In questa pagina trovate una lista di notizie reperibili su web (blog, notiziari, ...) che desidero condividere con voi.
di (author unknown)
Steve Jobs, 1955-2011.
di Alan Parekh
The picture above looks like a Samsung USB Hard Drive case right? Looks can be deceiving, the case actually contains come large nuts to make it feel like there is actually something inside and a USB thumb drive to provide some convincing operation. The USB drive has been made to simulate the large hard drive by showing up with a 500GB capacity even though the capacity of the drive is only 128MB. I am thinking some smart people in China made a custom controller for the drive to allow it to work in a loop mode which allows all of the most recent copied data to remain and the oldest data be overwritten. The TOC also works in an interesting way since even though an old file has been overwritten it would remain in the TOC to make it seem that the drive is functioning as it should.
di Paolo Attivissimo
Un'illusione classica diventa realtà
L'illusione della scacchiera e dell'ombra è un classico, eppure vederla in azione è sempre spettacolare. Però di solito la si presenta disegnata. Stavolta, invece, si tratta di un modello fisico, reale, tridimensionale. State a guardare questo video, segnalato da Bad Astronomy di Phil Plait.
No, non è un effetto digitale. Se non ci credete, provate voi stessi a confrontare i colori isolandoli dal contesto, per esempio mascherando il resto dell'immagine. È una delle più belle dimostrazioni di come il cervello non vede affatto come una macchina fotografica, ma interpreta la realtà sulla base di alcuni assunti. Di solito la interpreta egregiamente, ma quando questi assunti vengono meno, il sistema d'interpretazione va in tilt. E questi sono i risultati.
Ricordatelo, la prossima volta che qualche diversamente furbo strilla che non c'è bisogno di fare un'indagine scientifica ma basta guardare.
Vista l'incredulità di molti, ho realizzato personalmente (con l'aiuto di mia figlia) un modello che ricrea il fenomeno. Il video è qui.
di John Farrier
The small town of Riverside, Iowa (pop. 1,008) is tremendously proud of its most famous son, James Tiberius Kirk. So the citizens erected a monument to mark the site of his birth.
The local history museum, as you can see in the video, places a great emphasis on Kirk’s life and work. Who’s up for a road trip?
Photo by Flickr user Madolan Greene used under Creative Commons license
di (author unknown)
Google bids pi for Nortel's wireless patent stash, brings comedy to places you never thought possible
di Darren Murph
Enabling surfers to play Pac-Man instead of actually initiating the search they showed up to complete? Taking a stroll through an episode of Burn Notice? Throwing internet on a magical Indian bus? All relatively normal things from one Google, Inc., but it seems that Larry Page's deadpan demeanor is actually covering up quite the character. During the outfit's recent attempt to outbid the likes of Apple, EMC, Ericsson, Microsoft, RIM and Sony for a sliver of Nortel's coveted wireless patent portfolio, Reuters is reporting that Google's plays were... less than conventional. Reportedly, the company bid $1,902,160,540 and $2,614,972,128, better known by mathematicians as Brun's constant and Meissel-Mertens constant, respectively. Funnier still, Google decided to offer $3.14159 billion (you know, pi) when the bidding reached $3 billion. One of the unnamed sources summed up the bizarreness quite well:
Or, perhaps they're just supremely awesome?Permalink New York Times | Reuters | Email this | Comments
di Tyler Cowen
There is more here.
With those words, Tanzanian student Erasto Mpemba entered scientific history, and also sparked a scientific mystery and controversy that remains ongoing today, some 40 years later!
The phenomenon Mpemba found is now known as the Mpemba effect, and is the very counterintuitive idea that, under certain circumstances, a quantity of very hot/boiling liquid can freeze faster than an equal quantity of cold liquid!
How is this possible? The remarkable thing is that nobody really knows, even though the first observations were reported to the scientific community in 1969. The story of the discovery, and the consequent mystery, is worth a bit of exploration — and the Mpemba effect carries numerous important lessons about the nature and method of scientific discovery.
Mpemba made his accidental discovery in Tanzania in 1963, when he was only 13 years old and in secondary school. In spite of widespread disdain from his classmates, he surreptitiously continued experiments on the phenomenon until he had the good fortune in high school to interact with Professor Denis Osborne of the University College Dar es Salaam. Osborne was intrigued, carried out his own experiments, and in 1969 the two published a paper in the journal Physics Education.
This article is, in my opinion, one of the most remarkable in all of the history of physics. Aside from its title, “Cool?”, it is also unusual in being presented in two parts: Mpemba gives a first person account in his own words of his discovery in the first half, and Osborne picks up the story and describes the follow-up experiments in the second half. Mpemba’s own account is so charming and fascinating that it is worth quoting from liberally:
Here we have the beginnings of a classic story of science — an accidental discovery, scoffed at by the “establishment scientists”.
Mpemba might have given up at that point, but he encountered a friend who sold ice cream for a living, and that friend happened to mention that many vendors would use boiling water to make their ice cream! It was already common knowledge amongst them, apparently, that a boiling mixture could freeze quicker.
Here the high school teacher failed miserably — ridiculing a student is pretty much the worst thing one can do in a science classroom! Fortunately, Mpemba was not deterred:
Before he had this chance, however, Professor Osborne came to lecture on physics, giving Mpemba a valuable opportunity:
There are many remarkable points in this short passage. First of all, we see an admirable open-mindedness of Professor Osborne in his dealings with Mpemba, and that open-mindedness would quickly benefit them both. Conversely, we see a dangerous “groupthink” amongst Mpemba’s classmates regarding science, in which they are genuinely offended by Mpemba questioning the status quo. Mpemba shows great wisdom in his answer: “Theory differs from practical”. This is an important point for anyone studying physics: we like to create simplified models to explain nature, but those models often lose real-world aspects in the process of stripping them down.
Mpemba actually continued his experiments in a kitchen refrigerator, with the permission of kitchen staff, and convinced his classmates and the headmaster of his school of the accuracy of his findings.
At Dar es Salaam, Osborne was true to his word and looked into the phenomenon himself. As he notes in the continuation of the paper,
Osborne sets a great example for all physics educators! It can be difficult at times, but “No question should be ridiculed” would be a great part of a “Hippocratic oath” for teachers.
One other anecdote from Osborne’s account is worth quotation:
I leave it as an exercise to the reader to explain what is scientifically wrong with the technician’s attitude!
So what did Osborne’s research show? He placed a 100 cm³ beaker filled with 70 cm³ of water on a sheet of insulating foam in a freezer, and timed how long it took for the water to freeze. For temperatures up to 20 °C, the time was roughly proportional to the temperature above freezing, up to a maximum of 100 minutes at 20 °C. For higher temperatures, however, the time dropped dramatically, down to 40 minutes for 80 °C water!
Later experiments on the effect have been far less conclusive than Mpemba and Osborne’s. Some have seen similar results, while others have observed no effect at all! The appearance of the Mpemba effect apparently depends very strongly on the specific experimental circumstances, and is much harder to reproduce than the original paper would imply. A number of physicists seem skeptical that the effect truly exists at all!
With this in mind, however, it is worth noting that Mpemba’s observation was in fact noted earlier by others over the course of the past two thousand years. Around 350 B.C.E., Aristotle, in giving an explanation of hail in his book “Meteorology”¹, noted:
Others who argued for the existence of a Mpemba effect, under some circumstances, include Roger Bacon in the 13th century, and Francis Bacon and René Descartes in the 17th century. It was Mpemba and Osborne, however, who brought it to the attention of modern science.
Before delving into some of the explanations for the effect, it is important to explain why the natural objection to its existence is not necessarily applicable. A natural argument is as follows. ”Suppose we have two equal glasses of water, one at 100 °C and one at 30 °C. In order for the water at 100 °C to freeze, it must first cool to 30 °C. The time it takes to freeze is therefore the amount of time it takes to go from 100 to 30 plus the time it takes to go from 30 to zero. It therefore must take longer to freeze than the 30 °C water.”
The flaw in this argument is assuming that a body of water is only characterized by a single parameter: its temperature. In general, there are other characteristics of the water that could be changed due to its heating; for instance, the amount of gas in solution, the presence of other solutes, the presence of convection currents, gradients of the distribution of temperature in the container. Any, all, or none of these may in fact be the culprit, but it is important to realize that a body of water not in thermal equilibrium may have its behavior characterized by a number of different properties.
A number of hypotheses have been proposed as explanations, some based on the factors mentioned:
1. Convective heat transfer. When a liquid is heated, it can form convection currents that rapidly bring the hot liquid to the surface, where the heat is lost by evaporation:
Osborne noted that this convection will keep the top of the liquid hotter than the bottom, even when the temperature matches an initially cold liquid that doesn’t possess this convection cooling. This results in a faster rate of cooling that could, under the right circumstances, result in Mpemba’s observation.
2. Evaporation. A boiling or very hot liquid will lose some of its mass due to evaporation. With a lower mass, it will cool faster, possibly giving a “boost” to the Mpemba effect. Osborne already noted that evaporation alone could not account for all of the rate of cooling of the hot liquid.
3. Degassing. In a series of experiments in 1988, a Polish research group² reproduced the Mpemba effect and noted that the effect depended strongly on the amount of gas dissolved in the water. When the water was purged of air and carbon dioxide, the time to freeze became proportional to the starting temperature. The researchers suggested that the presence of gas was substantially slowing the rate of cooling, and that the heated water was purged of it.
4. Supercooling. In 1995, German scientist David Auerbach proposed³ that the Mpemba effect could be explained by supercooling, and performed experiments to back up the assertion.
Liquids begin to crystallize into a solid at the freezing point with the help of impurities around which the crystals can nucleate. In the absence of such impurities, the liquid can be cooled below its normal freezing point while remaining a liquid — it is “supercooled”. Auerbach suggests that the cold water will supercool to a lower temperature than the hot water, thus giving the hot water an “edge”. However, the reason that hot water has a higher supercooling temperature is unclear, and possibly related to the earlier noted effects.
5. Distribution of solutes. In 2009, an interesting explanation was proposed4 by J.I. Katz, who suggested that solutes present in the cold water slow the freezing process, as suggested earlier, but also that those solutes get driven from the freezing water into the as-yet unfrozen water, slowing the process further.
Other suggestions have been made, and other experiments have been done5, though no consensus seems to have been reached regarding the origins and generality of the effect. The confusion about the Mpemba effect is curiously reminiscent of the controversy around the “Archimedes death ray” controversy we have discussed earlier — the answer depends on the question being asked. Does hot water always freeze faster than cold? Almost certainly not! Are there some circumstances under which hot water freezes faster than cold? It seems likely, though nobody is sure exactly what those circumstances are. Does a negative result disprove the Mpemba effect, or was the experiment just done under the wrong circumstances? Nobody knows for certain.
The controversy no longer is of concern to Erasto Mpemba, however, who did not become a physicist himself but ended up studying at the College of African Wildlife Management. With his education, he eventually became the Principal Game Officer for the Tanzanian Ministry of Natural Resources and Tourism, working on wildlife management and conservation.
Erasto Mpemba, circa 1997 (source).
Regardless of the mystery, Mpemba’s discovery is a wonderful illustration of many important lessons in science: the significance of experiment over theory, the danger of clinging to preconceived notions, the difficulty in evaluating even seemingly simple real-world physics problems, and the importance of perseverance in the face of unreasoning denial. Furthermore, as far as I know the Mpemba effect is one of the only (if not the only) physical phenomena named after an African researcher! It is a wonderful and frustrating hint of how much intellectual potential lies untapped on the continent.
So what do I think of all the controversy? I’m not quite sure what to think! I’d love to test the Mpemba effect myself at home, but our freezer is packed to capacity6! Any enterprising experimenters out there want to give it a try? Describe your results in the comments!
(Thanks to Blake Stacey for providing one of the references used to write this post!)
¹ Aristotle, “Meteorology”, in The Complete Works of Aristotle (The revised Oxford translation), (J. Barnes, ed., Princeton University Press, 1984).
² Wojciechowski, Owczarek and Bednarz, “Freezing of aqueous solutions containing gases,” Cryst. Res. Technol. 23 (1988), 843-848.
³ D. Auerbach, “Supercooling and the Mpemba effect: When hot water freezes quicker than cold,” Am. J. Phys. 63 (1995), 882-885.
4 J.I. Katz, “When hot water freezes before cold,” Am. J. Phys. 77 (2009), 27-29.
5 Esposito, De Risi and Somma, “Mpemba effect and phase transitions in the adiabatic cooling of water before freezing,” Physica A 387 (2008), 757-763.
6 This is a clear indication of my current marital status. Back in my single days, the only thing you would find in my freezer would have been a pair of frozen pizzas!
E.B. Mpemba, & D.G. Osborne (1969). Cool? Physics Education, 4, 172-175
di (author unknown)
Shared by Mario
Se possedete una reflex Canon in grado di registrare video magari ve ne sarete già accorti o avrete sentito in giro che è meglio utilizzare un’impostazione degli ISO che sia multipla di 160.
Da prove che potete fare da soli, trovare online, o guardare dopo il salto è chiaro come ISO 640 abbiamo meno rumore persino di ISO 100 quando si tratta di registrare video. Per contro sembra che i multipli di 125 siano parecchio rumorosi. Questi risultati sono il risultato della sensibilità nativa del sensore utilizzato come avevamo approfondito la scorsa settimana con la spiegazione sul perché ad iso più bassi non corrispondano sempre immagini migliori.
La scala sembra essere questa: 160, 320, 640, 100, 200, 400, 800, 1250, 125, 250, 500, 1000, 1600, 2500, 2000, 3200, 4000, 5000, 6400. Quindi se avete luce a disposizione cercate di rimanere su iso 160.
Via | PhotographyBay