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Questions tagged [density]

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

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Matter density estimates in the 1980s

Liddle (2015, p.67) writes: "From the crude estimates that a typical galaxy weighs about $10^{11}M\odot$ and that galaxies are typically about a megaparsec apart, we know that the Universe cannot ...
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What is the so-called momentum density? [duplicate]

What is the so-called momentum density? I am reading the paper by Pitaevskii, in which he stated that it is well-known. He studied the nonlinear Schroedinger equation, but it seems that the concepts ...
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Why does the density of saturated $\rm NaCl$ and $\rm H_2O$ saline increases after reaching 90 Celcius? Values from www.omnicalculator.com

Values are obtained from www.omnicalculator.com/physics/water-density. The saline is always at maximum $\rm NaCl$ saturation at all temperatures.
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Would a cubic light year of ice cream collapse into a black hole? [closed]

I was looking in my Wolfram ChatGPT book and I happened upon the question, "How many calories in a cubic light year of ice cream?" The software used for the book could not answer so I tried ...
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What does Density really mean? [duplicate]

We say that density is mass per unit volume and if an object has high density that means it has more amount of matter contained per unit volume but as we know mass is not the amount of matter ...
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Current density and units

I have a question about current densities. My understanding is that there are 3, volumic $(\vec{J})$ , surface $(\vec{K})$ and linear $(\vec{\lambda})$. The first question is, that if J is volumetric, ...
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Gravitational binding energy of a sphere with radially-dependent density

As stated here, the gravitational binding energy of a uniform sphere is: $$U = \int_0^R -\frac{G}{r} \left(\frac{4}{3}\pi r^3\rho\right) \left(4\pi r^2\rho\right) \mathrm{d}r.$$ I want to know if the ...
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Is air less dense than chimney exhaust?

So there is this question in my text book that says us to arrange these following items by increasing density: Air, exhaust from chimneys, honey, water, chalk, cotton and iron. Everything is ...
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What does this equation for density mean?

What does this equation for density mean? $$\rho = \lim_{\Delta V\to\varepsilon^3} \ \frac{\Delta m}{\Delta V}$$
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Will a monolayer rise (or sink) in water?

I'm aware of the principles by which an air bubble rises in water . But what happens when a flat monolayer is placed beneath the surface, perfectly perpendicular to local gravity? Is there still ...
Carl Witthoft's user avatar
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Why is the 4-current a tensor rather than a tensor density?

I am trying to understand electromagnetism better in terms of tensors and differential geometry. First recall that (in the Lorenz gauge) the equation of motion for the four-potential $A^\mu$ is $$(-\...
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Where does the energy density of black holes fit in a figure with the different energy densities of the universe like matter and radiation? [closed]

Based on the black hole mass, giving that lower mass black holes have higher mass densities than higher mass black holes. Energy Densities `The density is dependent only upon the mass of the black ...
New's user avatar
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Does having a liquid (less dense than ice) above a floating (in water) ice cube, change the fact that the water level remains constant when ice melts?

An ice cube floats in water, with a liquid (less dense than ice) above it. When the ice cube melts will the water level go up, down or remain constant?
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The SI-unit of the cosmological constant (vacuum energy) is $\frac{1}{m^2}$. What does that have to do with Energy?

I just don't get how Energy is measured in $\frac{1}{m^2}$. Wasn't it measured in Joules? (source is https://en.wikipedia.org/wiki/Cosmological_constant#Equation)
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Water pressure at the bottom of a box changes drastically depending on whether a water column above is connected or not? [duplicate]

(For the purposes of this question I'm ignoring atmospheric pressure completely) Consider a hollow cubic box with side length $1\text{m}$ that we fill up with water. Now suppose we make a small ...
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What will happen if 10kg massed hydrogen ballon is released in air?

Generally, a hydrogen balloon would float when released in air, but what will happen when that hydrogen balloon has a mass of 10kg. Is it the density that decides whether an object floats in air or ...
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Balloon volume as it rises in constant atmospheric density

I've been considering an hypotetical situation and it's making me a bit confused. If anyone could give me any insight, I would really appreciate it! It is as follows: Let's pretend atmospheric ...
oquiefine's user avatar
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What will be the effect of density on relaxation time?

I am doing a Monte Carlo simulation of protein chains of various lengths. I change the densities of the simulation box. In my experiment, I found that density has no effect on relaxation time. Do you ...
user366312's user avatar
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For hot air at 150 °C have a density of 0.61 kg / m³, how to calculate the density for 160 °C? [closed]

I am working on a project draft and in an existing device, hot air up to 160 °C is generated for drying processes. I have found old documents where densities of air are reported at three different ...
Esat Becco's user avatar
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How did Newton determine the average density of Earth is twice the density of the surface rocks? [closed]

I've read that Newton determined the average density of Earth is twice the density of the surface rocks, but I can't find his computation anywhere. I presume he used the differential calculus in some ...
lee pappas's user avatar
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Electric current density definition

I'm just wondering why the current density $J$ is always defined as the amount of electric current traveling per unit cross-section area $J = \frac{I}{S}$, and not per volume unit $J = \frac{I}{V}$ so ...
ArziousYi's user avatar
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Is there no sense of 'absolute' in the universe?

Imagine we are talking about electric potential (e.g. gravitational potential or electric potential or whatever, it doesn't matter), then we have: \begin{equation} dV = \textbf{E} \cdot d\textbf{l}, \...
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Formula for air density given temperature, pressure, humidity

What is a good formula for air density (kg/m^3) given temperature (°C), pressure (hPa), and relative humidity? I tried implementing the formulas from here as follows: ...
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What is current density and is it a material property?

Current density has the units (Ampere/cm2) so by that is the total current flowing through a cross-section divided by the area of that cross-section. I'm working on developing a water electrolyzer ...
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Why does the density decrease equal the speed gain at Mach 1?

In this answer, it says : "At small Mach numbers, changes in speed cause negligible changes in density, but as Mach approaches unity, both are of similar magnitude. With Ma>>1 , changes in ...
Wyatt's user avatar
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Why does density decrease most above Mach ~0.3, and not as much below Mach ~0.3 in fluid dynamics?

In fluid dynamics, why does the density of air decrease more above Mach 0.3, and not as much below Mach 0.3? From what I've heard, it isn't a linear relationship but why is that? Asked differently, ...
Wyatt's user avatar
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Why is density considered constant over a length $dz$ but not pressure?

When deriving the formula for hydrostatic equilibrium in the atmosphere, one usually considers a slab of air at altitude $z$, of thickness $dz$ and cross section $A$, and then writes $$A(P(z)-P(z+dz)) ...
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Are there any physics reasons why the avoid-phase-transition technique used to make aerogel can't be done with a human immersed in xenon gas?

This question relates to this earlier question about what clever physics might be useful for subjecting a human to extremely high accelerations without damaging them. The specific idea here is similar ...
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When a ball is suspended in a fluid, why doesn't the pressure at the bottom of the container, and thus the hight of the fluid increase?

This was the answer to a question: When a metal sphere is dipped in a liquid of density $\rho$, with the aid of a thread, find the pressure at bottom of the vessel. Now I personally feel as the ...
math and physics forever's user avatar
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How to obtain spatial density from material density?

When you introduce the concept of the spatial density of an extensive property, a question of change of variables arises. Assume $\Omega$ is an extensive property, and $\omega$ and $\rho$ are its ...
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What's the difference between $\nabla\cdot(\rho v)$ and $\rho(\nabla\cdot v)$ as a physical intuition?

I'm currently learning on substantial derivatives in fluid mechanics and kind of understand how partial derivatives $\frac{(\partial\rho)}{(\partial t)}$ and substantial derivatives $\frac{(D\rho)}{(...
Lime nut's user avatar
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How can I find if it is gold or lead with waves? [closed]

I would find if a bowl is made of lead or gold know freqencies $w_{1}$ and $w_{2}$ and a vibrator induces waves at $z=0$ (and there are reflected waves). I do it with progressive plane electromagnetic ...
Dlouna.J's user avatar
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Archimedes' principle for gases

https://www.toppr.com/ask/question/archimedes-principle-does-not-apply-to-gases/ https://byjus.com/question-answer/archimedes-principle-cannot-be-applied-to-gases-true-false-1/ These two pages say ...
Apoorva Shukla's user avatar
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Confusion about the "Volume" in Archimedes principle

In Archimedes principle $F_b = \rho gV$. $\rho$ = density of fluid, $V$ = volume displaced by the fluid, and $g$ = gravity. If you have an object in the air, like a balloon, how does the volume work? ...
god david's user avatar
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How can I calculate the column density of Hydrogen of a galaxy?

I have the following data: Mass of HI of the galaxy and radius. I thought using the following equation: $N_{HI} = \int n_{HI} ds \quad$ where the number density would be $n_{HI} = \frac{M_{HI}}{m_H V}$...
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What is the effect of pressure differentials due to gravity on buoyancy force?

First of all, I am aware that this question has been answered in the past, however I have some follow up questions particularly regarded the argument posited in Why is Buoyant Force $V\rho g$? : When ...
darklyspaced's user avatar
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Why is the thermosphere more dense than the mesosphere or stratusphere?

According to my environmental science textbook and various sources on the web, the thermosphere has the second highest density of any part of the atmosphere. It falls only behind the troposphere which ...
Tyler Reynolds's user avatar
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Why is the density in GR equal to $\rho_0\dfrac{dx^0}{ds}\sqrt{-g}$?

In general relativity, the continuity equation says $$ \partial_{\mu}\left(\rho_0c\dfrac{dx^{\mu}}{ds}\sqrt{-g}\right) = 0 $$ with $\rho_0$ being the proper density, as seen by an observer who is at ...
K. Pull's user avatar
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Computation of Density for an Ideal Gas

I'd like to get some help deriving the following result: $$ \langle \rho(\mathbf{q}) \rangle = \frac{N}{V}$$ where $$\rho(\mathbf{q}) = \sum_{i}^{N}\delta(\mathbf{q}_i-\mathbf{q}) $$ and $\mathbf{v} = ...
Claudio Menchinelli's user avatar
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In fluid Pressure why we only take only force due to upper layer of fuids that is (density)$hg$?

Question In fluid Pressure why we only take only force due to upper layer of fluids that is (density)$hg$ But lower level also apply Force (up thrust from lower molecule of fluid) to maintain the ...
Vedant Date's user avatar
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How much salinity is needed for everyone to float?

Everyone floats in Dead Sea because it is so salty. Is it true in Great Salt Lake? How about Red Sea and Mediterranean Sea? Is it impossible for anyone to sink in these seas without added weight?
Michael Tsang's user avatar
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Radius of a ball to exceed the Schwarzschild Radius of the enclosed mass for a given density, for example universe's average density

Given a mass (energy) density of $\rho$, a ball of radius $R$ has a mass of $M(R)=4\pi/3\cdot R^3\cdot\rho$. The Schwarzschild Radius of this mass in dependence of $R$ is $$ r_s(R) = \frac{2GM(R)}{c^2}...
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Can we change coordinates to the Schwarzschild metric and interpret the result as the field corresponding to different matter distributions?

The Schwarzschild metric is the simplest solution to Einstein field equations. It describes the gravitational field around a static object with spherical symmetry. Perhaps a change in space ...
K. Pull's user avatar
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The linear growth factor of perturbations today should be 1 but I am unable to derive it

The definition of the linear growth factor $D(z)$ of perturbations in a cosmological setting is usually normalized to unity at redshift $z=0$. So, $$\delta(z) = D(z)\times \delta(z=0) \tag{1}$$ where $...
Matrix23's user avatar
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In a tokamak, is there a density-dependent limit on plasma current?

In a tokamak, the kink limit places an upper bound on the magnitude of the plasma current. For example, Parisi and Ball (2019) write this limit as (p. 246) $$I\lesssim\frac{1}{q}\frac{2\pi}{\mu_0}\...
SapereAude's user avatar
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Can the inertia factor of a black hole be used to infer its density profile?

The Sun's inertia factor of ~0.07 suggests a stark contrast between the density of its outer shells (very low density) and its core (very dense). The same applies to the rest of the solar system. ...
Mike Davis's user avatar
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Materials known to have higher density than Osmium at at high pressure and/or low temperature

This question is basically a modification of this. It's well known that Osmium is the densest material you can find at room temperature and pressure. I am curious at higher pressures (and lower ...
Sidharth Ghoshal's user avatar
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Measurement that takes into account air density and average kinetic energy?

I think in very thin air, high average kinetic energy of air molecules might still be perceived as cold because the number of impacts per unit time would be fewer. Is this a measure that combines ...
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Why are there 2 ways of predicting if an object will float or not?

I thought about it for a second, I have always thought that for an object to float it had to be less dense than water, and if it had more density then it would sink. But then if it sinks or floats ...
Alysid's user avatar
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Density of current in a Elementary Dipole

I can't understand this formula from my textbook, I'm trying to understand why the Density of Current is I*ΔZ(ΔZ is the lenght of the dipole) i mean a density of current would be I/Δs (with Δs the ...
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