Wikipedia:Reference desk/Archives/Science/2018 September 16

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September 16

Heating of solids

It's known that a temperature increase increases the speed of molecules/atoms. Since in solids molecules are tightly packed and tied by intermolecular forces of attraction, having little or no room to move, this means that they should experience very negligible to no temperature increase at all. So why do solids have different thermal conductivity and in general it's relatively easy to heat up a solid material? 212.180.235.46 (talk) 12:06, 16 September 2018 (UTC)

Actually solid water (Ice) is less dense than fluid water and altho that is rather unique that is enough to prevent setting a general rule of matter "packings". Also atoms and their state have almost no significance for thermal conductivity. The real significance lies in the molecular "static", aka their arrangement in 2 or 3 dimensions as crystal, net or string. The most thermal conductiv natural material for example is Diamond with 1000 watts per metre-kelvin (W·m−1·K−1) and the (known) most conductive artificial materials are Carbon nanotubes (3500 W·m−1·K−1) and (molecular 2-dimensional) Graphene (5300 W⋅m−1⋅K−1). So you see that thermal conductivity is much more complicated than just how atoms swing (aka move) more or less dependent on their temperature.

The third part of your question is dependent on the Volumetric heat capacity AND thermal conductivity, which again does not follow a simple rule and contrary is a very individual perk of materials in general. --Kharon (talk) 12:57, 16 September 2018 (UTC)

• It's known that a temperature increase increases the speed of molecules/atoms.

That's true as a statement, but it's not useful as a model unless you start to consider the laws which relate these two things. In a gas, kinetic theory does claim, "a temperature rise increases the speed", but it does so by showing the temperature to be proportional to the energy of each molecule, not its speed (i.e. the square of its speed, from the usual relation for kinetic energy).

It's similar in a solid. Temperature is proportional to the energy, not the speed. You might find phonons worth reading. The usual simple approximations begin by modelling the crystal lattice forces as atoms on springs, with a mathematical treatment as simple harmonic oscillators. This still works fine for a crystal or metal with a rigid lattice, but keep thinking about the energy for each atom (much of which is going into displacing that spring), not just their velocity, as if they were a free particle in a gas. Andy Dingley (talk) 13:20, 16 September 2018 (UTC)

Metal identity

Can the likely identity of the metal in these terminal blocks be inferred from the colour variations of the metal? 185.230.100.66 (talk) 15:58, 16 September 2018 (UTC)

• No. Those are screws you're looking at, not the blocks, and their colour is due to a coating on them, not the underlying metal.

The blocks inside are usually brass, but modern ones are becoming ever more cheaply made and might become steel at some point. They're usually tinned or plated to make their surface more corrosion-resistant and avoid poor contact problems. You can tell brass from steel easily with a magnet. It's quite likely that there are steel screws in a brass block.

These 'chocolate block' connectors are also becoming less widely used, in favour of connectors (such as the Wago range) with sprung connectors. They have a perennial reputation for poor contacts over time. Andy Dingley (talk) 16:57, 16 September 2018 (UTC)

This is the correct link to the Wago range of connectors. 194.174.73.80 (talk) 10:53, 17 September 2018 (UTC) Marco Pagliero Berlin

The surface appears to be Cadmium but terminal screws are almost never pure cadmium but rather cadmium plated. A magnet will tell you if there is steel underneath.

https://www.google.com/search?q=Cadmium+plating&num=100&tbm=isch will show you what cadmium plating looks like. --Guy Macon (talk) 17:55, 16 September 2018 (UTC)

It won't be cadmium. Cadmium is so toxic that even the Chinese have stopped plating things with it. Now it looks like cadmium, but so do many other modern plating and conversion coating processes. You can even get "super cadmium-looking" processes, for the car restoration crowd. Andy Dingley (talk) 18:12, 16 September 2018 (UTC)

Metallurgy is a very specialized Science and Metallurgists have a masters degree. They are also the highest paid specialists in material science, i learned by chance a long time ago. So you may have to pay the 500$ bill for the correct professional answer to your Question! Sorry, way above my pay grade. --Kharon (talk) 22:17, 16 September 2018 (UTC)

Then why do you keep answering questions here, if you have nothing to contribute? Andy Dingley (talk) 23:44, 16 September 2018 (UTC)

Makes sense to me. It keeps people from too easily believing answers posted by other people who are guessing. --76.69.47.228 (talk) 01:54, 17 September 2018 (UTC)

I'm a metallurgist and I don't have a masters degree. Neither do any of my colleagues. I'm reasonably well paid, but I'm not the highest paid person around either. 139.194.65.208 (talk) 22:19, 17 September 2018 (UTC)

Obesity

Is it genetic or determined by what you eat?86.8.203.189 (talk) 18:31, 16 September 2018 (UTC)

Have you considered that both factors may be in play, and that there may be others as well, for example gut flora? Matters of biology are rarely simple. {The poster formerly known as 87.81.230.195} 90.208.172.36 (talk) 18:54, 16 September 2018 (UTC)

It can't be only genetics because anyone can starve to death and 3 pounds of butter has 12,250 calories. Sagittarian Milky Way (talk) 18:56, 16 September 2018 (UTC)

The answer is "Yes". ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:07, 16 September 2018 (UTC)

Some people cannot get fat no matter how much they eat. 86.133.58.126 (talk) 19:12, 16 September 2018 (UTC)

Jack Sprat, for example. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:15, 16 September 2018 (UTC)

The nursery rhyme merely states that Jack couldn't eat fat, not that he couldn't get fat. It is certainly possible to get fat on a low-fat diet: the body is quite good at converting carbohydrates to fats. Wymspen (talk) 10:09, 19 September 2018 (UTC)

See Obesity#Causes. PrimeHunter (talk) 23:02, 16 September 2018 (UTC)

The more fat, refined grains and sugar you eat the less calories you can eat before you'll put on weight. This is because unlike whole grains, these food sources are empty calories that don't contain the nutrients the body needs for metabolism. The human body is a self-maintaining factory that cannot run on just fuel, it uses hundreds of thousands of different types of enzymes that need to be produced from the nutrients we eat. Count Iblis (talk) 16:24, 19 September 2018 (UTC)

Is the negative charge - the absence of the positive charge?

We used to say, based on the science - if I'm not mistaken, that the darkness is the absence of the light, because there're no photons at the medium of the darkness. True? So my question is about physics or chemical physics, is the negative charge is the absence of the positive charge? If it is, what is the explanation for that? (for example about light we say that there're are no photons but here in negative charge what is the actual absence in the micro-level?19:52, 16 September 2018 (UTC) — Preceding unsigned comment added by 93.126.116.89 (talk)

no, no, no. The attractive force is proportional to the negative charge times the positive charge, which can't be if the negative is just a lack of positive. Wnt (talk) 21:20, 16 September 2018 (UTC)

• 'Absence of charge' generally implies no charge. Only in some cases (where a negative charge carrier is expected) would it really be seen as implying a positive charge.

Charge is different to light (or mass). There are two forms of it, so there's a difference between 'negative' and 'absent' which doesn't arise for light.

The most common charge carrier (at least in discussion and description) is the negative electron (as electrons can travel freely in metals). But the 'natural' state of an atom is to be neutral (the number of positive and negative charges are equal). If a charge is removed, then yes, that's equivalent to gaining the opposing charge. So atoms in ionic bonding (how many molecules are formed) may easily lose or gain an electron (or two) to an adjoining atom; one becomes positively charged (an ion), one negatively charged. It's the electrons which move between atoms, not their protons, as the electrons are both less tightly bound and also lighter.

In semiconductors, both positive and negative charges are important. The idea of 'holes' is an importat one. It's just as important to account for a removed electron as it is for an excess electron. But this is the removal of an electron which was there, not merely the absence of one. Andy Dingley (talk) 21:55, 16 September 2018 (UTC)

Good answer. Excellent article. DroneB (talk) 16:03, 17 September 2018 (UTC)

Donated blood not clotting

Why does donated blood not clot? I thought, no air=no clot, but blood can clot inside our veins. Mix up some anti-coagulant into it? But, then wouldn't that be pretty inconvenient for recipients like accident victims and hemophiliacs? — Preceding unsigned comment added by 31.177.99.255 (talk) 22:14, 16 September 2018 (UTC)

I googled "why doesn't donated blood clot", and here's an answer:[1] ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:46, 16 September 2018 (UTC)

Yes, I saw that page too, when I googled it. It does not touch the topics of my sub-questions. --31.177.99.255 (talk) 22:54, 16 September 2018 (UTC)

Just enough citrate is added to the blood in order to chelate most of the calcium. Calcium (specifically Ca²⁺) is required for coagulation (factor 4 in the coagulation cascade), so without free Ca²⁺ the blood will not clot in the bag. When blood is then transfused, because there is little unbound citrate in the bag, there will be little effect on the plasma calcium concentration, and hence coagulation in the body is possible as normal. The kidneys can then effectively clear the citrate from the body, although in the interim it may have other biological effects. Klbrain (talk) 00:34, 17 September 2018 (UTC)

A reference for the actual amounts of citrate vs calcium in a donated unit would be important here. How much of the calcium is actually bound and how much free citrate remains? The plasma calcium concentration is also affected by dilution of the added no-free-calcium blood in the recipient. A donated unit is about 0.5 L and a person has about 5 L total volume, so if we are talking about someone suffering from blood loss, mixing one unit of whole blood with no Ca++ into "well less than 5 L" of whole blood is easily a 10% drop. If one really needs blood, it's obviously an acute situation and side effects or interactions with other conditions can be addressed later. Headed slightly off the OP's question, the citrate is also a potential problem for the donor in apheresis, where the blood is withdrawn, citrate added, certain components of interest removed, and the rest returned to the donor. That means dumping a bunch of calcium-depleted blood and possibly excess citrate into the donor for a several-hour period. pmid:26607494 is an interesting read. DMacks (talk) 03:13, 17 September 2018 (UTC)

Not a direct answer, but the citations in the "trauma" section of this review [2] presumably mention it at some point? That talks about standard practices in the management of bleeding during surgery. In brief, if someone comes in needing a massive blood transfusion after an accident, coagulation is very carefully monitored and a variety of infusions are available to keep the blood behaving normally. Someguy1221 (talk) 03:25, 17 September 2018 (UTC)

The American Red Cross notes[3] that there actually can be a possibly clinically significant amount of anticoagulant infused, although it is indeed metabolized promptly assuming normal liver function and circulation. And it concurs with my thought that that the transient drop in free Ca++ can be an acute problem. And it notes that the transient presence of citric acid and then the presence of its metabolic products can also be an acute problem (blood pH and Ca++-level effects) for those who have certain circulatory or metabolic problems. DMacks (talk) 03:29, 17 September 2018 (UTC)

For educational value: blood does not clot inside our veins, except when something's wrong, and if it does it can cause serious problems and needs treatment. Blood is only "supposed" to clot when it leaves the blood vessels, which only occurs when trauma damages them. --47.146.63.87 (talk) 08:23, 21 September 2018 (UTC)