# Questions tagged [density]

Density is defined as the mass per unit volume of a substance.

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### Why is the energy density of the dark energy component~magnitude as the density of matter now, when the two evolved in different time periods?

WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a ...
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### The moment of inertia of a hollow cylinder is lower than that of the "corresponding" solid cylinder (same density, outer radius, height):

This is a self-answered "question" for those scratching their heads after learning, e.g. in class, that the moment of inertia of a hollow cylinder is higher than that of solid cylinder. Note ...
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### Density, density operator, and number operator in quantum mechanics

Density is the number of particles per unit volume. But in quantum mechanics, the density operator is defined as $$\hat{\rho}(r) =\Psi^\dagger(r) \Psi(r)$$ here $\Psi(r)$ is the field operator. The ...
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### What does ultra-rarefied gas mean?

Without using any math, can you explain to me what a rarefied gas is? And then what an ultra-rarefied gas is? I'd like to understand it from a conceptual level if you can make connections to other ...
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### Why can't water flow fast when the impact speeds up? (the incompressibility)

I'm a high school student studying physics in Korea! Please understand my poor English skills. In the text below, the liquid is said to be destructive and incompressible. And he says, "I the ...
138 views

### A black hole made of ordinary water?

Years ago I read a book about black holes. It said it is theoretically possible to make a black hole from ordinary-density materials, like tap water. It would take a lot -- about the size of a galaxy ...
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### Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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### Why is the charge density of a rod in $\rm C/m$ and not in $\rm C/m^2$?

Is it because we consider the rod to be a 1-dimensional straight line? Or is there any other reason? I know it sounds like a stupid question but I am confused in this regard.