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

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Density of photons at distance, r, from point light source [on hold]

I am trying to derive an expression for calculating photon density $\left(\frac{Photons}{M^3}\right)$ at any given distance from a point monochromatic light source for fun (assuming previous knowledge ...
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What is the mass density distribution of a Schwarzchild black hole?

According to Schwarzschild metric the density element $$ \frac{\mathrm dm}{\mathrm dV} $$ becomes infinite on the event horizon if there is mass $$ \mathrm dm > 0 $$very near at the outside of ...
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Why don't the less denser steam move up, leaving the more denser water drop to fall on hot pan in Leidenfrost effect?

In Leidenfrost effect, the hot steam suspends water droplets above it and prevent the drop from touching the hot pan. My question is, as the steam is less denser than water, should the steam not ...
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1answer
57 views

Can we estimate the size of a neutron star based on direct observation? [duplicate]

Are there any neutron stars near enough for astronomers to get a good size-estimate, based on direct observation? I know that there are many theories regarding what happens inside neutron stars, but I ...
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24 views

How dense is the material in the core of a black hole? [duplicate]

If I would like to find out the answer of this question, I get two different one: The material is collapsed into a tiny dot. This density is called planck density, and it is so dense, all of the ...
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Calculating photon number density inside a volume on whose surface the energy flux is known

I have the power spectrum for a galaxy of radius $R$, which we can approximate as a sphere at some distance $D$. Now, on earth we can measure the power spectrum which is given as a list of tuples, ( $...
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26 views

Can a decompressed airplane cargo hold cause packaging air pads to burst / deflate?

I received a parcel from Germany to Chile recently. The contents were severely bruised due to a lack of packaging material. On second inspection I found a bunch of deflated air pads. It was a ...
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Is there a temperature at which ice is denser than water?

Normally ice would float on water because its density is less compared to that of water as a liquid. But is it possible that its density will increase due to a very low temperature or is ice in any ...
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Size and density of neutron stars

Most of the books which I looked at give approximately 10 km as the radius of a neutron star. Just yesterday I looked at a book by Dave Goldberg titled The Universe In the Rearview Mirror (2013) which ...
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37 views

Weight of sphere in kerosene [closed]

Two identically charged spheres are suspended by strings of equal length. When they immersed in kerosene, the angle between their string remains the same as it was in the air. Find the density of the ...
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How can matter be squeezed extremely dense [duplicate]

I'm fairly new to physics and currently reading A Brief History of Time by Stephen Hawking. Hawking is talking about black holes being the size of a few miles containing the mass of multiple times the ...
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1answer
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If all matter expanded [closed]

I know this is impossible (?), but what would happen if the electron orbit swelled, the gluons extended, and quarks lost density in relation to the gluons, would that affect the universe as we know it?...
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Particle density in k-space

Question: Given a many particle wave function $|\Psi\rangle$, how can I calculate the occupation numbers in k-space? Setup: I have a 1D chain of molecules which contains 4 sites per unit cell. Let'...
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59 views

How cold air is less dense in one case and more dense in another case when compared with warm air?

There are two different and contradicting explanations given. Which one is correct? Explanation 1: Cold air is more denser than hot air because ,when cold air of some density, say 'd' is heated, the ...
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A dead body floats with its head immersing in water . Why?

I found this question in my practice book . I want to know about it .
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Is the effect of mass density on the expansion of the universe linear?

The average expansion rate for large scales parsec and above measured is $\approx 73\,{\rm km}\,{\rm s}^{-1}\,{\rm Mpc}^{-1}$. This expansion is calculated for average densities of space volume across ...
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2answers
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Merging black holes makes them less dense, so

According to What is exactly the density of a black hole and how can it be calculated? (more specifically, John's answer here made me think: if you merge a whole load of chunks of an element heavier ...
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25 views

Finding density ratios with missing information? [closed]

How would you go about solving this problem? You can't find the densities since the volumes are missing.
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find the density of water only by its pressure?

Is it possible to find the density or specific volume of water, by only knowing its pressure? Or we need to know its temperature as well? I can't find any relevant table, so can you advise me please? ...
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Ionospheric electron density measurements

I've been working on measuring total electron column density (TEC) with multi frequency GPS data. In theory, signal delay caused by TEC differences should affect the code and carrier nearly evenly in ...
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Copper density modification

Can you increase the density of a material surface (metal) by surface treatment? For example commercial copper. If we take the ordinary OF copper (99.95%), I would expect not to have the tabulated 8....
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1answer
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Density of the universe [closed]

Why is the universe of the same uniform density at all distance and time scales? Why haven't cosmologists realised this simple fact disproves the big bang? Why do they keep pushing theories as ...
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1answer
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Consequences of general equation of hydrostatics $\nabla p= -\rho \nabla E$ in terms of surfaces where $p$ and $E$ are constant

I'm facing difficulties in understanding the consequences of the general equation of Hydrostatics in the case of conservative forces. $$\nabla p= -\rho \nabla E\tag{1}$$ Where $p$ is the pressure, $\...
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Speed of Sound in matter

So basically when it comes to the speed of sound, it is said that speed of sound in media is based on two main factors - 1)elasticity and 2)density from the formula V= $\sqrt{E/\rho }$ where E is ...
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Conversion from PPBV to PPBM [closed]

What is the conversion between PPBV and PPBM? I have a database that includes values of Parts-per-billion-by-volume (PPBV) as a function of altitude. I've been asked to convert this to "parts-per-...
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1answer
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Atmospheric pressure question [closed]

I'm asked to find at what altitude atmospheric pressure falls to 10% of the mean sea level value. I have found the expression for the variation of pressure with altitude to be $$p = p_0e^{-h \over a}$...
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Using $\sqrt{-g}$ in integrals of proper volume

I am a little confused over integration using proper volume element. When do we use $\sqrt{-g}$ in calculations? For example, in many calculations involving stars, say when using TOV equation, this ...
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Density of air using ideal gas law

I need to find the density of air using (I think) the ideal gas law. I have calculated $V$ of an ideal gas at s.t.p. to be $.0224m^3$. I am asked to find the density of air knowing it's temperature ($...
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Approximating Gas Density

So this was a random thought I had that I thought would be fun to mess around with using an Arduino and some sensors, but after the difficulty I've had I'm just curious to see if it can be done at all....
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25 views

Finding density using bulk stress equation

Basically, I'm using the equation $\frac{F}{A} = -B\times \frac{\bigtriangleup V}{V_o} $. Originally I had to find the change in volume given an initial volume (B and pressure ($\frac{F}{A}$) were ...
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Where does an object with density lower than outer space go?

Assume there was an object with lower density than that of the interstellar medium (which is extremely low already at just a few particles per cubic meter). If that object were released on Earth, ...
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Pressure question problem in forming the equation

A tank contains sea water of density $1030 \frac{kg}{m^3}$ The pressure at point M is $60 kPa$ more than atmospheric pressure. Calculate the depth of M below the surface of the water. My attempt: $P ...
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How does air density affect corona discharge?

I'm wondering how local air pressure and density affect the amount of ionizations occurring in a corona discharge. Also, does the threshold voltage required for a corona discharge vary with air ...
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Are black holes very dense matter or empty?

The popular description of black holes, especially outside the academia, is that they are highly dense objects; so dense that even light (as particle or as waves) cannot escape it once it falls inside ...
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Explanation for concept of Relative Density

I am in class $9^{th}$, I'm reading my physics book in which it is written that: $$\mathrm{Relative\ Density} = \frac{\mathrm{Weight\ of\ solid\ in\ air}}{\mathrm{Loss\ of\ weight\ of\ solid\ in\ ...
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Density solver for ideal mixture of real gases

I am referring the paper on GERG-2008 equation of state. I am interested in calculating the properties of ideal mixtures of real gases. The GERG-2008 EOS is in Helmholtz free energy and hence the ...
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If there were a planet with oceans tall enough saturn fitted in, would it be floating around? [closed]

In this morning I read an article that claimed (translated into english, so emphasis mine) Saturn is so slight1 it would float around in an ocean. So appart the missleading wording of its ...
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Standing Waves in a String of two Linear Densities

Given a string with sections of two linear densities like this: Does the point where the two linear densities meet have to be a node if a standing wave is produced? Are there alternatives? Could I ...
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1answer
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How does the gravitational field behave inside a star?

The interior gravitational field of a star with constant density is given by $ds^{2}=-\left(\frac{P_{c}+\rho_{0}}{P(r)+\rho_{0}}\right)^{2}dt^{2}+\frac{dr^{2}}{1-\frac{8\pi\rho_{0}}{3}r^{2}}+r^{2}d\...
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Is there a charge density in quantum mechanics?

Is it meaningful in quantum mechanics to speak of charge distribution? Some people say if you sovle the Schroedinger Equation for the hydrogen atom, the eigenfunctions represent a type of charge ...
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Confused about volume, density and mass, help!

I got into an argument with my friend, which cast confusion on my understanding of density and its relationship to volume. I'm hoping to get some clarity. The argument involved describing density in ...
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1answer
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Deriving a diffusion equation from a given mass flux density [closed]

I am given that the mass flux density $q$ of a substance in a medium obeys the following physical law. $$q=-D\frac{n^3}{n-n_0}\frac{\partial n}{\partial x}+nv,$$ where $n(x,t)$ is the ...
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What's the nature of intraction between hydrogen atoms in the interstellar medium?

Interstellar densities are about 1 atom/cm$^3$. Do these atoms even move? If they do, why do they move? What would be their average speed? (Free expansion)
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1answer
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Is it easier or harder to move moist air with a fan?

From what I understand, the molecular weight of water is lower than the molecular weight of nitrogen and oxygen. So, shouldn't it be easier for a fan to move moist air than dry air of the same ...
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Is there a limit to the concentration of photons in a given volume?

If you have a given fixed volume, is there some physical limit to how many photons can exist in that volume?
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Is there a limit to the size of black hole?

I have read answer by @John Rennie in regards to the size and density of black hole. In the last sentence he states that super supermassive black hole ...
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Changing the Density of a liquid while its surface tension is constant

I'm working on Plateau-Rayleigh waves and I want to investigate the effect of density of the liquid on the wavelength of these waves How can I change the density of the liquid while its surface ...
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How is refractive index related to the density of a medium (for example, air)?

I have a question regarding refractive index dependency on the density of a dielectric, specifically air. Background Let us start from Newton's second law form of driven harmonic oscillators $$-kx-...
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How do you calculate the number density of air at 300K? [closed]

I'm really new to this, any kind of help would be appreciated.
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Solid settling to the top of solution when centrifuged

I have a mixture of dense particles (about 4x that of water) and not-so-dense particles (maybe the same density of water or slightly less or more dense). When I centrifuge a tube filled with a ...