# Tag Info

## New answers tagged terminology

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It depends on what group you consider the starting point, and that depends on context. One context is the mathematical one (forget all you know about spins etc) where we start with the vector space $\mathbb C^2$ and the natural action of $SU(2)$ on it. If we then look at the projective vector space $\mathbb C^2/\mathbb C^* = \mathbb CP^1$, then the action ...

2

I don't think we should think about this in terms of definitions, and of a particular model being or not being a toy model, but rather it is a matter of the spirit with which a model is considered. A model usually qualifies as a toy model when it is considered mainly not as a (however rough) description of reality, but as a simplified version of a more ...

2

A toy model is simply a very simple model which nevertheless is able to explain qualitatively a certain phenomenon. A model should be able to explain natural phenomena in a quantitative way. Also, a toy model can be fundamentally flawed, mathematically or physically, or totally unrealistic. A model instead should be mathematically consistent and not ...

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When are spin exchanges said to be isotropic or anisotropic? I agree with the other answer, that the precise meaning depends on the context. My answer concentrates on the specific context of ab initio calculations in condensed matter physics/chemistry. Calculations are often performed using density functional theory (DFT) with on-site Coulomb corrections, ...

-2

perfect dipoles means forces at center of dipole is zero

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The best representative of a black body curve is the cosmic microwave background radiation. Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE, the most precisely measured black body spectrum in nature. The error bars are too small to be seen even in an enlarged image, and it is impossible to distinguish the ...

1

Is the Sun absorbing energy from it's surroundings? No, of course not in a net sense. The Sun loses far more energy than it absorbs from its surroundings. It is not in thermal equilibrium. The Sun is also not a blackbody at a single temperature, even though it most definitely absorbs nearly all radiation that is incident upon it. That is because the Sun is ...

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Speed of sound usually refers to the propagation velocity of a low amplitude vibration in a medium in equilibrium. A "sound" can travel far faster than the speed of sound if it is, for example, a shock wave

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it's likely that when you google the term in general, most pages treat about the "speed of sound" at the common human meaning (sound in air :-) ), while its generalization to all the various circumpstances of fluids in the universe might more often been spelt "sound speed". But here it's more linguistic than physics, and it wouldn't be physically incorrect ...

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The hydraulic radius is half of the geometrical radius. This is odd, no? The hydraulic diameter (in general) is defined to be $4A/P$ ($A$ being cross-sectional area; $P$ being the wetted perimeter). The hydraulic radius (mostly in thermoacoustics) is defined to be $A/P$ (check out Swift's book). Subbing in, you get \$\frac{\pi r^2}{2 \pi r} = ...

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I asked myself the exact same question. This PDF file from Harvard says that indeed diffraction gratings should be instead called interference gratings (page 13, first paragraph of "Remarks"). EDIT : Here is the paragraph from the link, as suggested by @Kyle Kanos : A diffraction grating should more appropriately be called an “interference grating,” ...

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You are right. Voltage is an electric potential difference. The concept of potentials is more general (e.g. gravitational potential) in physics.

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A pulse refers to a disturbance that travel from one location to another location through a medium. While, A wave refers to the disturbance or variation that travels through the medium.

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Rotational and turbulent motion cannot "deepen" the gravity well in a galaxy of galaxy cluster. The contribution of kinetic energy to the stress-energy tensor is negligible compared to the rest-mass energy in non-relativistic systems. Assuming you mean something like, 'how does rotational and turbulent motion contribute to the equilibrium configuration of ...

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A n-particle reducible diagram is a diagram that can be cut into two pieces if one cuts n or less lines. Conversely, a n-particle irreducible (n-PI) diagram cannot be cut into two pieces if one cuts n lines. The sunset diagram is 3-particle reducible, since it has 3 internal lines, but it is both 1-PI and 2-PI, and contributes to the self-energy (which ...

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