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Questions tagged [density]

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

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Is there a limit to the amount of mass we can concentrate in a region of space?

Black holes are solutions to Einstein's Field Equations. Black holes contain a singularity hidden from the outer universe. As I understand, these singularities are infinitesimal and contain a very ...
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What are typical atomic densities of solids and gases? [on hold]

What are average values for the number of atoms per cubic cm for typical solids and gasses at atmospheric pressure?
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volumetric density from linear densities in the three directions x,y,z

I have a cube and I know the linear density of particles along each axis (I mean- the number of particles per unit length along each axis). How can I get the volumetric density?
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How is water heavier than petrol, even though its molecular weight is less than petrol?

Molecular weight of petrol is so much higher than water, but when it comes to physical property, weight, one litre of water weighs more than one litre of petrol. How is it possible?
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Why does a helium balloon rise? [duplicate]

This may be silly question, but why does a helium ballon rise? I know it rises because helium is less dense than air. But what about the material of the ballon. It is made up of rubber/latex which is ...
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Does a non-buoyant (denser than water) object (such as a lead diving weight) weigh any less when submerged in water?

I've just been discussing this question with my eldest son, who is an extremely intelligent man, as well as being an engineer, a sailor and a scuba-diver, and he believes that an object heavier than ...
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1answer
21 views

Does optical density depend upon concentration of a solution?

Does optical density vary upon the amount of salts dissolved in water?
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1answer
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Does the density of an object play a role for it's gravitational effects?

My thought experiment: We repalce our sun with an object of identical mass, rotation, shape, etc. but not the same density, i.e. the object would have a radius of 100km. The "replacement-time" would ...
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What does Earth feel like when squeezed?

he average density is about 4g/cm^3 so if the earth was in the size of an orange, it would feel like a rather heavy ball. The crust is relatively thin, and the earth is said to be solid land masses ...
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Negative probability distribution function for Dirac equation

People say that the probability density function of the continuity equation for the Dirac equation is definite positive. I wanted to see it myself and followed the same path as Bjorken & Drell's ...
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2answers
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How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
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Is the universe be expanding at different rates depending on its temperature and density?

As I understand it, when observing different regions of the universe we get a different rate of expansion (the two Hubble constants), specifically the more distant observation seems to return a lower ...
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1answer
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Black hole gravity and light [duplicate]

When an object is squeezed to it’s swartzschild radius it becomes a black hole (made by density) and it’s mass does not change (it’s gravity doesn’t change), but if it’s mass doesn’t change (it’s ...
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Phase diagram of a fluid

How is it look like the phase diagram of a fluid in density-pressure plane? I came across the phase diagram in Pressure-temperature plane most of the time. But I can't find the phase diagram in ...
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Prove electric field is continuous at an external point $P$

I have a continuous charge distribution with density $\rho$. Can anyone please tell how shall I prove the electric field is continuous at an external point $P$? $\displaystyle \mathbf{E}=\dfrac{1}{...
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2answers
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Is it possible to have mass with zero volume?

As we had always studied that matter occupied space and has mass and our universe is made of matter so do that mean that there is no case where mass is present without volume .
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Why does sound not move through a wall? [duplicate]

I'm learning a bit about sound and was wondering: If the speed of sound is determined by the amount of matter the source is surrounded with, why doesn't it go through a wall? Example: Speed of ...
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1answer
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Cosmological densities when universe was/will be a different size

Supposing a spatially flat, matter-dominated universe, the expression for the Hubble parameter is: $$ H^{2}=H_{0}^{2}\left(\frac{\Omega_{m}}{a^{3}}+\Omega_{\Lambda}\right) $$ The previous equation ...
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2answers
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Is the Earth Atmosphere Model at this NASA site wrong?

I've been trying to get a decent numerical approximation (curve fit) of Earth's Atmosphere Model, and found what I was looking for at this NASA's web site. However, when checking some basic ...
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1answer
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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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0answers
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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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2answers
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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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1answer
35 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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1answer
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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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3answers
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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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3answers
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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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3answers
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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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1answer
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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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1answer
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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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1answer
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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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2answers
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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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1answer
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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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1answer
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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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2answers
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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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2answers
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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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1answer
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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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11answers
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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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1answer
99 views

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
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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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1answer
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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 ...