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

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Are densities only good approximations?

In a number of situations, continuous densities (e.g. for mass, electric charge, etc.) are used in calculations. However, for example, as far as I know, matter is not continuous. Also, for example, ...
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Determination of radiation pressure

Consider the incidence of an electromagnetic wave on the plane $x=0$. We have that: $$f_x=\frac{dF}{dV}$$ $f_x$ is the volumic force density on the medium. My doubt is purely mathematical. I ...
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why there is no temperature and density in the formula of the electrical resistance?

Why there is no temperature and density in the formula of the electrical resistance? We all know, that the electrical resistance depends on length, temperature, density( material). Why is the ...
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87 views

Why do denser things sink? Detailed justification needed [duplicate]

I would like a detailed explanation and justification if possible regarding the topic. I would like to know what happens at an atomic level. Most of the answers I have found were very basic and did ...
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28 views

The equation of continuity in isothermal system in spherical axis(transport phenomena)

My homework is about finding the equation of continuity in isothermal systems in spherical axis, I can't imagine a workaround for that since its a little complicated for me to understand velocities ...
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1answer
33 views

Reference values for viscosity and density in incompressible NSE

I come from a pure mathematics background, so I have very limited physics knowledge. I'm currently working out the non-dimensional form for the Navier-Stokes equations and have some questions. Where ...
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Does the pressure at the bottom change when two liquids mix and don't mix?

My question is when these two liquids above mix and don't mix, does the pressure at the bottom change ? When they don't mix the pressure is 2hdg + 3hdg = 5hdg (d:density) But what happens when they ...
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Is there an upper theoretical density limit?

Is there an upper theoretical density limit - density being mass in a finite volume, and the part consisting mass has also some finite volume, just with large space in between (particles, atoms, ...). ...
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Mass, density and gravity for 2D SPH

Having previously only worked with 3D sph, the 2d case is confusing me. The concept of mass and density as well as gravity is not easily converted, as I see it. As an example in 3D I take 1000 ...
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133 views

The Speed of Sound [duplicate]

Just a question about Physics I'm doing at school. If the speed of sound is inversely proportional to the density of a material, why does sound travel faster in solids (it is the most dense). I have ...
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1answer
43 views

Density profile in the atmosphere, which equations to use…? [closed]

I came across the following density profile in the atmosphere: ...
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1answer
51 views

Suffocation in a granular medium

I read of a recent accident in Alberta in which three girls fell into an open bed truck carrying canola seeds and suffocated. In that situation would it have been possible to keep breathing by using ...
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Does matter found at densities of white dwarfs “feel” more like a solid, liquid or gas?

I'm wondering what it would be like to touch or interact with very dense degenerate matter like this found at white dwarfs. I understand that white dwarfs are initially very hot, but for the sake of ...
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Is this charge density function, from a problem in Griffiths' book, a physically valid density?

This is from the book on electrodynamics by Griffiths: A sphere of radius $R$, centered at the origin, carries charge density $$\rho(r,\theta)= k(R/r^2)(R-2r)\sin(\theta)$$ where $k$ is ...
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224 views

Definition of a line charge with Dirac delta function [closed]

Is the following statement correct for a line charge distribution $λ(x)$? $$ρ(\mathbf r)=λ(x)δ(y)δ(z)$$ If yes - what does it say?
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Is it possible for an object to have mass but zero volume?

Can there exist a particle/object in the universe having mass but no volume? Is it possible that mass can exist without volume and density? We think we know that matter is anything having mass and ...
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188 views

Force on side of pool from water

Given a pool with dimensions $$\ell \times w \times h \, ,$$ I am trying to derive an equation that will yield the force by the water on the sides of the pool, namely $$\ell\times h \quad \mathrm{or} ...
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1answer
54 views

What special significance does the eigendecomposition of a mixed density operator hold over other pure state decompositions?

It is known that in general, a mixed state can have multiple pure state decompositions. However, it has a unique eigendecomposition in the absence of degenerate eigenvalues. What is the special ...
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Density of a substance [closed]

I have faced a question related to Archimides' principle which is all about a car sunk in water. They said calculate the average density of the car, but i really don't what does an average density ...
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1answer
42 views

Do siphons work for gasses?

There are videos of sulfur hexafluoride which show it is so dense (6.17 kg/m^3), an aluminum foil boat will float on it like water. I'm curious if gasses can be used in a siphon. I'm particularly ...
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119 views

Density of the Sun

Being either on the surface or somewhere inside; where is the density of the gases of the Sun equal to the density of the ground we stand on here on earth?
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Blackholes / density [duplicate]

If a star explodes to form a black hole how does the gravitational field become infinite from one state to the next? ie: it seems additional mass has been added or is this simply a function of ...
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0answers
23 views

How can I estimate the density of non-ideal vapor mixtures

I am trying to estimate the density of a non-ideal gas mixture and can't seem to find any resources for how to go about this. For some gas pairs like ammonia-water there are empirical correlations ...
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1answer
74 views

What is the highest density to be measured?

In the abstract of the recent article named "Transport properties and equation of state for HCNO mixtures in and beyond the warm dense matter regime" we can read that authors measured a density of ...
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Proof for Resistance is directly proportional to length and inversely proportional to the area of cross-section?

I have heard that Resistance is directly proportional to length and inversely proportional to area of cross-section. Can someone give me a practical explanation for this?
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1answer
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Could a vacuum airship be possible? [duplicate]

According to Wikipedia, a vacuum airship is a hypothetical airship that is evacuated instead of using a lighter gas. Really, once I first saw this, I thought, "Why did I never think about this ...
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1answer
106 views

How to produce a 3D density map of a time-depenent system of particles?

I have a time-dependent system of varying number of particles (~100k particles). In fact, each particle represents an interaction in a 3D space with a particular strength. Thus, each particle has ...
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57 views

Can a 0.1mm iron cannonball fired with a proportional cannon damage a tumbler (glass)? [closed]

A teammate just used the well known phrase "a storm in a teacup". I looked at my tumbler (full of water) and asked this question: "If a pirate ship the size of a nutshell had cannons with iron ...
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If a super massive black hole captures a very small black hole, would the smaller black hole be immune to spaghettification?

I think to answer this question we would have to fully understand the nature of black holes. It seems to me that the smaller black hole could have a lower density than the larger one. In this case the ...
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33 views

How can a singularity have infinite density? [duplicate]

I've heard many times that black holes have infinite density at their center, but I've also heard astronomers quote mass estimates for black holes. How could the singularity have infinite density if ...
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24 views

Is density of an element directly proportional to atomic number or atomic weight? [duplicate]

Is the density of an element directly proportional to any power/order of atomic weight or atomic number? I know very less, probably only highschool level physics, after which my brain has been ...
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17 views

Density and pressure of the earth core [duplicate]

According to some sources the density of the earth core is about 9.5kg / m^3 and the pressure: 4*10^11 Pa I know the gravitational force decreases, if you get closer the center of the earth, ...
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1answer
417 views

Disc and hole - moment of inertia [closed]

Determine the moment of inertia of a homogeneous disc with density $\rho_0$ of radius R with a circular hole of radius R/2 and central radius R/2 regarding an axis perpendicular to the surface and ...
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192 views

Why are densities not fields?

I have read (in Statistical mechanics of lattice system 2: exact, series and renormalization group methods by D.A. Lavis and G.M. Bell pg 2 ), that intrinsic variables are either fields or densities. ...
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180 views

What exactly is a one particle density?

In Density Functional Theory (DFT) we derive the Grand Potential as a functional of a so-called one particle density (OPD). I have trouble imagining what exactly that is. Could someone help me with ...
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879 views

Why is it difficult to mix helium and nitrogen gases?

I recently learned an interesting fact: That it's difficult to mix helium and nitrogen gases in a compressed gas cylinder. Gas suppliers that need to mix the two gases have to rotate the cylinders for ...
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2answers
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Are atoms compressible?

Liquids are not compressible, but what does compressible mean? Does it mean the increase in the density or decrease in volume? Or something else? Why are liquids incompressible, in a way nothing ...
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1answer
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An example on computing the density of a typical nucleus

An example from my text is as such: Compute the density of a typical nucleus, and find the resultant mass if we could manufacture a nucleus with a radius of 1cm. I will only provide a partial ...
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How does one write density as a form?

In vector calculus, given the density $\rho$ of a body with volume $V$, it's total mass $M$ is simply $M=\int_V \rho dV$. If density $\rho$ is a form, say dm it would need to be a volume form to ...
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Density of states and fermi energy

I am trying to find the fermi energy and density of states. From the equation: K_F^3=3*(pi)^2*n and n=NAvagadrosdensity/m what is the big N here?
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Meaning of negative density

Charge density depends on the area/volume and the amount of charge. Before electromagnetism I used to think of density as being positive only. What is the intuitive meaning of negative density? Does ...
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Energy density question

I have looked for these formulas everywhere but cant seem to find them... What is the formula for the energy density of the electric current ,and the energy density of the mechanical wave?
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880 views

Why can liquid mercury sit on salt?

A video shows liquid mercury sitting on top of table salt. Even though the mercury is much denser, it does not sink to the bottom. Why does this happen? Is the mercury just so thick that it is like ...
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1answer
50 views

Will this balloon rise?

A helium balloon contains mostly helium which is an inert gas that is less dense than the air around it, suppose I drained all the air molecules from the balloon and note that this special balloon can ...
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How do we measure mass distribution in an object?

Given an object I want to know the mass distribution among different section. What are the inexpensive methods to know this ?
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107 views

Speed of sound in an inifinitely dense medium

Curious thought i just had. The speed of sound is affected by a few factors, but the density plays a large role. As density increases, does this mean that sound could approach the speed of light? If ...
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2answers
142 views

Schwarzschild radius in black hole density

The textbook from which I teach physics at the end of secondary school has a question about the density of a non-rotating black hole. Because the density at the singularity is perhaps infinite or ...
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How much water is needed for Saturn to float in?

Saturn floats because it's density is less than that of water. Just how much water would be needed? Assume a cuboid tank holding it. And when we actually put saturn in that water, how far in the ...
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296 views

Physical Significance of the Planck Density

The Planck density is the Planck mass devided by the Planck volume, approximately 1093 g/cm3. Does this quantity have any known physical relevance? The Planck mass is believed to be the smallest ...
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490 views

Packing Fraction vs. Packing Density

Hello: I am confused if Packing fraction and packing density mean the same thing. I have been led to believe they are different. I would like help in sorting the confusion. Thanks. Also is packing ...