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November 26

Current in two circuits

Hello. I have a question from McMaster University's Physics Contest, which can be found here. For that last question, #10, I assumed that the current follows conventional current and that the "electrons" from the positive side of the left battery would flow in a counterclockwise direction through the bottom light bulb, up to the top light bulb, and to the negative side of the battery. Thus, the resistance in the first circuit would be greater than the second, and since the voltage is the same for both circuits, since V=IR, if resistance (two bulbs vs. one) increases, the current decreases, so I2 should be greater than I1. The answers, however, state that no current flows through the top light bulb at all! Can someone shed some light on this? Thanks! 74.15.5.210 (talk) 04:28, 26 November 2014 (UTC)[reply]

The classic way to solve these problems is to calculate the voltage at each junction, and the extrapolate current using Ohm's law. When a wire splits, voltage is identical. Let's say the batteries are at 5 V. If you look at that top bulb, that means the voltage on one side of that bulb = +5V, and the voltage on the other side of that bulb = +5V. If the voltage on one side = voltage on the other side, there is no potential difference, no EMF, so no current. Now, in BOTH circuits, the voltage at the top juncture = +5V, while the voltage at the bottom of each circuit is 0V. That's a voltage difference, so that means the bulb has a current. Since that voltage difference is the same in both pictures, the two currents have to be identical as long as they have identical bulbs. Don't try to figure out how electrons "move" or "flow". Just find voltage across every component and use ohm's law to find the current. --Jayron32 04:46, 26 November 2014 (UTC)[reply]

Assuming they are perfect batteries, the bottom picture is very straight forward and I₂ = V/R. Looking at the top picture, the same voltage is applied to the vertical light bulb by the perfect source on the right so I₁ = V/R. This means the currents are equal. When working through these types of problems, there are forced voltages with the battery. What's also identifiable is that the voltage across the second (horizontal) lightbulb in Figure 1 is 0 so this means the current through that bulb is 0. The difference is that in the bottom circuit, both batteries split the load while in the top circuit, only the right battery supplies current (assuming ideal sources). The answer is C). I1=I2≠0 --DHeyward (talk) 08:00, 26 November 2014 (UTC)[reply]

Confirm: C is correct. To understand it: If the left battery nears empty, the right will particially backup, but only up to a quarter of energy on each bulb. The lower bulb well emit less light by time, the upper bulb more until euqual. It is a quarter on energy on each bulb. The bulbs are in line. This halves the resistive load (doubles the ohms Ω walue) on the right battery causes half current. Half voltage and half current is a quarter of energy per bulb, see Ohm's law. --Hans Haase (talk) 10:42, 26 November 2014 (UTC)[reply]

Work hardening

I've read your article on work hardening and dislocations etc but how do you see this on a stress visualisation animation which changes as load is applied to a material over time. The visualisation is limited to a maximum of the materials yield stress. 194.66.246.125 (talk) 10:34, 26 November 2014 (UTC)[reply]

Diagramms are always limited and these are not much more that an example. Might even been put in there so the article isnt just text ;) Material science is a highly underestimaten and very complicate field. This starts with the fact that "stress" is not an uniform unit or measurement (shear-, bend-, swing-, notch impact-, torrision-, tension-, weight-"stress" etc..) and thus you would have to add a lot more diagramms. Additionally this all does change with each material, and even with each alloy. So what example would you like more? --Kharon (talk) 15:41, 26 November 2014 (UTC)[reply]

Rescue dog awareness of selection and rejection?

Are dogs in rescue centres likely to realise they're being selected and rejected? Are they trying to appeal to be chosen and disappointed when someone walks past or are they likely to have no idea at all what's going on? --78.148.108.62 (talk) 13:01, 26 November 2014 (UTC)[reply]

Im not an expert but dogs are mostly focused on their relation to the "leader of the pack". In the case of rescue duty the woman or man they always work with. From that point of view there is likely no difference between a "rescue team" and a "family". But to be certain we would have to ask the dogs. --Kharon (talk) 15:50, 26 November 2014 (UTC)[reply]

To clarify, you mean what our article describes as rescue dogs, not search and rescue dogs, right ?

I doubt that the dogs in a the pound know they are facing the gas if they don't get adopted. However, dogs have been bred for thousands of years to appeal to humans, so them acting friendly is the expected behavior, with exceptions for some that are in the pound because they are "defective" in that regard, or perhaps were abused to the point where they now fear all humans.

If the dog pound was full of wolves, instead, which are genetically almost identical to dogs, save the thousands of years of breeding for traits humans find desirable, then very few of them would act in a friendly manner (and those few would be the mutations). StuRat (talk) 16:07, 26 November 2014 (UTC)[reply]

Have you ever walked through a place where you can adopt a dog? I think Kharon's right that they will likely already be a bit focused on their caretakers, and Stu's right that they have no concept of death if they are not chosen soon enough. But-- they still are often bored and restless, as the caretakers don't have the manpower to give all the dogs all the time and care they need to have stimulating life. So, some dogs at shelters will indeed perk up and wag their tails, lick hands, or whine in pleasure as people come by. It's not so much that they have a concept of being chosen but I think many of them just want to make friends. If they are chosen and led out by new people, they look extremely happy. Seriously, go check out a dog adoption center. You aren't obligated to take any home, and you'll learn a lot about the variety of dog behavior in that context. SemanticMantis (talk) 18:22, 26 November 2014 (UTC)[reply]

I pet the cats at the local humane society whenever I've nothing better to do. (At first I went for at least a couple of hours every week; then I got more work and a pair of cats at home.) (Other volunteers take the dogs for daily exercise.) Most of the dogs seem more alarmed than pleased to see me; maybe they can tell I'm a vampire. —Tamfang (talk) 06:24, 27 November 2014 (UTC)[reply]

What are the differences between Thimmamma Marrimanu, Pando, Armillaria solidipes in Oregon, Posidonia oceanica in Mediterranean Sea?

I was reading about the largest organisms and I don't understand how the plants (like those 4 mentioned above) differ from each other. I would like an explanation for a layman as I'm not familiar with the subject.

For example, how is the way the banyan "spreads"/grows different from the aspen? They both seem about the same to me.

Are these plants only considered 1 single large plant because they are physically connected? Can 2 physically disconnected parts of the plant be reconnected again?

Thanks.

138.75.150.164 (talk) 15:29, 26 November 2014 (UTC)[reply]

An organism can be defined, very broadly, as a contiguous living system. Of course, in reality this definition is almost invariably restricted to only include contiguous living systems that possess the same or essentially the same genetic material throughout their extent, and/or to only include contiguous living systems where all parts are not just in contact with each-other but actively exchange some resources between them. That is, for example, a flock of sheep huddled together is not a single organism. A pine grove is not a single organism either, as the pines merely grow next to each-other. By contrast, an aspen grove (a clonal colony) constitutes a single organism when all individual trees are interconnected at the roots and can exchange water and chemicals with each other via the interconnected root system. Plants can form interconnected clonal colonies by various means. For example, some plants (like many grasses) send out roots or rhizomes that sprout new plants when they reach the surface. Some plants (like strawberry) send out runners that root at certain intervals. Some plants (like raspberry) produce roots when their branches touch the ground. Some plants (like banyan) send aerial roots down from its branches, and some of these roots with time become secondary trunks. Fungi ("mushrooms"), too, form interconnected clonal colonies by sending out their hyphae (mycelium) and producing new fruiting bodies from them; this often looks like a circle of mushrooms, and can be quite large. Does this answer your question? Please let us know. All the best, --Dr Dima (talk) 19:08, 26 November 2014 (UTC)[reply]

To your question on whether 2 physically disconnected parts of the plant be reconnected again, the answer is yes. You can do this experiment yourself, it is called grafting. --Dr Dima (talk) 19:13, 26 November 2014 (UTC)[reply]

I think you've explained it pretty well. OP and others may like to read up on ramets and genets, perhaps rhizomes and stolons. The general concept of plants spreading/growing through various non-sexual means is vegetative reproduction - some of those methods can leave the "parent" and "clonal offspring" connected as one organism, some end up separated. In the most confusing cases, whether the clone becomes a distinct organism is often a matter of chance, e.g. in the well-known spider plant. (I also took the liberty of linking the terms in the header for convenience.) SemanticMantis (talk) 20:17, 26 November 2014 (UTC)[reply]

Yes it answers my question. Thanks for the help! I have a few related questions: Is it theoretically possible for humans to cultivate/grow an artificial plant/fungi that is even larger than what nature has naturally made? (given sufficient land space) If it were discovered that there was a disconnect/break in the underground roots/mycelium/etc. of the organisms mentioned above (thereby reducing its total size), would grafting them back together make them one organism again? Thanks. — Preceding unsigned comment added by 138.75.128.131 (talk) 15:56, 27 November 2014 (UTC)[reply]

Yes, it is possible to grow such an organism artificailly, but this will take a lot of time and space. Yes, once disconnected from each-other, the root systems of two trees (from two parts of the clonal colony) may reconnect naturally when the roots are allowed to grow in contact with each-other. It is also possible to graft roots artificially; you may, for example, take a look at the techniques used for root grafting in bonsai plants. --Dr Dima (talk) 00:52, 2 December 2014 (UTC)[reply]

Von mises

Does von Mises yield criterion take into account material shape or does it only consider the fact that a particular material with a material shape has a yield stress which is a factor of sqrt3 more than that of the shape in pure shear? — Preceding unsigned comment added by 194.66.246.125 (talk • contribs) 16:05, 26 November 2014‎

In other words how do you mathematically predict the yield stress of a structure if you only know the material yield stress, Young's modulus and poissons ratio. Am I right in assuming this isn't enough information to predict this? What else do I need?

The von Mises criterion is useful when you have more than one stress acting on a point. For example, if you have tensile stresses acting along both the x and y axes, you might imagine that even with each stress being lower than the yield stress, the 2 acting in combination might be enough to cause yielding. The von Mises criterion gives an estimate of how big the 2 stresses can be before yielding occurs. Before applying the von Mises criterion, you need to find out what stresses are acting on the point in question, and this will require knowledge of the loading and geometry (unless, for example, the stresses are given to you in a class problem).

If you want to find the magnitude of a pure shear stress that will cause yielding, you set sigmav = yield stress (sigmay) and plug the shear stress (e.g. sigma12) into the von Mises criterion (the last equation in the mathematical formulation section of the von Mises article). This gives sigmay = sqrt(3)*sigma12. So the factor of sqrt(3) relating shear stress to the yield stress (tensile) is one case of the von Mises criterion.--Wikimedes (talk) 06:49, 27 November 2014 (UTC)[reply]

I'm actually trying to find a way to validate a finite element model of the von mises stresses on a structure with a uniform load applied. I'm trying to do this with theoretical values but I don't know how to produce theoretical values using what I have. 194.66.246.26 (talk) 17:55, 27 November 2014 (UTC)[reply]

100 billion coincidence

There are 100-140 billion stars in the Milky Way, probably more than 170 galaxies in the Universe, and about 100 billion neurons in the human body. The similarity of these numbers astonished me. What a coincidence that each of the stars in the galaxy corresponds to a galaxy. It also led me to think about the Anthropic_principle, where the constants of nature are so finely tuned that (apparently) to shift even one would make human life impossible. The fact that there are 100 billion neurons in the human body doesn’t fit into that equation, though, since the number of nerve cells is not a prerequisite to life but rather a part of life. Still, this is a very interesting coincidence. Not a question, I just couldn’t resist putting such an interesting factoid before you to see what you think. --Halcatalyst (talk) 20:12, 26 November 2014 (UTC)[reply]

As factoids go, I'd have difficulty accepting that it was 'interesting'. There are many (approximate) numbers one can find in relation to the human body, from the number of heads (one) to the number of electrons (~~which I will leave someone else to figure out...~~ 2.3*10²⁸ [1]). That one of these numbers is somewhere near the number of stars in the Milky Way is accordingly entirely unsurprising. AndyTheGrump (talk) 20:20, 26 November 2014 (UTC)[reply]

Once again, the user per his user contribution s is basically a single purpose account; the purpose being to pose ref desk questions. So long as the OP is suggesting there might be as many as 17o galaxies in the universe, I think a private chuckle is perhaps the best response. μηδείς (talk) 20:23, 26 November 2014 (UTC)[reply]

What is with this weird criticism that pops up again and again? There's nothing wrong with using an account to mostly post questions here. To insinuate that that makes someone unwelcome is doing a disservice to the ref desks. I'd rather answer "asking-questions-only" than IPs, if only so I can more easily keep track of who I'm talking to. SemanticMantis (talk) 20:27, 26 November 2014 (UTC)[reply]

On top of that, a registered user does not run the risk of exposing their IP. The only policy criterion they seem to flunk is "not here to build an encyclopedia" but this one is a mild example. I'd be more upset about users who insert nonsense into articles.

You can't say that "probably more than 170 galaxies in the Universe" is wrong, either. That probability is a damn high one if you ask me. 217.255.180.86 (talk) 07:29, 1 December 2014 (UTC)[reply]

That there are roughly as many galaxies as stars in the milky way is at least an easily remembered heuristic. It's unclear on whether these "mean" anything, but you may wish to read up on pareidolia. Our pattern recognition skills were (and are) crucial to our survival, so it's not too surprising to see that ability produce many false positives. SemanticMantis (talk) 20:27, 26 November 2014 (UTC)[reply]

Don't you think it's interesting that the scale is from human to (what could be called) the basic unit of the Universe to the Universe itself? Another interesting factoid is that, starting from one meter (human size), there are about the same number of powers of ten going up to the size of the Universe and down to the size of a neutron. --Halcatalyst (talk) 20:37, 26 November 2014 (UTC)[reply]

That could help explain how the Greeks were able to invent the atom. On the other hand, how the article Anthropic principle's size is related to that of Sapience remains undetermined. --Askedonty (talk) 20:49, 26 November 2014 (UTC)[reply]

The OP may find the Dirac large numbers hypothesis interesting. Tevildo (talk) 20:52, 26 November 2014 (UTC)[reply]

+++ I have never posed a question to the reference desk (though I'm aware the practice stands somewhere between discouraged and forbidden, depending on your taste.). Visitors might appreciate a little courtesy from the reference librarians.

I was looking for intelligent answers, which I generally get to questions I put to Wikipedia. --Halcatalyst (talk) 20:59, 26 November 2014 (UTC)[reply]

If Dirac relates "ratios of size scales in the Universe to that of force scales," I guess I can do something of the sort too. BTW, I was just interested in thoughts, not looking for meaning. Thanks for those who have contributed thus far. --Halcatalyst (talk)

You said it yourself: it's a coincidence, nothing more or less. To read about crackpots who read too much meaning into coincidences, see numerology. --Bowlhover (talk) 02:05, 27 November 2014 (UTC)[reply]

You may want to check out Benford's law. The apparently disproportionate quantity of ones as the leading digit when measuring items is a much-studied phenomenon and not at all related to numerology or pseudoscience. Matt Deres (talk) 19:33, 27 November 2014 (UTC)[reply]

Well the OBVIOUS conclusion is that it's not a coincidence but the universe is in fact a giant brain!! And of course there's only one brain it could be: It's the brain of GOD! Of course I don't I don't believe any such thing, but just demonstrating how easy it is to dive down the rabbit hole. Vespine (talk) 03:50, 28 November 2014 (UTC)[reply]

so my fart could be God's brain fart?!66.87.116.95 (talk) 16:09, 28 November 2014 (UTC)[reply]

Physicist Richard Feynman has a great quote relevant to such coincidences: "...on the way to the lecture, and I came in through the parking lot. And you won't believe what happened. I saw a car with the license plate ARW 35W. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing!"

The joke, of course, is that the probability of this coincidence is in fact very low if it were predicted (pre-observation); but post-observation it ceases to be a probabilistic situation at all.

If the rough approximation of the order of magnitude of the count of stars did not match, would you have noticed or remarked on its likelihood at all? What about all the other irrelevant cases where some value is approximately equal to a few hundred billion, plus or minus a couple orders of magnitude? Are these coincidences as well? Can you meaningfully compute the probability of such coincidental observations? Nimur (talk) 19:25, 28 November 2014 (UTC)[reply]

Freezing temperature for water

When the temperature hits 32 degrees Fahrenheit (or lower), the liquid form of water turns to ice (i.e., it freezes). When I look at my "weather app" on my computer, it always says something like this (for example): "Today, the temperature is 53 degrees, but it feels like 43 degrees." And I believe it is the wind chill factor that makes the temperature "feel" colder than it actually is. So, let's say that on some given day, my weather app says: "Today, the temperature is 38 degrees, but it feels like 28 degrees." What happens to water in that case? Does it not freeze because the temperature is higher than 32 degrees? Or does it freeze because the temperature "feels" like it's less than 32 degrees? Thanks. Joseph A. Spadaro (talk) 23:06, 26 November 2014 (UTC)[reply]

Related question: Wikipedia has an article for boiling point, but not for freezing point? Joseph A. Spadaro (talk) 23:11, 26 November 2014 (UTC)[reply]

The water would still stay liquid. It feels like 43 degrees to you because you are hot and loose heat quickly when the wind is constantly moving cold air past you. Humans can't actually sense temperature; we sense how quickly heat is being transfered (which is why a block of metal feels colder than a book at the same temperature). To water near 38 degrees, the air isn't that much colder, so it looses heat much more slowly than you (although the wind still makes it cool faster). As for the second question, it's because melting point and freezing point are the same thing. I hope this helps! --T H F S W (T · C · E) 23:44, 26 November 2014 (UTC)[reply]

No offense, but your answer actually confused me even more. Joseph A. Spadaro (talk) 00:58, 27 November 2014 (UTC)[reply]

Let me try to unconfuse you: 1) Wind chill factor has to do with how quickly heat transfer happens from your skin to the air. The reason why you feel colder when the air is moving is that the moving air carries heat away from your body faster than still air would. Thus, if the weather says "38 degrees F, with a wind chill factor of 28 degrees F", what that means is that your body will lose heat as though it were a windless day at 28 F, though the actual air temperature is 38. The difference is not in actual temperature, the difference is in how fast two bodies of different temperatures equilibrate. When the air moves over the warmer body, it will cool off faster. But it can never cool off to a temperature lower than the ambient temperature. Thus, if the air is at 38 F, the water will never freeze. If the water is at, say, 50 F, it will get down to 38 faster if there is wind than if the air is still, but it can never drop to below the ambient temperature, which is why it will never freeze, no matter WHAT the wind chill factor is. 2) Freezing point and melting point is the exact same thing: the only difference is the direction the temperature is moving. If the temperature is going up, we call the temperature the "melting point". If the temperature is going down, we call it the "freezing point". But the two are identical. --Jayron32 02:17, 27 November 2014 (UTC)[reply]

Water can indeed become colder than the ambient temperature, because it loses heat through the latent heat of evaporation and not just by sensible heat. This can be enough to cause the water to freeze when ambient air temperature is above freezing. Peggy LeMone has a nice little article here. Short Brigade Harvester Boris (talk) 03:22, 27 November 2014 (UTC)[reply]

Yes, Jayron's usually excellent answers have a glitch here because a wet cloth, or a windscreen, can freeze in a cool breeze that is above 32 F. See Wet-bulb temperature for the technicalities. Dbfirs 13:24, 27 November 2014 (UTC)[reply]

There's no freezing "point" of water unless one looks only at temperature, without regard to the energy of the system. Water at 32F can be liquid, solid or slush. Water requires more energy to be removed from the system to go from liquid at 32F to solid at 32F than it does to drop a degree from 33F to 32F liquid or from 32F solid to 31F. See water and enthalpy of fusion. μηδείς (talk) 22:56, 27 November 2014 (UTC)[reply]

Thanks. All the answers were very helpful. Thank you. Joseph A. Spadaro (talk) 16:54, 28 November 2014 (UTC)[reply]