Wikipedia:Reference desk/Archives/Science/2012 July 17

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July 17

First law of Thermodynamics

Ive tried to calculate and got an answer for a seemingly simple problem yet I don't think it's correct. I have a cylindrical container with a radius of 0.05m which has a vertical frictionless piston of mass 10kg. Air is trapped inside. I want to know what the change in internal energy is when 300J of heat is transferred to the air causing te piston to move upwards by 0.2m. As this is a closed system, I'm assuming there's no mass flow and I'm quite confident that the change in heat energy is 300J so that leaves the change in work done. My initial guess was that this is 10kg x 0.2m but I don't think this is correct. Where an I going wrong? I'm sure that once I have the correct change in work done, I need too add this to the change in heat energy to get the change in internal energy. 82.132.249.136 (talk) 12:25, 17 July 2012 (UTC)

Because the air is doing work on the piston, the resulting change in its internal energy (without the heat) is negative. Meanwhile, check your units: mass times distance isn't energy. --Tardis (talk) 12:42, 17 July 2012 (UTC)

Thanks according to how I've interpreted what you've said, the answer should be -[(10x9.81)x0.1]+ 300. However, I still don't think this is correct. I think it may be to do with pressure but I'm not sure how to apply this. Thanks. 82.132.249.125 (talk) 15:18, 17 July 2012 (UTC)

I don't quite understand the question. Are you asking how much energy you need to add to the system in order to increase the heat energy of the air by 300J (which will be greater than 300J, due to some of the energy going to lifting the piston)? In which case, you were almost there, it's 300J+10kg*9.81m/s*0.2m. --Tango (talk) 20:40, 17 July 2012 (UTC)

Damselfly identification

I photographed a damselfly this morning in Liaoning Province, NE China Imgur Link. If someone can help me identify it, I will gladly add it to Commons and the related article (if one exists). I figure the white leggings should make for easy identification? Those are pretty unusual... The Masked Booby (talk) 15:28, 17 July 2012 (UTC)

There is a White-legged Damselfly (Platycnemis pennipes), already but it doesn't look like your beautiful capture. I think yours is more the Seriously White-legged Damselfly. Sorry I can't be more helpful. Richard Avery (talk) 15:49, 17 July 2012 (UTC)

Wow, based on the description that's definitely it! That article already has a few good photos but they might appreciate a clearer look at the namesake legs... Great job, Richard! The Masked Booby (talk) 21:51, 17 July 2012 (UTC)

I've gone ahead and added it to Commmons. Thanks again. The Masked Booby (talk) 01:11, 18 July 2012 (UTC)

Please check my maths

Hello. I've calculated the amount of energy spent by holding 9 kg in your arms for a minute. I calculated it as follows.

1. 9 kg * 9.8 (g) = 88.2 N.
2. 88.2 N = 88.2 W
3. 88.2 W = 88.2 J/s
4. 88.2 J/s = 5,292 J/min

But the result seems quite low to me. Am I getting it wrong somewhere? If so, where? Thanks. Leptictidium (mt) 15:34, 17 July 2012 (UTC)

You need to check the dimensions of energy. Holding a mass does not expend any energy. If you lifted the mass thru a certain height, you will have then added some potential energy to the mass. If this does not make it clear, imagine youself holding that 9kg for 1 hour. Your arms might then feel sore from having to maintain the force required, but force is not energy, and you'll still be breathing normally. Now, imagine yourself lifting 9 kg weights from the floor to a table top every second for an hour. Bet you can imagine yourself getting puffed out - heck you won't last an hour. That's because your muscles had to release some chemical energy, consuming oxygen to do so, and converting that chemical energy into mechanical energy to transfer as potential energy in the weights. Wickwack58.167.241.156 (talk) 15:47, 17 July 2012 (UTC)

Muscle usage is a strange duck, and analysis of it more rightly belongs with biology than "assume-a-spherical-cow" physics. While you're correct in that there is no additional energy imparted to the 9 kg mass by holding it in a fixed location, (and certainly a table or a mannequin will not expend any energy doing so), you're incorrect that holding something in your flexed arms doesn't expend any energy. If your muscles are in tension, you're expending energy to keep them that way, even if by a rough, entry-level-physics calculation you wouldn't be. The reason for this is that on a microscopic level your muscles have little bits that are constantly grabbing and releasing to maintain that tension, constantly burning chemical energy to do so. (Much like if you start and end with a mass at the same place on a table you don't impart any net energy to the mass, even if you expend much energy raising and lowering the mass repeatedly while the mass is temporarily hidden behind a curtain.) This is why it's often misleading to use muscle-based intuition to judge how much energy or force is required to do something. -- 71.217.5.199 (talk) 16:02, 17 July 2012 (UTC)

Well, yes, my original aim was to find out how many calories you burn by holding 9 kg for 1 minute.Leptictidium (mt) 16:34, 17 July 2012 (UTC)

Clue: the concept in question is isometric exercise. If you Google "isometric exercise calories" then you will find various numbers, but not much information on how they were obtained. I expect they were obtained empirically by measuring CO2 and H2O output from human subjects, since physics won't tell you the answer. As Wickwack said, step 2 is wrong and there's no way to make it right, since the two sides of the equation are related by an unknown function that describes how muscles work. --Heron (talk) 19:03, 17 July 2012 (UTC)

When googling as suggested by Heron, I found that the top sites returned are hardly definitive, but most confirm that isometics burn very little calories. While 71.217.5.199 is certainly correct in what he said, there's the matter of scale. Certainly just holding 9 kg, although making you muscles sore if you hold it long enough, will not alter your breathing rate one iota (try it and see!)- confirming that the additional energy consumed is just about negligible compared to the calories you burn anyway just by being alive, warm, and awake. Wickwack124.178.145.103 (talk) 02:46, 18 July 2012 (UTC)

The way the OP set out his question strongly suggests he's seeking to understand the physics and needs to understand the difference between force, power, and energy - round cow physics if you like, rather than trying to undertstand muscle efficiency and physiology. It's up to Leptictidium to come back to us and let us know which it is - then if he still needs some help, we can tailor it accordingly. Wickwack124.178.145.103 (talk) 02:46, 18 July 2012 (UTC)

Um the OP already came back and said "Well, yes, my original aim was to find out how many calories you burn by holding 9 kg for 1 minute" which suggests he's much more interested in the biological answer (not necessarily how the answer is derived) rather then trying to understand the physics. Nil Einne (talk) 14:31, 18 July 2012 (UTC)

If you say so. It's virtually identical to his original question - no new points in it. I think it neither confirms nor denies he has no understanding of the physics. It neither confirm nor denies he understands that muscles, being an imperfect machine, actually burn calories to exert a force, but those calories are very small in quantity and he wants to estimate them neverthless. Wickwack58.170.181.203 (talk) 15:32, 18 July 2012 (UTC)

What flower is this? (Seen in Chihuly Garden)

The poster of this photo on Flickr wants to know: what flower is it? (I've saved it as a .jpg file but don't know how to paste it here; advice welcome :-) -- Deborahjay (talk) 17:27, 17 July 2012 (UTC)

It looks to be an Eryngium, but not sure which species. Mikenorton (talk) 17:52, 17 July 2012 (UTC)

(edit conflict) It's a false purple thistle also known as "sea hollies", belonging to the genus Eryngium. That's likely to be an amethyst sea holly (Eryngium amethystinum), though like most cultivated ornamental plants that's difficult to tell as it could be a cultivar of another species.-- OBSIDIAN†SOUL 17:54, 17 July 2012 (UTC)

Satellites

In a stable orbit, acceleration remains tangent to velocity

If they are in orbit around Earth and are subject to its gravitational pull, why aren't they continuously accelerating at 9.81m/s²? Is there drag or a terminal velocity for satellites? Ankh.Morpork 21:09, 17 July 2012 (UTC)

Because in a stable orbit, the acceleration is always tangent to the velocity vector, so the magnitude of the velocity does not change. BigNate37_(T) 21:41, 17 July 2012 (UTC)

Great simulation! I forgot that a change in direction counts as an acceleration. Thanks. Ankh.Morpork 21:52, 17 July 2012 (UTC)

In a circular orbit, the acceleration is perpendicular to the velocity. —Tamfang (talk) 04:57, 18 July 2012 (UTC)

Well their velocity changes it's speed does not changeDja1979 (talk) 21:50, 17 July 2012 (UTC)

They accelerate at slightly less than 9.81m/s² because they are further away from the Earth, but they are constantly accelerating towards the Earth. They are moving sideways as well, though, which means they keep missing the Earth (the direction that is towards the Earth changes as they move, so gravity causes them to go in a circle). It's just like when you have a weight on a string and swing it around your head really fast. You're constantly pulling on the string, pulling the weight towards you, but if never hits you (until you stop spinning!). --Tango (talk) 21:51, 17 July 2012 (UTC)

Presumably the larger the diameter of the orbit, the faster a satellite travels if they all have similar orbital periods. Is this the case or can two satellites at different heights travel at the same speed, just the lower one has a quicker time of orbit? Ankh.Morpork 22:06, 17 July 2012 (UTC)

Intuitively, the further a satellite is from the mass it orbits, the weaker the force exerted by gravity. A weaker centrifugal force implies less centrifugal acceleration, so I would assert that velocity must also be lower to maintain the orbit. If I understood Kepler's Third Law better, I would explain the exact relationship between radius and speed of an orbit, but I don't and I can't. BigNate37_(T) 22:22, 17 July 2012 (UTC)

We have Orbital speed. It states low earth orbit satellites travel roughly 7-8km/s, the moon only travels 1km/s. Vespine (talk) 23:43, 17 July 2012 (UTC)

You are correct, objects in higher orbits move slower. Kepler's Third Law says that the square of the period is proportional to the cube of the semi-major axis, ie. $P^{2}\propto a^{3}\,$ where $P$ is the orbital period of the planet and $a$ is the semi-major axis of the orbit (for a circular orbit, that's just the radius). You might find it easier to think of it as $P\propto a^{\frac {3}{2}}\,$. Put another way, if you double the size of the orbit, you multiply the period by $2^{\frac {2}{3}}\approx 2.83$. --Tango (talk) 23:47, 17 July 2012 (UTC)

There's also the Newton's cannonball article, which describes how Issac Newton first determined that orbits are just objects falling in a circle (or eclipse). 62.56.63.195 (talk) 10:32, 19 July 2012 (UTC)

quaternary ammonium compounds

I recently read a study saying that 70% of all hospitals use quaternary ammonium compounds to clean the hospital beds. Personal experience in a hospital by myself has proven this to be correct. They use a dilute bleach solution to clean Most items in the Room except for the bed which they use a quandary ammonium compounds such as Lysol to clean. Why is this? My understanding is that quaternary ammonium compounds do not kill spores, such as Clostridium difficille or ringworm spores. Wouldn't it make sense to clean the bed with a product that kills such spores?--64.38.226.90 (talk) 21:16, 17 July 2012 (UTC)

Many disinfectants can cause skin irritation and are therefore not used when they are likely to come in contact with the patient's skin. Ankh.Morpork 21:32, 17 July 2012 (UTC)

why don't they just use the dilute bleach solution to clean the beds with?--64.38.226.81 (talk) 21:56, 17 July 2012 (UTC)

Bleach can cause metals to corrode, stainless steel can turn rusty with bleach as it contains chloride ion and an oxidiser. Graeme Bartlett (talk) 22:06, 18 July 2012 (UTC)

Is there a list of yeasts that infect Humans?

Is there a list of different types of yeast Human beings can become infected with Besides candida yeast? — Preceding unsigned comment added by 64.38.226.81 (talk) 21:55, 17 July 2012 (UTC)

You could start by reading Candida (fungus) and see where it leads you. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:25, 17 July 2012 (UTC)

Or just scroll to the bottom and click on Category:Mycosis-related cutaneous conditions. 75.166.200.250 (talk) 03:24, 18 July 2012 (UTC)

The Candida article is rather incomplete though. I understand that Candida growing inside the eyeball in otherwise healthy people is almost always from "cutting their junk" with lemon juice before injecting. Closet addicts are always shocked when their ophthalmologist diagnoses the condition... --BozMo talk 06:50, 18 July 2012 (UTC)

citation needed --TammyMoet (talk) 08:32, 18 July 2012 (UTC)

The first half dozen turned up by google scholar [1] all look reasonable. "Candida endophthalmitis after intravenous drug abuse" is notable enough to be worth its own article if anyone can take time out from Linux articles to write it. --BozMo talk 17:07, 18 July 2012 (UTC)