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181

I think that mere touching does not bring the surfaces close enough. The surface of a metal is not perfect usually. Maybe it has an oxide layer that resists any kind of reaction. If the metal is extremely pure and if you bring two pieces of it extremely close together, then they will join together. It's also called cold welding. For more information: What ...


127

They do, as Feynman said. If you have two copper pieces perfectly polished and you put them in contact, they will weld automatically (the copper atoms won't know what piece they belonged to). But in real life, oils, oxides and other impurities don't allow this process. Found it! Read Feynman's own words: If we try to get absolutely pure copper, if ...


53

Why do most metals appear silver in color, with gold being an exception? It is hardly surprising that the answer to this question relies heavily on quantum theory, but most people will be surprised to hear that the full answer brings relativistic considerations into the picture. So we are talking quantum relativistic effects. The quantum bit of the story ...


50

You asked two questions; I am going to give a very long answer. But the TL;DR is: As posed in the title: "Was Titanic's captain's decision correct?" - the answer is "probably yes" As posed lower down: Was there a chance of Titanic to escape from its tragic fate?:- the answer is "once they had hit the iceberg the way they did, no". The Titanic was ...


32

Instead of a circular hole, let's think of a square hole. You can get a square hole two ways, you can cut it out of a complete sheet, or you can get one by cutting a sheet into 9 little squares and throwing away the center one. Since the 8 outer squares all get bigger when heat it, the inner square (the hole) also has to get bigger: Same thing happens ...


26

David Zaslavski's answer is correct and complete. But I want to propose a different way to look at the problem. Think of the disc that was cut out, and imagine that you heat it too, exactly as you heat the plate. After heating, the disc will fit in exactly to the hole, just as if it was first heated and then cut out. Therefore, the hole will expand.


24

Good question! Assuming the disc is uniform and isotropic (the same in different directions), the hole will expand in the same ratio as the metal. You can see this because the thermal expansion equation $$\mathrm{d} L = L\alpha\mathrm{d}T$$ applies to all lengths associated with the metal, including the circumference of the hole, since the edge of the hole ...


21

I believe this is essentially what happens in gilding, owing to the special properties of gold (malleability and lack of corrosion). Extremely flat surfaces can get stuck together due to Van der Waals forces as well as air pressure. I once accidentially stuck two quartz optical windows together, and had a hell of a time separating them.


21

Two reasons: Oxides The roughness of the surface If the surface is rough, then the majority of the surface is touching the air gap between the two, not the opposite surface. A bond may form at the touching "peaks", but it will be weak compared to the rest of the metal because a very small fraction of the surface has actually bonded. In addition, metal ...


20

Because the electrical force on an electron is around 10^39 times that of gravity. Given the equivalence between gravitational and acceleration forces, you would have to shake it quite hard. Before you got to the point where an electron would drop out the entire material would disintegrate and all kinds of other phenomena would take precedence over you ...


16

D electrons in metal allow optical transitions in the visible regime. Visible light can be absorbed by elements, having unbound valence electrons in d shell. So Chemistry: optical d->s$^2$ transition Iron [Ar] 3d$^6$ 4s$^2$ Tin [Kr] 4d$^{10}$ 5s$^2$ 5p (full d shell) Aluminium [Ne] 3s$^2$ 3p$^1$ (is a special case: no d valence electrons, but Aluminium ...


15

Because there is an energy barrier between the metal and vacuum. Consider the ions in the metal as a uniformly distributed positive charge. Near the metal surface, the free electron wave function spread out a little into the vacuum, thus near the surface of metal the electric dipole forms with the electric field points to vacuum. Thus a gradual potential ...


15

I would suggest that the bottom penny does not have the same composition. A zinc penny is mentioned in the comments (BTW, what a tedious video) and zinc has a much lower melting point than copper (420 C vs 1085C). Copper plated zinc pennies were introduced in 1982. Given that it started to melt almost straight away, I think that is your answer. At about 10 ...


14

If the gold leaf is thin enough it will allow light to pass through it. Gold reflects the yellow end of the spectrum, which means the blue end of the spectrum is not reflected and can pass through the thin gold leaf. So if you view the gold leaf with transmitted light it will appear blue. In reflected light it still appears gold even when only 25nm thick - I ...


14

If you worked in an auto shop, you'd know the answer already. When an axle gets stuck in a ball bearing, one way to pull it out is to heat up the bearing with a welding torch. The whole bearing, including the hole in the middle, expands and allows you to pull the axle free.


14

I can't comment since I don't have the reputation for it, but I do have some relevant knowledge from my research in materials science. To add to what DumpsterDoofus said, it is very easy for two pieces of glass or polymer to bond if you clean them extremely well and ionize the surface. Look up plasma polymerisation. Moreover, you'd be surprised how much ...


11

Metals with perfectly clean surfaces WILL bond together just like you explained, but that isn't the case in real life because there is a thin layer of oxygen blocking the metal's surface. Much like how rust forms, thin layers of oxygen coat every metallic surface upon contact.


10

Materials (including some non-metals) that are strongly attracted to magnets are known as ferromagnetic. If you Google for this, or just search this site, you'll find lots of articles on this subject, thoughly surprisingly I don't think the question how does ferromagnetism arise has been asked before. Electrons have a magnetic moment so they interact with ...


8

Here is a table I made for you listing the elements with a density higher than 10 g/cm$^3$ and their approximate price per kg: I couldn't find any prices for Einsteinium or Actinium and some of the other prices might come from poor sources, but take it as a rough guide. Now you only have to figure out how much you need and your budgetetary constraints, ...


8

You've asked a lot of questions there, and I'll try to answer them one by one. First, though, I want to ask what post you're reading about metallicity in the core vs. out here in the 'burbs because I don't think it is correct. Obviously, for example, we exist and we're ~26,000 light-years (half-way out) from the galactic center and we have a fair amount of ...


7

The characteristic feature of a conducting medium is its presence of free electric charges on its surface, which are unbound. Unbound, free electrons do not have a restoring force and therefore, have no natural frequencies; however, they will always oscillate at the driving frequency. When an impinging EM wave oscillates these free electrons at optical ...


6

The densest elements (metals) in the Earth fall to the center, due to the gravitational force. The densest elements are radioactive, Earths core is radioactive, Uranium-238 is one radioactive species at the center of the earth. U-238 decays to Thorium via alpha emission, due to the electric force $$^{238}_{92}\text U_{146}\to\quad^{234}_{90}\text ...


6

The "metallicity" of a star simply means how much elements other than hydrogen or helium it contains. In this case, a "metal" means anything that's not on the first row of the periodic table of elements. Thus, a "metal-rich star" is one that contains lots of (for example) carbon, oxygen, nitrogen, etc. The term does not refer to metals in the strict ...


6

Yes, it is possible to magnetically levitate molten metal. This is not due to ferromagnetism however. As seen in the below references, the metal sample is placed within a tapered conducting coil, which carries alternating electric current in the ~400kilohertz range. This sets up a magnetic field gradient inside the coil and causes eddy currents in the ...


5

In principle yes, but the electrons will respond at around their natural frequency of oscillation. This is the plasma frequency and for most metals is around the frequency of visible light or about $10^{14}$ Hz. So the electrons will only be displaced for a few fractions of a picosecond. The analogy with sound is that the motion creates a sound wave that ...


5

An atomic species defined by its number of protons (usually denoted $Z$) and its number of neutrons (usually denoted $N$) is called a nuclide. For atomic species the number of electrons is the same as the number of protons (i.e. $Z$). You are right to assume that the nuclide of a single nuclide solid will typically determine its melting point and hardness ...


5

Conductors are defined by the freedom of some of the charges inside to more with little resistance. So, if there were a non-zero field, what happens? Answer: some of the free charges move until the field is again zero. You might be wondering if there are limits to this claim, but a introductory book of that sort is not worrying about extreme situations. ...


5

Your big problem is that metallic alloys are so numerous that it very hard to do in general. Your big advantage is that people mostly use things that are common, easy to get and cheap. This is especially true for something as mundane as a weight. Who's going to spend a lot of money sourcing something unusual? (Special case answer, when you need a lot of ...


5

Although the question has been partially answered, there is a superb reference on this topic which will certainly give you some of the deep, and not so deep insights needed to understand the answer to this question. http://pubs.acs.org/doi/abs/10.1021/ed068p110 Nevertheless, both the contraction of the s(1/2) orbitals predicted by the Dirac equation, and ...



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