Wikipedia:Reference desk/Archives/Science/2007 November 22

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November 22

Cooling down -- drink or pour?

I do not seek medical advice, and neither is this a homework question. I have seen people pouring liquid over their heads (presumably in order to cool down). The question is: if it was a very warm day at 30 C, and I had 1 liter of water at, say, 2 deg C; would it be more cooling for me to drink it, or (bearing in mind most heat is generated/lost in the head/brain) for me to pour it over my head( slowly) —Preceding unsigned comment added by 79.76.162.232 (talk) 02:09, 22 November 2007 (UTC)

By drinking it the heat of your body goes into heating up the water, thus cooling you somewhat. By pouring over your head the heat of your body (and the air) go into heating the water. By pouring it over your head you also get the benefit of evaporative cooling which will be much more significant than the cooling you get from just heating the water up. So pour over your head..... unless you are dehydrated, then you should drink.Shniken1 (talk) 02:19, 22 November 2007 (UTC)

But it will also constrict you blood vessels so less heat can be transferred away from your skin, therefore leaving you hotter in the long run. --antilived^{T | C | G} 04:20, 22 November 2007 (UTC)

Also, not much of the water you pour over your head gets evaporated: most of it lands on the ground, whereas all that you drink gets warmed to body temperature. Dribbling water slowly over your head or body might be more helpful than pouring it all at once, but of course that keeps your hands busy while you're presuambly trying to do something else.

Another point is that if you get dehydrated enough, you'll stop sweating, leading to more overheating and possible heat stroke, which can kill you. Drinking the water will prevent that -- unless you drink so much that you suffer water intoxication (hyperhydration), which can also kill you. Giving specific guidelines, of course, would be medical advice. --Anonymous, 05:25 UTC, November 22.

We've had similar questions in the past, such as why it is that dousing yourself in cold water after an intense workout only cools you down for a short period. As explained above, the chief culprit is vasoconstriction. What you need to decide with your bottle in hand is first, do you really need to cool down or do you need to hydrate, and second, how cold is that bottle of water? If the water is as cold as you mention, I would rinse the stuff in my mouth and spit it out. Some of the (now warmed) water would get swallowed to help hydration and the mouth can deal with temperature changes better than, say, your scalp, when it comes to simply cooling. Matt Deres (talk) 17:46, 23 November 2007 (UTC)

• A subjective, originally-researched answer: I'm a marathon runner in the southern USA, where it gets quite warm, and I have found that pouring some ambient-temperature water on my head (which is covered by a hat made of technical fabric) has a *much* greater subjective cooling effect than just drinking it. --Sean 22:01, 23 November 2007 (UTC)

physics

ships r made of iron and other heavy metals so why is it that when a ship is anchored it doesnt sink? —Preceding unsigned comment added by 59.95.15.157 (talk) 03:53, 22 November 2007 (UTC)

Yes, ships are often made of materials that are heavier than water. However, much of their volume is lighter than water, since they contain air. The weight of the ship is less than the weight of water contained in volume equal to the volume displaced by the ship, allowing for buoyancy. If you fill a boat/ship with water, it will sink. moink (talk) 04:04, 22 November 2007 (UTC)

(ec)

First off, it's not the case that lightweight things automatically float and heavy things automatically sink.

If you had a ten pound piece of wood, it would float, and if you had a metal coin that weighed two ounces, it would sink.

Try this: find a metal can, such as soup or canned vegetables come in. Remove the top completely. Fill it with water, and put it in a sink or bathtub full of water. It will sink, as you expect: the steel that the can is made of is "heavier" (or as we'll see, denser) than water, so it sinks.

But then: pick the can back up and empty half the water out, so that it's now half full of water. Put it back in the sink or bathtub, right-side up. It will float! Even though it's still "heavier" than water, and even though it's half full of water! How does this work?

The answer is, you've just made a crude steel-hulled boat. The only reason I had you leave it half-full of water was so that it would stay upright, instead of tipping over, filling all the way up with water, and sinking. But it's still pretty tippy -- water is a lousy ballast; the boat has a terrible righting moment. (There's a reason you don't want your real boat half-full of water!)

So now, try one more experiment. Empty all the water out of the can, and put some coins in the bottom. For a 15 oz. can, I find that using fifty U.S. pennies works well. (Your mileage may vary, depending on the size of can you use and the coins you have available where you live.) With the coins in the bottom, the can floats even better! But this seems really strange, because the coins are heavier than the water was. Or are they? The same volume of coin metal is heavier than the same volume of water, but the volume of the coins is a lot less. So actually the fifty coins in the bottom of the can weigh less than the half-can's worth of water.

So what we're finding out here is that it's not so much how heavy something is, but rather how dense it is, that matters when we're deciding whether it will float in water. It turns out, the rule is simple: if it's denser than water, it sinks, and if it's less dense than water it floats. That's why, everything else being equal, metal sinks and wood floats.

But, when you make a boat out of metal, the boat is obviously not solid metal. (Otherwise it would sink like, well, a stone.) The inside of the boat is full of air, and it's the overall density of the boat+air system that matters. If the boat plus the air inside weighs less than the same volume of water would, its overall density ends up being less than water, and it floats. And if the boat is heavier at the bottom, it will float well, and stay upright, and not tend to tip over. (It turns out that real boats usually have something heavy -- extra metal, or rocks -- down in their keel, as ballast, so that they work this way, just like our can with coins in the bottom. You can read more about this at center of buoyancy.) —Steve Summit (talk) 04:40, 22 November 2007 (UTC)

Take a hypothetical. What if you have bucket of water in a vacuum/airless room. Then you put the empty metal can on the water. Will the can sink? Since there's no air, we don't count the air mass and volume into the density calculation, right? And since the can itself, without air, is greater than that of water, the can should sink, right?199.76.152.229 (talk) 03:23, 24 November 2007 (UTC)

No, It would float even more. It would not have the mass of the air, but it would now it would contain a certain volume of empty space. Empty space is even lighter than air. 72.10.110.107 (talk) 17:05, 26 November 2007 (UTC)

Steve - that was a really good answer! DuncanHill (talk) 06:13, 22 November 2007 (UTC)

(Thanks! —scs 00:20, 24 November 2007 (UTC))

Because math is good, I'd like to add that Archimedes's Principle is that the weight of water displaced is equal to the weight of the object displacing it. So if I have a 2000-ton ship, it will displace 2000 tons of water, which is about 2000m^3 in volume. But suppose the ship has a total volume of 5000m^3 -- then it will float, and the water line will only come as far as about 2/5 of the side if it's a barge-like ship. SamuelRiv (talk) 11:56, 22 November 2007 (UTC)

Zoom lenses

Why do some zoom lenses have a constant f-number? As focal length increases, the entrance pupil must also increase to maintain the same f-number. Since this is the case, do such lenses actually have an enterance pupil that physically expands when the lens is zoomed? If the entrance pupil can expand, why not keep it at its largest, so that, a lens that is a, say, 70-210mm f/4 (at 210mm, the pupil is 52.5mm) is a 70-210mm f/1.3-f/4 (52.5mm constant pupil diameter), so that the lens is faster at its wider end? 70.156.49.65 (talk) 04:03, 22 November 2007 (UTC)

I don't think the pupil changes size, I have a 70-210mm f/4-5.6 (NOT constant aperture) and if I set it on say f/11 at 70mm (supposed pupil size = 6.4mm), the aperture size doesn't change as I zoom, finally becoming f/16 at 210mm (supposed pupil size = 13mm), but it still doesn't solve why the effective aperture changes when the pupil size doesn't. --antilived^{T | C | G} 07:04, 22 November 2007 (UTC)

over working the kidneys

In the past I used to eat a lot of salads but when my schedule kept me from sitting down to eat I followed the great idea of adding water and pureeing my salads to consume as a drink. Now I do the same with yogurt, fruits and vegetables to the point that I must be drinking at least a gallon ever two hours. Can I hurt my kidneys by drinking so much liquid or does it make them work even better? 71.100.5.134 (talk) 08:19, 22 November 2007 (UTC)

• You should probably ask your doctor, but since that would equate to nearly 12 litres in a working day alone, I'd expect it to be unhealthy. You only need around 2-3 litres to stay hydrated and drinking too much can indeed have an adverse effect on your health, especially if you do it in such short periods. - 131.211.161.119 (talk) 09:00, 22 November 2007 (UTC)

If you have a schedule that prevents you from sitting down to eat you need, seriously, to address your life style. Fact - People do die from drinking excess liquids, usually water. Richard Avery (talk) 10:54, 22 November 2007 (UTC)

Well usually it is alcohol :D, but yes water intoxication is possible Shniken1 (talk) 12:32, 22 November 2007 (UTC)

A gallon every 2 hours, that's about 4.5Litres right? So in a day? That's 18L in an 8 hour work day, or if you're talking about your waking hours it's more like 36L! Wow, that's dedication! Water intoxication occurs when the amount of fluid you ingest causes electrolyte washout to such an extent that cell membrane physiology becomes irreparably disturbed and cell death follows. To get it just from drinking you need to drink plain water only, and lots of it. You're also consuming a lot of electrolytes in the puree so that would go a long way to helping you maintain electrolytic homeostasis. Normal kidneys should be able to excrete at least half to 2/3 of that water volume, but you'll also have water loss through perspiration, respiration etc. You're probably peeing like a racehorse and/or sweating like a pig to compensate. If you weren't, you'd probably be rather oedematous by now. The figure of 2-3L of water being required per day is a figure arrived at by looking at the metabolic requirements of a sedentary 70kg man. So if you're very physically active your water requirements will be much higher. Plenty of people don't find time to sit and eat regularly - ask any mother! And while it would be nice to say society is to blame for your salad smoothies and resultant concerns, getting the world to slow down really needs to be addressed in other venues. But the important point is:

• • See your doctor if you're worried. A simple blood test will priovide all the answers to your concerns.

Cheers! Mattopaedia (talk) 03:11, 23 November 2007 (UTC)

Anti-aliasing in human vision

Are there aliasing effects in human vision and what plays the role of the anti-aliasing? Keria (talk) 09:56, 22 November 2007 (UTC)

Correct me if I'm wrong but I always thought our retina is like film, with all the receptors are pretty much randomly distributed at a very high resolution, so that it's pretty much unnecessary. I still see aliasing outside my fovea though, so I guess the resolution outside the fovea is low enough to be able to see aliasing. --antilived^{T | C | G} 10:08, 22 November 2007 (UTC)

I associate aliasing with geometric optics, but the fovea operates pretty close to the diffraction limit. I think the blurring you get from diffraction will have about the same effect as oversampling in 3D graphics. -- BenRG (talk) 12:04, 22 November 2007 (UTC)

There are distortions that occur in human vision that are caused by a lack of spatial (an optical illusion book has tons of these, or even looking through a fine-grained mesh can cause a similar effect) and temporal (a propeller blade that appears to spin backwards) resolution. It's not quite as simple as this, because illusions can occur either as a true aliasing effect in the eye, as a pre-processing effect in the thalamus, as a processing and integration effect in the occipital lobe, or even as a post-processing effect in the rest of the brain as images are linked with memories and higher judgement. All of these areas can cause illusions as all of them also serve to resolve possible illusions in vision, which is itself technically an optical illusion. So I'd say one of the clearest examples of "anti-aliasing" would be our pattern-matching, which fails when we see a triangle when shown three circles with wedges cut out of them. SamuelRiv (talk) 12:08, 22 November 2007 (UTC)

I don't think I've ever seen an optical illusion resulting from spatial or temporal aliasing in the visual system, except perhaps outside the fovea. Looking through a fine-grained mesh can make a moiré pattern, but that's a result of "sampling" by the mesh. I see nothing in the wagon-wheel effect article to suggest that it's ever caused by temporal aliasing in the visual system. The "discrete-frame theory" sounds unlikely to me. The circles-with-wedges illusion has nothing to do with aliasing, not that it isn't interesting in its own right. -- BenRG (talk) 12:08, 23 November 2007 (UTC)

Anti-aliasing is something that is only necessary when you scale an image down, and the eye doesn't do anything like that, so there is no anti-aliasing in human vision. Also, human vision is mostly analog, so our eyes don't really use image sampling, which is what causes spatial aliasing, therefore I'm pretty sure human vision can't cause spatial aliasing. Looking through a real fine-grained mesh (not one on TV) will not produce a Moiré pattern unless there are two imperfectly overlapping meshes, though you may see a pattern anyways if the holes in the mesh are not completely flat and regular. In either case that pattern is not an optical illusion or a product of aliasing, it's simply a varying pattern of blocked and unblocked light. Also, we generally only see temporal aliasing aliasing due to a flickering light source, such as from a TV or a fluorescent lamp, which causes a sampling effect. So, aliasing generally isn't a problem caused by human vision. -- HiEv 15:13, 22 November 2007 (UTC)

You can see what is called the wagon-wheel effect without a flickering (stroboscopic) light source, however there are various explanations why this effect can occur (see here). -- HiEv 15:28, 22 November 2007 (UTC)

Anti-aliasing is not only necessary for downscaling images. It's necessary (or at any rate useful) any time you sample a signal, whether or not that signal was reconstituted from higher-frequency samples. I suppose one could argue that the laws of physics are discrete, and every sampling is a downscaling. -- BenRG (talk) 12:08, 23 November 2007 (UTC)

String theory and the shape of the Universe

String theory seems to imply a particular shape of the Universe. Therefore, could it be that local geometry of the Universe is 3-dimensional (in an Euclidean, spherical or hyperbolic form) but the global geometry of the Universe is quadridimensional?

In such a scenario, the Universe would be a kind of anomalous sphere: just like in our Earth, in such an Universe, one could go westward and return to the starting point from the East; one could go northward and return to the starting point from the South; BUT, differently from Earth, one could also go upwards into space and, after eons of travelling, one would return to the starting point from beneath. In other words, the universe would be a simply connected sphere folded on itself in such a way that each point is in contact with its antipodal point. Is this view consistent with string theory? Is this even plausible? -- Danilot (talk) 10:43, 22 November 2007 (UTC)

It sounds like you're describing a 3-sphere, which has been a popular choice of topology for the universe since the earliest days of modern cosmology. I don't understand what you mean by quadridimensional; the space you're describing has a local dimension of 3 and a global dimension of 3. Also, I don't think this has anything to do with string theory, which doesn't to my knowledge predict the shape of the universe. -- BenRG (talk) 11:57, 22 November 2007 (UTC)

You might be interested in shape of the universe. Someguy1221 (talk) 16:34, 22 November 2007 (UTC)

piezoelectricity

I would like to build a rudimental piezoelectricity sensor (generator) for highschool project. I intend to apply pressure on a sample of Rochelle salt (which I have) and show the resulting current generation using a voltmeter. Is this going to work? Can somebody show me a simple diagram of how to apply the wires which would conduct the current? What is the best metal to use for these conductors? Thanks, Curious Student —Preceding unsigned comment added by 67.189.247.193 (talk) 11:15, 22 November 2007 (UTC)

The short answer is, it depends. The current produced will be transient—just a little blip of current when pressure is applied or released, after which the potential across the crystal will be back at equilibrium. The current is likely to be fairly small for reasonable pressures and crystal sizes. I don't know if you'll be able to see a small, short-lived current, and I suspect that it will be difficult for you to measure with any accuracy (unless you've got some specialized instruments).

I wonder if some sort of rudimentary electrometer might not be a better bet for you—squeeze some charges out of the crystal and into gold-leaf electroscope for your demonstration. (Caveat—I haven't tried this stuff, and can't tell you if it will work.) TenOfAllTrades(talk) 14:48, 22 November 2007 (UTC)

Incidentally, you might find something useful in our articles on piezoelectricity and piezoelectric sensors. TenOfAllTrades(talk) 14:51, 22 November 2007 (UTC)

You could use a gas ignition lighter that makes a spark. You would then just have to wire to the outlet. Beware the voltage is high on the order of several thousand volts, so you will need some special kind of voltmeter, such as TenOfAllTrades electrometer. Graeme Bartlett (talk) 20:31, 22 November 2007 (UTC)

It's the mechanics of the ignitor that result in the high voltage. Absent the over-center mechanism that delivers the sudden impact to the piezo portion, they can produce low voltages quite nicely.

Atlant (talk) 16:06, 26 November 2007 (UTC)

In the classroom, measure voltage, not current. Just get a cheap piezo buzzer (about $2.00USD), remove the piezo element, and place it under stress to produce the voltage, which you can measure with a voltmeter. An actual generator must be a dynamic system, so you will need an oscilloscope to show the varying voltage. Alternatively, you could use the piezo as a microphone and rectify the AC voltage via a full-wave bridge to charge a capacitor, and measure the (relatively small) current as you discharge the capacitor through a resistor. -Arch dude (talk) 05:20, 23 November 2007 (UTC)

Burglar alarm glass breakage detectors are usually piezoelectric as well; you might want to investigate one of those as a "prototype" for your project. If you mention what you're doing, an alarm installer might give you one for free. Old, inexpensive phonographs also used piezo pickups. Interestingly enough, certain ceramic capacitors are also piezoelectric, leading to undesireable microphonic effects when they are used in certain electronic circuits.

Atlant (talk) 16:01, 26 November 2007 (UTC)

Glassware for baking

There is a kind of bowl made of glass that you can put into the oven or into a microwave, primarily for baking vegetable-based food. These are made of a special kind of glass as far as I know. What's the name of them? Is there an article about them? If no, what property of the glass makes them usable in an oven? I'm not interested in how they are created, but the resulting physical properties. Thanks, – b_jonas 19:01, 22 November 2007 (UTC)

I believe you are talking about Pyrex, which is made of soda-lime glass, but originally was constructed of borosilicate glass. --80.229.152.246 (talk) 19:37, 22 November 2007 (UTC)

Glass recyclers consider it to be ceramic material. They hate to find pieces of it in recycled glass containers because it screws up their process: when "regular" glass is melted this ceramic material is still solid. When pouring the glass melt in whatever shape this endproduct has to be rejected because of the ceramic shards enclosed in it. VanBurenen (talk) 22:12, 22 November 2007 (UTC)

Why should Pyrex be primarily for vegetables? --Seans Potato Business 00:13, 23 November 2007 (UTC)

Because in the regular oven, I can use metallic containers, and I rarely use the microwave for baking meat or cookies. Rice or green peas, however, can be cooked just fine in the microwave. – b_jonas 10:07, 23 November 2007 (UTC)

It could also be because acidic vegetables (? Well, acidic fruits, maybe) may pick up flavors from some reactive metals. --Mdwyer (talk) 17:22, 23 November 2007 (UTC)

Besides Pyrex, you can use ceramic, glass-ceramic, and silicone cookware in microwave ovens. See Cookware#Non-metallic cookware. -- HiEv 19:34, 23 November 2007 (UTC)