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Questions tagged [density]

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

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Black hole and its density

I'm an amateur to astrophysics but I'm slowly teaching myself things and I have these three doubts. How does more compression relate to a stronger gravitational pull. Like, when we say that a black ...
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Do different neutron stars have different density?

When stars go supernova, a neutron star may be left over if there isn't enough mass to create a black hole. Based on the mass present, will a neutron star have a varying density or is the radius ...
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Density of ice < water?

In the situation of an ice cube placed in water and eventually melting, it is supposed that natural convection is the process in which heat transfer from water to ice occurs. However, natural ...
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30 views

Relationship between volume and surface density

I was not sure if it fits better here, or in the math forum, but this is the problem I am trying to solve: Say you have a full cylinder of radius $R$, height $L$, and its uniformly charged with ...
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Density of supermassive black holes

So I know that supermassive black holes can have "densities" less than water because black hole density scales as $1/m^2$ since $R_s = 2GM/c^2$. I am trying to reconcile this with the fact that black ...
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Can a single photon have an energy density?

In one question, which is further irrelevant for thís question, the comment was made that a single photon can have an energy density. I didn't agree. Off course the wavefunction is spread out in ...
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In a ball with random thread/strings, how does the density of threads/strings change with radius?

A large plastic ball full of holes is given. (So the holes are in a plastic shell.) Straight threads connect these holes randomly, by passing through the interior of the ball/shell. For a big ball or ...
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What is the physical origin of buoyancy?

Imagine a horizontal cross-section $\Delta S$ of a cylinder containing a liquid of density $\rho$. The downward pressure on $\Delta S$ at the depth $d$ from the surface of the liquid is $P_{down}=\rho ...
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Why is current-density a vector? [duplicate]

I was told that current has a direction but does not follow vector-laws, so it is a scalar quantity. That's okay. I thought it through Kirchoff's junction law: assume two currents joining at 90 deg, ...
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Can a sound wave begin with rarefaction?

Some digital recording samples (audio files) of recorded acoustic sounds present sound waves which begin with rarefaction. Is this an actual phenomena that can occur or is it a result of sound ...
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Maximum density of water

The maximum density of water in its liquid form occurs at around 4°C (under normal pressure p = 1013,25 hPa). If the pressure is increased (or decreased) would the point of maximum density still be at ...
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An issue with thermodynamic properties of infinite systems

I have the following problem. Given a finite system, let us say a box of side $L$ containing $N$ "ideal gas" non-interacting particles, properties such as the entropy can be defined, in this case, $S ...
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Invariant density of Harmonic oscillator

In general, dynamical systems described by a pure Hamiltonian can have an infinite number of invariant densities. In fact, each initial state determines exactly a closed path in phase-space and the ...
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Electron density in DFT (*ρ*(r)) and probability density (wave function squared)

Are the electron density in density functional theory, ρ(r), and probability density, defined as wave function squared, the same quantities?
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Density $\rho$ in the Friedmann equations

In the Friedmann equations: $$\ddot{a}=-\frac{4}{3}\pi G(\rho+\frac{3p}{c^2})$$ $$\dot a^2+Kc^2=\frac{8}{3}\pi G\rho a^2$$ I didn't understand if $\rho$ is the mass density deriving from $m_0$ (the ...
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$\rho(t)\gt or \lt \rho_{critical}(t)$ depends upon $k$ for expansion or contraction in cosmology?

From friedmann equation $$1=\frac{\rho(t)}{\rho_c(t)}-\frac{k}{a^2H^2},$$$$\dot a(t)=+-\sqrt\frac{k}{\frac{\rho(t)}{\rho_c(t)}-1}$$ for $k\gt0$,$$\rho(t)\gt\rho_c(t)$$ and for $k\lt0$,$$\rho(t)\lt\...
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Cause of change of tea volume during cooling

At work I often drink tea, and i usually fill my cup to the brim with boiling water. When I let it cool though, until it's ready to drink, the liquids surface has sunken by about a fingers width. To ...
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The Speed of Light and Its Medium's Density

When reviewing a refractive index chart there is a relationship to the density of the medium and the velocity of electromagnetic radiation through the medium. The lower the density the greater the ...
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103 views

Finding the expression for probability density (the Klein Gordon equation)

Source: Quantum Field Theory for the Gifted Amateur by Tom Lancaster, Stephen J. Blundell. I am struggling to understand the logical step from the outline of the 'proof' in the footnote, to the fact ...
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How does density affect gravity?

Say we have two masses, mass A and mass B. These two masses are identical in every dimension. The only difference is the density. Do they not curve the same amount of space-time, and if not, why?
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How can I calculate the density of Mars' atmosphere during dust storms?

I have been searching to try to estimate the air density of the atmosphere on Mars during a dust storm. I am trying to use this dynamic pressure equation to calculate the pressure on a theoretical ...
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2answers
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What is the meaning of effective density in porous media? Is the density of air inside the pore space not same as density of free air?

I am trying to understand the physical meaning of using effective density in porous media. Is it a fictitious value? Can't I use the density of solid and fluid as it is while modeling porous media?
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By which physical mechanism does the continuity equation in fluid mechanics work?

The Navier-Stokes equations consist of the momentum equation and the continuity equation. Consider the incompressible versions for the purpose of this question. Continuity is always talked about as a ...
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How to find density of a wire when we know information of it in polar form

We know that the wire has the "path" in the shape of the polar graph $$r = \theta, \quad 0 \leq \theta \leq \pi/2 $$ and we know at point $(\theta,\theta)$ the density of the wire is $2\theta$. I ...
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Are there physical properties that can be used to differentiate stainless steel from copper in a home environment? [closed]

So the backstory is that I purchased a reusable drinking straw that is copper coloured, but is advertised to be stainless steel. That got me thinking about whether I could be sure it was one or the ...
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How to calculate the normal force of a box in the bottom of a bucket of liquid? [closed]

I have been trying for some time now, and seem to be unable to come up with the correct answer. The setup for the problem is as follows: After finding a similar question and rechecking the buoyancy ...
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1answer
68 views

How much harder is a proton theoretically speaking than a diamond? [closed]

It's generally said that a diamond is the hardest substance known to man (apparently there are a few materials known to be harder). However, one ought to expect that a proton or neutron should be ...
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1answer
29 views

How do fluids behave in space with regards to bouyancy and density?

Will a fluid loose it's bouyancy in zero-gravity. What would happen if an object is immersed in a liquid in space (if the liquid is still very much together and not spilling as big bubbles or droplets)...
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3answers
90 views

why do fluids become less dense when heated?

I have been told that fluids expand and become less dense when heated, but i never been told why. So my question is why do fluids become less dense when heated? What is going on in a microscopic level ...
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Number density of photons and the total energy of the universe

I was reading over some assignment solutions that my lecturer put up and one of them said that instead of calculating the number of photons in the universe with the familiar method of $$\int^{\infty}...
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0answers
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What is the equation of state of foam, on a macroscopic scale?

Consider a large amount of soap foam (or any other substance producing foam), of mass density $\rho$ in a gravityless environment. What is the internal pressure $p$ of that foam, viewed as a fluid on ...
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1answer
34 views

Effect of temperature on specific weight

How much of an impact has a temperature difference when the specific weight of an object in water is measured, e.g. the weight of a mussel in seawater. Would the two scenarios yield the same specific ...
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Charge density of sea water

If sea water contains salt ions and some H+ and OH- ions, what are the charge density of a certain volume of sea water? And is there a way to calculate?
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Non-relativistic limit of the cosmological constant

Usually, when we apply the non-relativistic limit ($c \rightarrow \infty$) to relativistic equations, the cosmological constant $\Lambda \sim \mathrm{L}^{-2}$ is simply offhandedly neglected by ...
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638 views

BH singularity? Infinite density

How can the density of a region of space go from finite density to infinite when there are no numbers larger than any Aleph0 number but smaller than any Aleph1 number (no decimal point in front of it, ...
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1answer
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Why does the pressure change on uniformly mixing two liquids?

The two cylinders are connected the upper cylinder has a cross-section of A and the lower one has a cross-section of 2A (I've taken the cross-sections to be A and 2A as they are easier to work with ...
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Which weighs more in atmosphere, $1\,{\rm kg}$ of steel or $1\,{\rm kg}$ of feathers?

I'm having a discussion at the moment regarding the mass of $1\,{\rm kg}$ of feathers and $1\,{\rm kg}$ of steel. The person I'm arguing with states that $1\,{\rm kg}$ of feathers will be lighter ...
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Does the amount/speed a balloon rises vary with the temperature of the gas inside, given a constant type and amount of gas?

I'm attempting to conduct an experiment with a balloon. I fill it up with some amount of helium. If I cool the balloon down (say with liquid nitrogen), the balloon's volume will decrease (because of ...
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5answers
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If gravity accelerates all objects the same, why does a ball in water sink? [closed]

I recently learned the law of universal gravitation: F = GmM / d². It dictates that all objects should fall at the same acceleration. But how is density explained ...
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Scaling of mass density

Let distances scale as $L'=kL$, $k$ is the scaling factor. Then spherical volume will scale as $V'=k^3 V$. Say, the sphere is filled with mass with constant density. If we scale the model with ...
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87 views

Why the body is drowning in the swamp?

Is it correct explanation why does the body sink? Swamp is not a liquid, but particle suspension, that's why Archimedes principle not working and upward buoyant force not applied to the body.
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By how much does space-time curvature modify the density of an object?

Let's imagine a transparent object with a density $\rho_0$ [$kg/m^3$] (A sphere of glass as example). Now I send through this object a long flat impulse of electromagnetic plane wave with frequency $\...
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5answers
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Do the weights of two liquids not add when mixed?

I was given an interesting dilemma today. A co-worker saw me adding a liquid (Diisopropyl ethylamine AKA DIPEA) to a flask filled with another liquid (Tetrahydrofuran AKA THF). I needed to weigh out ...
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Silly question about Schwarzschild Black holes and scientific communication

Well, in scientific communication literature often we encounter the following phrase about black holes: A Black Hole is a body that have infinite curvature and infinite density So, with basic GR ...
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How can the Friedman Equation produce negative pressure or density?

As I understand it, the Friendman Equation provides the driving mechanism for Inflation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right)$$If $\rho$ or p is negative enough, you ...
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376 views

Relation between Young's modulus and density?

Can I calculate Young's modulus from density? Is there any mathematical relation between Young's modulus and density?
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Why are the volumes above and below the center of mass of a uniform cone not the same?

So today in my physics class we derived the center of mass of a uniform cone, and it all made sense, but near the end of class a student asked, "If you were to split an object into two parts with ...
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2answers
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Density of Sea Water [duplicate]

Understandably, there is a clear-cut difference between liquids and gasses. Gasses being more compressible, liquids less so. If you bring a volume of air sealed in a container from sea level to 10k ...
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2answers
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Neutral buoyancy for cylinder/bladder in water

A 25 liter flexible plastic bladder consists of three parts: an air chamber located on top of the bladder, a large middle water chamber, and a lower weighted area that serves as a counterbalance and ...
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1answer
103 views

Milky Way Density

It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA ...