically expensive and bulky. There are exceptions such as near-infrared cameras — silicon imaging chips are quite sensitive to near infrared and ordinary digital cameras filter it out to avoid presenting odd-looking images. In other words, the camera would naturally see farther into the infrared than our eyes do and would thus present us with images that don’t look normal.

In summary, techniques for visualizing many of the invisible portions of the electromagnetic spectrum exist, but making them small enough to wear as glasses… that’s a challenge. That said, it’s probably possible to make eyeglasses that image and detect infrared or ultraviolet light and present false-color views to you on miniature computer monitors. Such glasses may already exist, although they’d be expensive. As for making them small enough to wear as contact lenses… that’s probably beyond what’s possible, at least for the foreseeable future.

How do gas bubbles wash carbon dioxide out of freshly fermented wine?

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In a wine tank we use Nitrogen (N2) to sparge both excess Oxygen (O2) and Carbon Dioxide (CO2) out of the wine solution. The sparger injects Nitrogen (N2) into the wine in very small bubbles at 20L/min to remove both Oxygen and Carbon Dioxide. Why does this work? — JT, Australia

During wine making, the amount of dissolved carbon dioxide (and possibly oxygen gas) can easily exceed its equilibrium concentration. That means that the liquid contains more dissolved gas than it would have if exposed to the atmosphere for a long period of time and had thereby reached its equilibrium concentration of the gas. Having too much dissolved gas does not, however, mean that this gas will leave quickly. For example, when you open a bottle of carbonated beverage the carbon dioxide is out of equilibrium. Although the gas was in equilibrium at the high pressure of the sealed bottle, it instantly became out of equilibrium when the bottle was opened and the density of gaseous carbon dioxide suddenly decreased. Nonetheless, it can take days for the excess carbon dioxide to come out of solution and leave. You’ve probably noticed that carbonated beverages take hours or days to “go flat.”

Part of the reason why it takes so long for the dissolved gases to come out of solution is that the gas can only leave through the exposed surface of the liquid. In an open bottle of carbonated beverage that may be only a few square inches or a few dozen square centimeters. The dissolved gas has to find its way to that exposed surface and break free of the liquid. That’s a slow process. The same thing is happening in your wine: the dissolve carbon dioxide and oxygen gases must normally find their way to the top of the tank and then break free to enter the gaseous region at the top of the tank — another slow processes. To speed the escape of dissolved gases, you can enlarge the exposed surface of the liquid by bubbling an inert gas through the liquid. Here, inert gas is any gas that doesn’t dissolve significantly in the liquid and that doesn’t affect the liquid if it does dissolve. Nitrogen is great for wine because it doesn’t interact chemically with the wine. As you let bubbles of nitrogen float upward through the wine, you provide exposed surface within the body of the liquid wine and allow carbon dioxide and oxygen to break free of the liquid and enter those bubbles.

The spherical interface between the gas bubble and the surrounding liquid is a busy, active place — gas molecules are moving between the gas and liquid in both directions. Because carbon dioxide is over-concentrated in the liquid, it is statistically more likely for a carbon dioxide molecule to leave the liquid and enter the bubble’s gas than the other way around. It takes a little energy to break those carbon dioxide molecules free of the liquid and that need for energy affects the balance between dissolved carbon dioxide and gaseous carbon dioxide at equilibrium. The harder it is for the carbon dioxide molecules to obtain the energy they need to escape from the liquid, the greater the equilibrium concentration of dissolved carbon dioxide — the saturated concentration. But your wine is supersaturated, containing more than the equilibrium concentration of dissolved carbon dioxide, so carbon dioxide molecules go from liquid to gas more often than the other way around.

When the degree of supersaturation (excess gas concentration) is high, the transfer of gas molecules from liquid to gas bubble can be fast enough to make the bubbles grow in size significantly as they float up through the wine. You can see this type of rapid bubble growth in a glass of freshly poured soda, beer, or champagne. In beer, champagne, and your wine, however, the liquid surface of the bubble contains various natural chemicals that alter the interface with the gas and affect bubble growth. The “tiny bubbles” of good champagne reflect that influence.

Another way to provide the extra exposed surface in the wine and thereby allow the supersaturated dissolved gases to come out of solution would be to agitate the wine so violently that empty cavities open up within the wine. Although that approach would provide lots of extra surface, it would probably not be good for the wine. Bubbling gas through the wine is a much more gentle.

The exact choice of gas barely matters as long as it is chemically inert in the wine. Argon or helium would be just as effective, but they’re more expensive (and in the case of helium, precious). The temperature of the gas doesn’t matter significantly, but the temperature of the wine does. The cooler the wine, the higher the concentration of dissolved carbon dioxide and oxygen it will contain at equilibrium so you’ll remove more of those gases if you do your bubbling while the wine is relatively warm.

How can an ant survive inside a microwave oven?

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Upon removing a cup of coffee I’d heated for one minute in a microwave oven, I noticed a small ant running about, apparently unharmed. Curious, I gave it another one minute ride and when the door was open, it was still running about. How come an ant is apparently unharmed after two minutes in a microwave? — KMB

Most likely, the ant never left the floor or walls of the microwave oven, where it was as close as possible to those metal surfaces. The six sides of the cooking chamber in a microwave oven are made from metal (or painted 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 to their 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 incident wave. Because of that cancellation at the conducting surfaces, the intensity of the microwaves at the walls is zero or very close to zero.

The ant survived by staying within a tiny fraction of the microwave wavelength (about 12.4 cm) of the metal surfaces, where there is almost zero microwave intensity. Had the ant ventured out onto your cup, it would have walked into real trouble. Once exposed to the full intensity of the microwaves, it would not have fared so well.

How can food catch fire in a microwave oven?

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My wife makes blueberry pancakes for my daughter daily. Twice recently she noticed and brought to my attention a curious event in the Microwave oven. Frozen Blueberries placed inside a microwave oven to thaw, caused a popping sound and a small flame to appear amidst the blueberries. The flame self extinguishes. There is no apparent damage to the blueberries or the bowl they were contained in. — HA, New Jersey

I think that you’ve rediscovered an experiment in which people cut a grape almost in half, open the two halves like a book and lay it flat on a plate. In the microwave, the thin bridge between the halves carbonizes and 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 burn up. 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 plasma balls in the oven (lots of fun, but not great for the oven… you can scorch its top surface because those plasma balls rise and skittle around the ceiling of the oven). Anyway, you can probably find the carbon areas if you look closely enough, but they’re no worse than a little burnt toast.

Keeping an open soda bottle fresh

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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, CO

Yours is actually a complicated question. After you open the soda, the CO2 dissolved in the soda is no longer in equilibrium with the gas above soda. When you cap the bottle, CO2
will gradually escape from the liquid until it forms a dense gas so that CO2 molecules from that gas return to the liquid solution as often as they leave the solution for the gas. In other words, the equilibrium between dissolved CO2 and gaseous CO2 has to be reestablished.

By shrinking the volume of gas over the soda, your boyfriend reduces the number of CO2 molecules that must enter the gas phase in order to reestablish that equilibrium. BUT, when dense gas develops in the squeezed bottle, the high pressure of that gas will reinflate the bottle to its original size. The benefits of shrinking 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 squeezed bottle stays squeeze. That’s hard to do. You’re probably better off pouring the soda gently into a smaller bottle, one that just barely holds all of the liquid. That smaller bottle won’t expand as a dense gas of CO2 forms above the liquid soda and the soda will reestablish its equilibrium without losing too many of its dissolved CO2 molecules.