Author: Lou Bloomfield

Do brownouts or other power outages damage appliances?

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If a home looses some of its power during a power outage and the lights shine dim, will it burn up the motor in the refrigerator? Will it damage other appliances (TV, VCR. stereo. etc)? Should the main disconnect be shut off? — J, Ohio

Power outages come in a variety of types, one of which involves a substantial decrease in the voltage supplied to your home. The most obvious effect of this voltage decrease is the dimming of the incandescent lights, which is why it’s called a “brownout.” The filament of a lightbulb is poor conductor of electricity, so keeping an electric charge moving through it steadily requires a forward force. That forward force is provided by the voltage difference between the two wires: the one that delivers charges to the filament and the one that collects them back from the filament. As the household voltage decreases, so does the force on each charge in the filament. The current passing through the filament decreases and the filament receives less 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 behaves approximately according to Ohm’s law: the current that flows through it is proportional to the voltage difference between its two ends. The larger that voltage difference, the bigger the forces and the more current 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 current and receive so much power that they’ll overheat and break. Voltage surges, not voltage decreases, are what kill lightbulbs.

The other appliances you mention are not ohmic devices and the currents that flow through them are not simply proportional to the voltage supplied to your home. Motors are a particularly interesting case; the average current a motor carries is related in a complicated way to how fast and how easily it’s spinning. A motor that’s turning effortlessly carries little average current and receives little electric power. But a motor that is struggling to turn, either because it has a heavy burden or because it can’t obtain enough electric power 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’t surprise 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 devices operate on specific internal voltage differences, all of which are DC — direct current. Your home is supplied with AC — alternating current. The power adapters that transfer electric power from the home’s AC power to the device’s DC circuitry have evolved over the years. During a brownout, the older types of power 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 if their batteries are worn out.

But the most modern and sophisticated adapters are nearly oblivious to the supply voltage. Many of them can tolerate brownouts without a hitch and they’ll keep the electronics working anyway. The power units for 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 assumptions about 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 the potential overheating problem. The electronic appliances will either misbehave benignly or ride out the brownout unperturbed. Once in a while, something will fail during a brownout. But I think that most of the damage is down during the return to normal after the brownout. The voltages bounce around wildly for a second or so as power is restored and those fluctuations can be pretty hard some devices. It’s probably worth turning off sensitive electronics once the brownout is underway because you don’t know what will happen on the way back to normal.

Is it safe to operate a microwave oven with a torn front screen?

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My husband put a large metal bowl in our new microwave oven and tore a small hole in the oven’s metal screen while trying to close the door. My husband isn’t concerned, but the oven is mounted over the stove at face level and it certainly concerns me. Can we use it? — E, Ontario, Canada

That 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 much microwave leakage. The oven’s microwaves have a wavelength of 12.4 centimeters and they’ll reflect from conducting surfaces with holes much smaller than that wavelength. A foot from your oven, there probably won’t be any significant microwave intensity, although the only way to be sure is with a microwave leakage meter.

The evanescent wave problem is more likely. When any electromagnetic wave reflects from a conducting surface that has small holes in it, there is what is known as an evanescent wave extending into and somewhat beyond each hole. It’s as though the wave is trying to figure out whether or not it can pass through 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 the hole. The extent of the evanescent wave increases with the size of the hole. The microwave oven’s screen has very small holes and it is located inside the glass window. The evanescent waves associated with those holes 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 extend farther through that screen and perhaps out beyond the surface of the glass window. If you press your hand against 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 electric currents. The microwaves’ electric fields push electric charge back and forth in the conducting screen and it is that moving charge (i.e., electric current) that ultimately redirects the microwaves back into the cooking chamber as a reflection. Those electric currents in the screen are real and they’re not going to take kindly to that tear. It’s a weak spot in the conducting surface through which they flow. Weak electrical paths can heat up like lightbulb filaments when they carry currents. Moreover, charge that should flow across the torn region can accumulate on sharp edges and leap through the air as an arc. If either of these processes happens, 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 the window to never cause injury, and the tear could never heat up or arc. But the risk of operating this damaged microwave oven is not insignificant. Since it’s an installed unit, I’d suggest replacing the screen or the door. There are a number of websites that sell replacement parts for microwave ovens and I have used them to replace the door on our microwave oven.

Does a bigger camera lens always produce a better picture?

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Is it true that the bigger the lens on a camera, the more light goes through it and the better the photo or video? My film teacher says that while this idea is logically correct, he didn’t know if it was true. Your lecture slides say the answer is yes, but my teacher still doesn’t believe it. We were wondering about your source for this material. — PJ

I’ll assume that by “bigger lens” you mean one that is larger in diameter and that therefore collects all the light passing through a larger surface area. While a larger-diameter lens can project a brighter image onto the image sensor or film than a smaller-diameter lens, that’s not the whole story. Producing a better photo or video involves more than just brightness.

Lenses are often characterized by their f-numbers, where f-number is the ratio of effective focal length to effective lens diameter. Focal length is the distance between the lens and the real image it forms of a distant object. 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 lens is 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 images of equal brightness onto their image sensors and the smaller the f-number, the brighter the image. Whether a lens is a wide-angle or telephoto, if it has an f-number of 4, then its effective focal length is four times the effective diameter of its light gathering lens. Since telephoto lenses have long focal lengths, they need large effective 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 simply dividing the lens diameter by the distance between the lens and image sensor won’t tell you much. Many of these 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 image brightness while zooming.

You might wonder why a lens would ever choose to discard light. There are at least two reasons for doing so. First, there is the issue of image quality. The smaller the f-number of a lens, the more precise its optics must be in order to form a sharp image. Low f-number lenses are bringing together light rays from a wide range 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 and Canon sold a remarkable f0.95 lens in the early 1960’s. The lowest f-number camera lens I have ever owned 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 of angles and those rays only meet at one point before separating again. Since objects at different distances in front of the lens form images at different distances behind the lens, it’s impossible to capture sharp images of both objects at once on a single image sensor. With a high-f-number lens, this fact isn’t a problem because the light rays from a particular object are rather close together even when the object’s image forms before or after the image sensor. But with a low-f-number lens, the light rays from a particular object come together 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 constant while zooming from telephoto to wide angle, its f-number would decrease during that zoom and its depth-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 that assumes that they are high-quality and that they can shrink their effective diameters in ways that allow them to imitate high-quality lenses of smaller diameters when necessary. But flexible characteristics always come at some cost of image quality and the very best lenses are specialized to their tasks. Zoom lenses can’t be quite as good as fixed focal length lenses and a large-diameter lens imitating a small-diameter lens by throwing 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 is part of the basic toolbox of an optical physicist (which I am). You can, however, look this stuff up in any book on geometrical optics.

Can I warm plates in my microwave oven? Will the microwaves heat them?

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Can I warm plates in my microwave oven? — AC

Yes, but it’s not a good idea. Depending on the type of plate, you can either damage your microwave oven or 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 perfectly transparent to microwaves, the electromagnetic waves streaming out of the oven’s magnetron tube bounce around endlessly inside the oven’s cooking chamber. The resulting intense fields can produce various types of electric breakdown along the walls of the cooking chamber and thereby damage the surface with burns or arcs. Furthermore, the intense microwaves in the cooking chamber will reflect back into the magnetron and can upset its internal oscillations so that it doesn’t function properly. Although magnetrons are astonishingly robust and long-lived, they don’t appreciate having to reabsorb their own emitted microwaves. In short, your plates will heat up slowly and you’ll be aging your microwave oven in the process. You could wet the plates before putting them in the microwave oven to speed the heating and decrease 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 absorbs the 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 plates you’re warming. If you heat a plate too long, bad things will happen. It may only amount to burning your fingers, but some plates can’t take high temperatures without melting, cracking, or popping. Unglazed ceramics 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 seem safe to me. And if a plate absorbs microwaves nonuniformly, then you’ll get hotspots or burned spots on the plate. Metalized decorations on a plate will simply burn up and blacken the plate. Cracks that contain water will overheat and the resulting thermal stresses will extend the cracks further. So this type of heating can be stressful to the plates.

Breathing Underwater through a Hose

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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 through that 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 much more pressure outside your lungs than inside them that you won’t be able to draw in any more air. Your muscles just won’t be strong enough.

The water pressure increases quickly with depth because each layer of water must support the weight of all the water layers above it. Since water is dense, heavy stuff, the weight piles on quickly and it takes only 10 meters (34 feet) of descent to increase the water pressure from atmospheric to twice atmospheric. In contrast, the air in the hose is light, fluffy stuff, so its pressure increases rather slowly with depth. Even though each layer of air has to support the weight of all the layers of air above it, the rise in pressure is extremely gradual. It takes miles of atmosphere above the earth for the air pressure to build up to atmospheric pressure near the ground. The air pressure in your hose is therefore approximately unchanged by your descent into the water.

With the water pressure outside rising quickly as you go deeper and the air pressure in your mouth rising incredibly slowly as you go deeper, you quickly find it hard to breathe. Your muscles can push your chest outward against a modest pressure imbalance between outside and inside. But by the time you’re a few feet below the surface, you just can’t draw air into your lungs through that hose anymore. You need pressurized air, such as that provided by a scuba outfit or a deep-sea diver’s compressor system.

Ice Cubes and Freezer Smells

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Would ice in the freezer absorb the smell in the freezer? — ML, Auckland NZ

Despite the freezer’s low temperature and the motionlessness of all the frozen foods inside it, there is still plenty of microscopic motion going on. Every surface inside the freezer is active, with individual molecules landing and leaving all the time. Whenever a molecule on the surface of a piece of food manages to gather enough thermal energy from its neighbors, it will break free of the surface and zip off into the air as a vapor molecule. And whenever a vapor molecule in the air collides with the surface of another piece of food, it may 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 means that 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 the scent 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 normally exposed to the air in the freezer, keeping the odor molecules trapped in their own sealed containers keeps the freezer air fresh and the ice cubes odor-free.

I was told the holes in the front door of a microwave oven were shaped round bec…

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I was told the holes in the front door of a microwave oven were shaped round because the microwave beam is shaped as a square. Thus, this means that a square shape object cannot pass through a round shaped object. Is this a true statement or not? — BH, Texas

No, there is no square-peg in round-hole effect going on in microwave ovens. Microwaves reflect from conducting surfaces, just as light waves reflect from shiny metals, and they can’t pass through holes in conducting 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 the microwaves are reflected by that mesh.

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

In reality, electromagnetic waves consist of electric and magnetic fields (influences that push on electric charge and magnetic pole, respectively) that point up and down in a rippling fashion, but nothing actually travels up and down per say. The spatial structures of these fields are governed by Maxwell’s equations, a set 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 there are 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 they try 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 pretty tightly in a surface while leaving their conducting boundaries relatively robust. And round holes treat all electromagnetic waves equally because they have no wide or narrow directions.

What happens when sheets of paper, long rolled up into a tube, are unrolled but …

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What happens when sheets of paper, long rolled up into a tube, are unrolled but simply won’t ever lie flat again? — PD

Paper consists mostly of cellulose, a natural polymer (i.e. plastic) built by stringing together thousands of individual sugar molecules into vast chains. Like the sugars from which it’s constructed, cellulose’s molecular 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 its molecular pieces didn’t cling so tightly. These effects are why it’s so hard to reshape cellulose and why wood or paper don’t melt; they burn or decompose instead. In contrast, chicle — the polymer in chewing gum — 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. Like ordinary sugar, cellulose is attracted to water and water molecules easily enter its chains. This water lubricates 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 taking advantage 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 fibers of even brittle materials can bend significantly without breaking. If you bend paper gently, its fibers will bend 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 will begin 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 at room temperature. Over months or years in a tightly rolled shape, a sheet of paper will rearrange its cellulose fibers until it adopts the rolled shape as its own. When you then remove the paper from its constraints, it won’t spontaneously flatten out. You’ll have to reshape it again with time, moisture, and/or heat. If you press it in a heavy book for another long period, it’ll adopt a flat shape again.

Why is a car’s rear window put and kept under stress, and what has this to do wi…

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Why is a car’s rear window put and kept under stress, and what has this to do with polarization? — BD, Leuven, Belgium

The rear window of a car is made of tempered glass — the glass is heated approximately to its softening temperature 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 to tear. But once a tear does manage to propagate through the compressed surface layer into the tense heart of the glass, the entire window shreds itself in a process called dicing fracture — it tears itself into countless little cubes.

The stresses frozen into the tempered glass affect its polarizability and give it strange characteristics when exposed to the electromagnetic fields in light. This stressed glass tends to rotate polarizations of the light passing 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.

Why must you “shake down” a mercury fever thermometer? I was told by one manuf…

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Why must you “shake down” a mercury fever thermometer? I was told by one manufacturer that mercury expands but does not contract. Also, is it true that the rounded glass acts as a magnifier because the bore is so small? — JB

Mercury 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 as they get hotter, the mercury experiences a larger increase in volume and must flow up the narrow channel or “capillary” inside the glass to find room for itself. Mercury is essentially incompressible so that, as it expands, it pushes as hard as necessary on whatever contains it in order to obtain the space it needs. That’s why a typical thermometer has an extra chamber at the top of its capillary. That chamber will receive the expanding mercury if it rises completely up the capillary so that the mercury won’t pop the thermometer if it is overheated. In short, the force pushing mercury up the column can be enormous.

The force pushing mercury back down the column as it cools is tiny in comparison. Mercury certainly does contract when cooled, so that the manufacturer is telling you nonsense. But just because the mercury contracts as it cools doesn’t mean that it will all flow back down the column. The mercury needs a push to propel 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 modest narrowing of arteries can dramatically impair blood flow in people. The capillaries used in fever thermometers are so narrow that mercury has tremendous trouble flowing through them. It takes big forces to 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 keep the column together. That’s because the only thing above the column of liquid mercury is a thin vapor of mercury gas and that vapor pushes on the liquid much too feebly to have a significant effect. And while gravity may also push down on the liquid if the thermometer is oriented properly, it doesn’t push hard enough to help much.

The contracting column of mercury takes hours to drift downward, if it drifts downward at all. It often breaks up into sections, each of which drifts downward at its own rate. And, as two readers (Michael Hugh Knowles and Miodrag Darko Matovic) have both pointed out to me in recent days, there is a narrow constriction in the capillary near its base and the mercury column always breaks at that constriction during contraction. Since the top portion of the mercury column is left almost undisturbed when the column breaks at 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 the constriction so that it rejoins into a single column. In effect, you are making the glass accelerate so fast that it 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 of the 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 of water, it acts as a cylindrical lens that magnifies the narrow sliver of metal into a wide image.