# Tag Info

20

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.

20

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 ...

19

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 ...

19

This is a problem in the theory of cracks, but let me try to give an intuitive discussion on the level of linear elasticity. Imagine a rectangular sheet of material with two opposite ends (which I'll call East and West) being pulled apart slowly. The stress in the material due to the tension from the boundary conditions will be symmetric across the North ...

17

When you look at a surface like sand, bricks, etc, the light you are seeing is reflected by diffuse reflection. With a flat surface like a mirror, light falling on the surface is reflected back at the same angle it hit the surface (specular reflection) and you see a mirror image of the light falling on the surface. However a material like sand is basically ...

14

In addition to the question of bend radius - there is also an effect of surface scratches. Most materials are very strong - they fail because a surface flaw allows a stress concentration - ie a crack to form. glass fibre has a very smooth surface because of the way it is made and can be put under high stress without cracking. You can show this with a thick ...

11

Although I don't know anything about this, using some rough estimates I think I can get the right order of magnitude: Volume of graphite in a pencil: $10 cm$ cylinder of $1 mm$ thick = $0.314 mm^3$ (error: ~factor 2) Maximum surface a pencil can write: $50 km$ $\times$ $1$ mm = $10 m^2$ (error: ~factor 5) Thickness of the graphite layer: Volume / Surf. ...

10

Yes, it is possible to predict the color of a substance, but it is, in some cases, very complex. The color of a substance is decided at various levels. The most "trivial" level is the molecule in gas phase. You have a molecule, all by itself, and when you send some white light on it you provide all the colors of the spectrum. The electronic configuration ...

10

The thing is that paper fibers are really transparent (unless the paper has been painted some color, of course). The only reason paper blocks light is that its fibers are all “immersed” in air. Try to imagine what you would see with a very potent microscope: various clear tubes going in all directions. What happens to a ray of light entering this maze of ...

9

As far as I remember, there is some truth behind this statement: glass is inherently extremely strong, but it is fragile in practice because of microcracks on its surface. In water, glass dissolves to some extent. As a result, microcracks partly disappear, and glass becomes much stronger (at least for a while). I remember reading that A. F. Ioffe (I guess ...

8

I'm afraid that outside the cartoon world vibrating won't allow a solid object to pass through another solid object. Solids won't pass through each other because the electrons in one solid interact with electrons in the other solid. There's a question about this already, How can I stand on the ground? EM or/and Pauli?, and there are lots of answers to it ...

7

See http://en.wikipedia.org/wiki/Thermal_conductivity In metals, I think it generally has to do with the higher valence band electron mobility, but it gets more interesting elsewhere. In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law, as freely moving valence electrons transfer not only ...

7

Typically, a laser will damage an optical surface in one of two ways. The first is just what you would expect: the laser heats the material up until something bad happens. The second is also pretty simple, but less common because (AFAIK) it is really only a problem with very short pulses (on the order of femtoseconds). In this case a small but rapidly ...

7

No. Or at least, not without special caveats. Edit After comments by Mark Beadles, David Zaslavsky, and Ron Maimon, I should clarify that you cannot have a plain window that lets all light through in one direction but reflects all light coming the other direction. Thus, you cannot have a window that doesn't absorb radiation at all and also has the ...

7

Suppose you bend a perfect, i.e. unscratched, piece of glass, the forces on it look like: The top of the glass is in tension and the bottom in compression, but the stress is spread over a large area of glass so the local stress at any point isn't enough to break the glass. Now put a scratch in the top surface and bend it again: This time the stress is ...

6

When you say "anti-gravity material", the closest thing I can think of is the hypothetical concept of negative mass: In theoretical physics, negative mass is a hypothetical concept of matter whose mass is of opposite sign to the mass of the normal matter. Such matter would violate one or more energy conditions and show some strange properties such ...

6

Yes, evidently. A. B. Khanikaev, C. Wu, H. Mousavi, and G. Shvets, "One-Way Slow Light in Nonreciprocal Low-Symmetry Metamaterials," in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper QThA6. http://www.opticsinfobase.org/abstract.cfm?URI=QELS-2011-QThA6 But of course this is with a ...

6

No. The two glasses are made out of different materials: sapphire is aluminum oxide, and BK7 is silicon dioxide with some additives including about 10% boric oxide (Wikipedia). Therefore they refract light differently. Sapphire is also slightly birefringent, refracting differently polarized light waves differently. You can check refractiveindex.info for this ...

6

Forever_a_Newcomer is on the right lines, but it's not like water dissolving salt. Paper is mostly made from cellulose fibres (depending on the type there may also be filers and glazes like clay). Cellulose molecules bristle with hydroxyl (OH) groups, and these form hydrogen bonds with each other. It's these hydrogen bonds that make the individual fibres ...

6

These are "fake monopoles", in the sense that the north and south poles are not actually separated. They are the ends of thin tubes which behave like Dirac strings - like long thin twisted magnets. The tubes are formed due to geometrical frustration, which forces the magnetic field to be orientated either toward the outside or toward the inside of the ...

5

No. Of course, to argue if a definition applies, we must first agree on a definition. Wikipedia gives this one: A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Humans are certainly solid-ish, and our ...

5

You're looking for something called a "light diode" or "optical isolator". The simplest is two polarizing sheets 45 degrees apart, with a 45 degree Faraday rotator in between. Here's why it doesn't violate the second law of thermodynamics. One of the components--the Faraday rotator--is thick and bulky and usually covers only a small area. So I guess it ...

5

Thanks to @Manishearth for the edit In normal phosphorescence, the atoms are in a "metastable" state--where electrons are in a higher energy level, but do not immediately come to ground state due to partial stability. The electrons come down slowly, giving rise to the (relatively) long lasting glow. The IR light frees away the electrons from the shallow ...

5

Ah, such an innocent question. In a previous job I spent six months trying to understand what controlled the stickiness of polymer solutions, before admitting the task was beyond the funding I had available (such is industrial research, but to be fair when they bought my soul they paid me a lot for it :-). Some things are obviously relevant. When a liquid ...

5

Blaise Gassend has created this simulation of "An elevator that breaks at the counterweight.": More discussion of various possible failure modes of a space elevator: Blaise Gassend discusses and simulates other possible failure modes at Animation of a Broken Space Elevator by Blaise Gassend. Bradley Carl Edwards. "A Hoist to the Heavens". IEEE Spectrum ...

5

Please see this short paper as well: Influence of Water on Crack Growth in Glass In the mid-1890s, Brodmann conducted experiments on the strength of glass and found that specimens whose surfaces were etched in HF solution were significantly stronger than specimens tested without etching. That result provided the first reported evidence that glass ...

5

Although it's not strictly what happens, you can think of the bonds around a carbon atom as repelling each other because the electrons localised into those bonds want to get as far away from each oither as possible. That's why when a carbon atom forms three bonds you get the bonds separated by 120º. When you have four bonds they arrange themselves into a ...

4

Well, I think that whatever distortion of the magnetic field is caused by paramagnetic materials - such as aluminum - is also caused, with the opposite sign, by diamagnetic materials - such as bismuth (whose effect should be almost exactly opposite to that of aluminum). I am convinced you may neglect all those materials. Only paramagnetic materials and ...

4

I will just answer the first part of question: is phonon attraction stronger than Coulomb? Short answer: No. Longer answer: Nothing (in condensed matter) is ever stronger than the Coulomb force. Longest answer: There are two aspects to consider. First is the self-screening of the electrons, which will add a mass term to the photons, giving a Yukawa-esque ...

4

It's important to note that color is only defined given knowledge of three things, see especially this section of the wikipedia article which has a nice discussion. 1) Light absorbing / reflecting properties of a material 2) Properties of the light source 3) Properties of the human perception of light In general, 1 is determined by the interaction of the ...

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