Wikipedia:Reference desk/Archives/Science/2014 December 14

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December 14

depression

Some people that suffers from clinical depression experience feelings of tiredness and feel the need to sleep much more than usual. a. How and why is feeling of tiredness and lethargy connected to depression? b. Is it better to fight off the feeling of tiredness and sleepiness or to go along with it (eg. to sleep much more than usual) ? — Preceding unsigned comment added by Syadeela (talk • contribs) 10:59, 14 December 2014 (UTC)

Define "better". Also, read about the life of Theodore Roosevelt and how he dealt with it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:06, 14 December 2014 (UTC)

I would remind everyone not to give medical advice in replying to this question. See WP:RD/G/M. To the OP, for advice about depression or any other medical condition, you should consult an appropriate professional. See also Clinical depression. Tevildo (talk) 11:10, 14 December 2014 (UTC)

Even aside from the medical advice issue, he's asking for an opinion as to what's "better". And if he's talking about himself, that's between him and his doctor to decide. No one here can tell him what "better" means for him. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:32, 14 December 2014 (UTC)

Question b is clearly out of our scope, but question a is legitimate. --65.94.50.4 (talk) 11:43, 14 December 2014 (UTC)

And to that, Tevildo has already posted the appropriate link. ←Baseball Bugs ^{What's up, Doc?} carrots→ 12:15, 14 December 2014 (UTC)

Jellyfish snacks

A jellyfish stings with its tentacles. It has a stomach of sorts. How does it put the food on its tentacles into the stomach? The prey might be metres away from the bell or "hubcap" part of the organism. Can it wind up the tentacle the way a spider winds up a web? If so, does the jellyfish eat its own |"foot" in the process? If so, can it grown a new tentacle?

Jellyfish tentacles are able to move quite well - perhaps YouTube is best for something like this, for example [1]. Appreciation of the motility of jellyfish can be hindered by the "diploblast" concept, since normally we think of the mesoderm as where muscles are found, but whether the intermediate layer is regarded as "true" mesoderm or mere mesoglea, it is present, as is the occurrence of striated muscle cells which in other organisms are unhesitatingly assigned to the mesoderm. On the other hand, there is good evidence these evolved convergently, based on ancient mechanisms of contraction that date back to single celled organisms; and effectively the mesodermal and endodermal gene networks are combined in the gastrodermis. That particular dispute in interpretation about mesoderm goes back to the mid-1800s; see [2]. But while there are many differences in the overall specification of the third layer and how the myocytes migrate to the mesoglea, we see from the video that these are real animals, not functionally incomplete. Wnt (talk) 14:37, 14 December 2014 (UTC)

Nature finds a way, with functional appendages on even the tiniest microbes. Consider the very small tail or Flagellum on a sperm cell. Or the extremely small leg-like fiber "tails" on the Bacteriophage. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:59, 14 December 2014 (UTC)

Tips for ACS Test?

I am preparing for the ACS (American Chemical Society) test for my final exam. Any suggestions/tips to aid in my studying or test taking? — Preceding unsigned comment added by Pinterc (talk • contribs) 15:03, 14 December 2014 (UTC)

Which ACS exam? There are several (general, organic, and physical, just to name a few). --OuroborosCobra (talk) 15:26, 14 December 2014 (UTC)

Physical Chemistry, focusing on Quantum Mechanics. — Preceding unsigned comment added by Pinterc (talk • contribs) 20:04, 14 December 2014 (UTC)

Post-translationational modification

What is "post-translationational modification"? My guess is that it's a typo; it's not mentioned here on Wikipedia except at Tryptophan tryptophylquinone, and a Google search <"post-translationational modification" -wikipedia> returns nothing except for Wikipedia mirrors that don't happen to mention their source. If it's a typo, what's the correct phrase, and do we have an article on it? Nyttend (talk) 15:19, 14 December 2014 (UTC)

Well, we have an article on something called posttranslational modification, so the instance you found would seem to be a case of dittography (mistakenly typing "ation" twice instead of once). Deor (talk) 15:27, 14 December 2014 (UTC)

Spelling fixed. Thanks! Nyttend (talk) 15:46, 14 December 2014 (UTC)

Bending stresses in a beam

When calculating bending stresses in a beam using Max Moment/section modulus, does it matter where that maximum moment occurs on the beam for this equation to work? I assume it doesn't. 194.66.246.66 (talk) 16:17, 14 December 2014 (UTC)

That's right, the BM diagram is independent of the location of the point of application of the BM. That's a general result from solid mechanics, not just structures. The point of application of a torque to a rigid body makes no odds, only its direction and magnitude matter. Greglocock (talk) 21:56, 14 December 2014 (UTC)

I'd quibble with that answer, Greg (either with the wording, or the idea behind it, I'm not sure). The bending stress distribution in a beam does depend on the point of application of the loads. But I'm not sure that's what 194.66.246.66 is asking about, as the question is a little unclear. 194.66.246.66, Mmax/S calculates the bending stress only at the location of maximum moment. The bending stress at any other point can be different than this, and is just the (moment at that point)/(section modulus at that point). Assuming its a uniform cross section so S is the same everywhere, the maximum bending stress in a beam is the bending stress at the point of maximum moment, but that's kind of axiomatic, so I wonder if you've asked the question you think you've asked. --Floquenbeam (talk) 22:11, 14 December 2014 (UTC)

Yes I see what you are getting at. I think I may have misinterpreted the OP's question, on re reading it. Greglocock (talk) 23:49, 14 December 2014 (UTC)

Battery charger: are they fail-safe?

If a battery charger (of a laptop, cell-phone, or something like that) is broken, could it start letting AC current through and basically damaging a device or the people who touch the output plug?--Noopolo (talk) 17:25, 14 December 2014 (UTC)

Broken mains-supply electrical devices should never be used. AndyTheGrump (talk) 17:34, 14 December 2014 (UTC)

What if the damage is not visible? Someone drops the charger and the case remains intact, although internally something has gone wrong?--Noopolo (talk) 17:42, 14 December 2014 (UTC)

See Double insulated, Underwriters Laboratories, and CE marking. _If_ the charger bears a suitable safety marking, _and_ that marking is genuine, it will have been tested to ensure this sort of thing doesn't happen. Not all chargers, particularly the cheaper ones, satisfy these conditions. Tevildo (talk) 17:48, 14 December 2014 (UTC)

Anecdotally, my wife and I have had 4 or 5 laptop power supplies fail over the course of 10 or 15 years. In every case, the failure caused the box to simply go dark (not functional with no apparent residual output). I do recall a couple of times where there was a sizzle sound that suggested internal electrical arcing, but no attached equipment was damaged. As with anything, your results may vary. And it should be noted that even if the device is designed to fail safely, that probably doesn't account for what could happen if the device is crushed or pierced with an external object. so definitely don't use it if the case has suffered significant damage. Dragons flight (talk) 18:14, 14 December 2014 (UTC)

Yes, the question is when you don't see any evident external damage. — Preceding unsigned comment added by Noopolo (talk • contribs) 18:27, 14 December 2014 (UTC)

The standard conversion method from AC to DC involves two steps:

a "step-down" transformer to reduce the voltage from mains level (220-240V in most of Europe, Australia and Asia, 110-120V in the US and most of the Western Hemisphere) to a lower voltage, and

a diode "bridge" to rectify the AC sine wave into a relatively "flat" voltage (which still has some "hum" due to the fact that the diodes basically invert the troughs of the incoming AC sine wave to be positive-going peaks; capacitors condition this into a much flatter direct current voltage).

A short circuit between the windings of the step-down transformer is so drastically unlikely assuming a well-made transformer that it would be the result not of drop damage to the adapter, but a manufacturing error.

I've actually seen, however, adapters not work after being dropped owing to damage to the diode rectifier bridge - generally a diode will either have broken (the older ones were made with glass housings, not plastic) or its connection to the other diodes in the bridge will have broken (bad solder joint, generally, is the problem there - a manufacturing error, again, but a much more common one than a cross-windings short in a transformer). That failure mode wouldn't pass mains current to the user or device - at worst, you'd get a reduced, half-wave secondary ("low" side of the transformer) AC voltage or no voltage at all. So, no real health threat or threat to the device being supplied.

Since these days, rectifier bridges are packaged as monolithic plastic blocks with the diodes sealed inside, the sort of damage you're talking about is much less likely.

But I've also seen and owned incredibly cheaply made power adapters for smart phones whose covers are only fastened by an friction fit between the halves of the covers, so that they've opened up after being dropped, exposing bare wires connected directly to the wall outlet. It's what happens when you outsource production of these things to factories run by cronies of Asian politicians, not engineers. loupgarous (talk) 18:32, 14 December 2014 (UTC)

One factual correction - most battery chargers these days won't have a transformer, they'll directly rectify the mains to high-voltage DC and use a switched-mode regulator to provide the output. Vfrickey's other points are good. Tevildo (talk) 19:40, 14 December 2014 (UTC)

Which type of material is the most friendly for human body in transplantation?

I mean which of the artificial materials is considered as the most friendly for human body in transplantation?149.78.233.188 (talk) 22:16, 14 December 2014 (UTC)

See Implant, Biocompatibility, and Biocompatible material, although, frankly, none of them is among our best articles. The biocompatibility article has the longest reference section, so that might be a good place to look for more detailed information. Tevildo (talk) 23:00, 14 December 2014 (UTC)

Humanities

close duplicate question, moved answer to science desk
The following discussion has been closed. Please do not modify it.
Are sciences and humanities really two opposite intellectual fields? — Preceding unsigned comment added by 70.231.229.215 (talk) 23:29, 14 December 2014 (UTC) This was asked early at the humanities desk, here. μηδείς (talk) 23:55, 14 December 2014 (UTC)