# Tag Info

151

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 ...

112

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 ...

33

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 ...

21

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 ...

21

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.

21

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 ...

17

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.

15

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. I addition, metal ...

12

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 ...

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 ...

8

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 ...

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

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, ...

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 ...

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 ...

5

X-ray diffraction is certainly possible for metals! Most X-ray diffraction experiments are carried out with hard X-rays (photon energies of about 4keV or higher); among the most frequently used photon energies in laboratories are 8.04keV (Cu K$\alpha$ fluorescense) and 18keV (Mo K$\alpha$), for example. There are mainly two reasons for choosing hard X-rays: ...

4

http://www.physicsforums.com/archive/index.php/t-37701.html says "Most of the strength of a cylinder comes from the outer portions. I think the contribution goes like the cube of the radial position. So, if you took a solid rod and drilled out a half the volume from the center, you do not lose half the strength. Strength to weight ratio is ...

4

It is well-known that x-rays are blocked by metal. [Ref: Kid's Science] Obviously the doctor wants to look at your internal organs, unobscured by a fuzzy outline of your house keys and pendant. So, the sign is requesting that you removing metal items from the external parts of your body, to allow visibility to the internal parts. (MRI is a totally ...

4

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. ...

4

Physically what is happening is this: When you touch the positively charged source to the conductor (the metal sphere), electrons leave the conductor through the point of contact. This leaves the point of contact on the conductor with a large deficit of electrons, and thus the point has a positive charge density. The positive charge density produces ...

4

By metal-production, I think you're mentioning nuclear fusion (like those that take place in hydrogen bombs and core of stars) by which elements can be produced. If you look at the Wiki article I've linked, you can see a quote: to be about the same temperature as the surface of the Sun - approximately 5700 K I don't think such a low (relative to sun) ...

3

The Fermi level is supposed to be the highest occupied state at zero temperature. For fermions at zero temperature, they fill up these states with multiplicity one starting with the ground state up to the Fermi level. This is the lowest energy configuration that abides Pauli exclusion. At positive temperature (or quantum mechanically) the fermions can be in ...

3

The "first level" answer was given by nibot in a comment. The entire conductor must be equipotential. If there were a potential difference from one part of a conductor to another, free electrons would move under the influence of that potential difference to cancel it out. However, since I have similar curiosity myself I'm going to try to answer in ...

3

It expands. Take a normal disc, and draw a circle on it. The circle expands. Take a new disc, draw a hole, and make a score on that circle. It still expands. Repeat with deeper scoring. It still expands. Keep making it deeper. It still expands. At one point, your razor(or whatever) will start poking through the metal. It still expands. Cut out the circle, ...

3

Yes, x-ray crystallography has been applied to minerals and metals for almost a century. Wikipedia notes Linus Pauling's work on $\text{Mg}_2\text{Sn}$ leading to his theory of complex ionic crystals in the 1920s. In general, crystallography is a powerful tool in material science research. Why do you ask? I should note that x-rays are typically broken ...

3

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 ...

3

There is much more to bonding than exchange of electrons. Especially because electrons that are part of electron cloud doesn't take part in crystal bonds. All metals are basically crystals --- they have proper lattice, to weld two parts you'd have them to build common lattice (at least in the are where they are welded). When you weld properly you create ...

2

Gold has its 6s valence shell unfilled, so is more reactive than Hg. thallium has a valence electron in the 6p shell which again is unfilled. The valence shell of mercury, 6s2 is filled, and is drawn closer to the nucleus because the proximity causes the electron to move at relativistic speeds thereby causing its rest mass to increase, which draws it closer ...

2

Im a machinist. We commonly heat holes to expand them in vatious applications. For example to install bearings that demand press. We use liquid nitrogen to also freeze the bearings. When both objects return to ambient temps the results are the hole shrinks. We can even control within some tolerance how much. Consider on an atomic level what is taking ...

2

Well, the metals are good conductors more or less by definition. However, really heavy metals like uranium or plutonium are not as good as usual. The opposite is not true. There are lot of good non-metallic conductors. Semiconductors may be very good conductors, there are conducting polymers (not as good as copper or aluminum but properly produced they ...

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