Questions tagged [density]

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

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Why is the energy density of the dark energy component~magnitude as the density of matter now, when the two evolved in different time periods?

WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). This is equivalent to a ...
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The moment of inertia of a hollow cylinder is lower than that of the "corresponding" solid cylinder (same density, outer radius, height):

This is a self-answered "question" for those scratching their heads after learning, e.g. in class, that the moment of inertia of a hollow cylinder is higher than that of solid cylinder. Note ...
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Density, density operator, and number operator in quantum mechanics

Density is the number of particles per unit volume. But in quantum mechanics, the density operator is defined as $$ \hat{\rho}(r) =\Psi^\dagger(r) \Psi(r) $$ here $\Psi(r)$ is the field operator. The ...
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What does ultra-rarefied gas mean?

Without using any math, can you explain to me what a rarefied gas is? And then what an ultra-rarefied gas is? I'd like to understand it from a conceptual level if you can make connections to other ...
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Why can't water flow fast when the impact speeds up? (the incompressibility)

I'm a high school student studying physics in Korea! Please understand my poor English skills. In the text below, the liquid is said to be destructive and incompressible. And he says, "I the ...
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A black hole made of ordinary water?

Years ago I read a book about black holes. It said it is theoretically possible to make a black hole from ordinary-density materials, like tap water. It would take a lot -- about the size of a galaxy ...
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Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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Why is the charge density of a rod in $\rm C/m$ and not in $\rm C/m^2$?

Is it because we consider the rod to be a 1-dimensional straight line? Or is there any other reason? I know it sounds like a stupid question but I am confused in this regard.
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Do we know what causes the deviation in nuclear radius?

Wikipedia: The radius of a typical nucleus, in terms of number of nucleons, is $R=A^{1/3}R_0$ where $A$ is the mass number and $R_0$ is $1.25\space\rm{fm}$, with typical deviations of up to $0.2\...
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Given the mass density $\rho(x):=1/|x|$, a ball centered in the origin contains a finite amount of mass

Consider the following charge/mass distribution: $$\mathbb{R}^3\ni x\mapsto\rho(x):=\frac{1}{|x|}$$ One can show that a ball centered in the origin contains a finite amount of charge/mass, i.e. that $$...
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Can a body, made of semi-light material, theoretically float on water (ocean) if it has enough airtight space under it? What are the risks? [closed]

Hello scientists and amateurs, Basically, if I put a quite big airtight container on water and use it as a type of float to wander around ocean (Let's say pretty big, size of a half a football field). ...
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Why are fluid properties continuous with respect to spatial variables?

I get the intuition behind the continuum hypothesis in which we may treat a fluid as a continuous distribution of matter if we’re studying the fluid at a macroscopic scale. But the continuum ...
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Why is the buoyant force of some object equal to the gravitational force of the fluid displaced? [duplicate]

I’ve done the derivation for the buoyant force using the difference in pressures and after everything is said and done the buoyant force of some object in a fluid is equal to the gravitational force F=...
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The relationship between the pressure at the base of a container and the normal reaction on the container

This is NOT a homework question. The question here is just an example. In this question, after both the liquids are mixed and a homogenous mixture is created, I found the effective density of the ...
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Why doesn't an object with less density than a fluid float 100% above the fluid level?

If an object is less dense than that of a fluid, then why doesn't it float completely over the fluid level? Why does some part of the object remain submerged in it? Can there be an object that can ...
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Material which becomes less dense as pressure is applied

Is it possible that a material may become less dense as more pressure is applied, without a phase transition? My initial layman thought is no because it seems to me that "density" is a form ...
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When light ray travels from rarer medium to denser medium with angle of incidence =0 , does its speed or direction changes?

The speed of the light will change , it doesn't remain constant and it will decreases , but what about the direction of light, will it change ?
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Fluid pressure at a point [closed]

If there be $n$ fluids arranged in strata of equal thickness and the density of the uppermost be $\rho$, of the next $2\rho$, and so on, that of the last being $n\rho$: find the pressure at the lowest ...
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Boat and Rock Problem: Is there a possible scenario in which the opposite will occur, in that the boat would decrease and the water level rise?

My question concerns the person in a boat with a rock brain teaser. You are in a rowing boat on a lake. A large heavy rock is also in the boat. You heave the rock overboard. It sinks to the bottom of ...
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How can the mass of a teaspoonful of nuclei be equal to the mass of all the people on the earth?

In our topic regarding density, a calculation was shown where the mass of one teaspoonful of nuclei was equal to the mass of all the people on the Earth. I understood the calculation, but couldn't ...
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Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe than the 'present' closer part of the universe?

Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe?If the universe is expanding, the galaxies should have been much closer in the past so should we see a larger ...
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Why do individual helium atoms rise?

Balloons full of helium rise because the buoyant force is great than the gravitational force and the buoyant force is due to a pressure difference between the top and bottom of a ballon but why ...
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Calculating density of a liquid based off 2 pressure readings at a known distance

I'm working on a project at home where I want to track the change in the density of a liquid over time (as it converts sugars to alcohol) using 2 submerged pressure sensors at a known distance apart. ...
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Can every global conservation law be written as following? [duplicate]

Consider a physical quantity $\phi$ that is globally conserved. From Feynman's argument (in his volume 2 I think), which states that local conservation follows from global conservation due to special ...
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Name of property where density function after thermal expansion is related to the one before

I have a one dimensional rod of non uniform density $\mu(x)$ and length $l$, upon heating, it expands into a rod of length $l'$. Under heating, the density function may change form to some other ...
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Would helium-3 be a better lifting gas than normal helium?

Buoyant force is proportional to the mass of the fluid displaced minus the mass of the volume doing the displacing. Thus the best choices of lifting gas are nothing(vacuum), hydrogen, and then helium ...
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Does gravity pull anything up?

I was watching a smoke simulation tutorial (VFX), the trainer mentioned that gravity needs to be turned "on" to have a smoke simulation rise. He mentioned turning gravity "off" the ...
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Is muon-catalyzed fusion a form of pycnonuclear reaction?

Muon-catalyzed fusion is a form of fusion where the electron in a hydrogen atom is replaced by a much heavier muon, which orbits 196 times closer than an electron and allows it to approach other ...
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How would the density of a sphere of uniform composition vary with depth due to gravity?

Let's say we have a planet-sized sphere of some uniform material floating isolated in space, not rotating, with the only significant gravity acting on it coming from its own mass. How would the ...
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When elements inside the Sun form plasma is there any stratification as densities of different cores are practicaly same?

When elements inside the Sun form plasma is there any stratification as densities of different cores are practicaly same?Where do electrons accumulate? I pressume it may depend on electron to core ...
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Conditions of Bernoulli's equation: Do they matter?

Bernoulli's equation: $P+\frac{1}{2}\rho v^2+\rho gh=\text{constant}$ There are some conditions to use Bernoulli's equation. But often we neglect them. The fluid must be incompressible. But we use ...
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Fluid buoyancy why is $F_{\text{buoy}}=W_{\text{air}}-W_{\text{w}}$? [closed]

I am having trouble understanding the reasoning for a solution for a buoyant forces question. The density of a human body can be calculated from its weight in air $W_{\text{air}}$, and its weight ...
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Will a penny ever stand still in the water at a certain depth?

Let’s say I drop a penny in the deepest part of the ocean having a certain depth. Would the penny become buoyant enough to stand still in the water, since the density of water increases with depth? ...
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Would a Pythagorean Cup (Greedy Cup) work with oxygen?

Since oxygen is denser than air (From google: The density of air is 14.4 and that of oxygen is 16.0) could I in Theory and/or in practice fill a greedy cup with ...
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Uniform density representation in space in terms of Dirac delta and step functions in different types of coordinate systems of different shapes

As we know, the presence of a particle at a point $\textbf{r}_0$ could be represented by the density function, $$\rho(\textbf{r}_0)=c\delta^3(\textbf{r}-\textbf{r}_0),$$ Whare, $c$ is a constant and, $...
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Current density of given current

Given is the current $I(t)=I_0\cos(wt)$ on a thin line on the $z$-axis from $z=[-l,l]$. I guessed now that the current density must be $\vec{j}(t)=\delta(x)\delta(y)I_0\cos(wt)\hat{e}_z$. But im not ...
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Given current densities of dark energy, dark matter and normal matter in universe currently, calculate ratio of density of dark matter at CMB to now? [closed]

I stumbled upon this question while preparing for the astronomy Olympiad. It is question #17 in this pdf. Given that dark energy is vacuum energy, and that the densities of dark energy, dark matter ...
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Independence of Gravitational acceleration with depth if local density of earth is 2/3 of average density

Show That the acceleration of gravity in a vertical mine shaft is independent of the depth if the local density of the Earth is 2/3 of the average density. Assume that the Earth is a spherically ...
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U-Tube Manometer vs Application

I am running an active vacuum distillation process on ethylene glycol (EG) and I am getting what I would call weird results. The setup is as shown in the picture. The vessel is held above 145°C to ...
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Is there a way to calculate how high the water will go? [closed]

Calculate how high the pump pumps water into the reservoir to fill it to 68O hl. When filling the reservoir, there will be a work done of 9MJ. (ρ = 1000 kg / m2) I am having a real hard time with this ...
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Is it correct that a sphere of water the size of Venus's orbit would instantly be a black hole? [duplicate]

Let's say I have a genie and wish for a sphere of water at earth density the size of Venus' orbit - would it be instantly indistinguishable from a supermassive black hole due to the schwarzchild ...
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Which system has the higher density (mass per unit volume): dry or humid air?

I wonder if wet and dry air have the same density, I think they must have the same because both are "air" however, if we set both with the same pressure and temperature to try to standardize ...
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Why speed of sound in gas depends on density but not pressure?

The book tells that with decrease in density there is an increase in speed of sound in gas but with increase in pressure there is no change in the speed of sound in gas. Why? If pressure increases ...
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It is quite weird for me that the equation of areas are represented in a trignometric way of electric flux density

I've been handling the electric flux tube above. $$ S_{1} :=\text{ area of the upper vertical cross section } $$ $$ S_{2} :=\text{ area of the upper vertical cross section } $$ $$ S_{} :=\text{ ...
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How to calculate charge density [closed]

I am trying to solve this exercise: Using this formula calculate electric intensity $$ \vec{E} = \frac{\vec{c}\times \vec{r}}{r^4} $$ $\vec{c}$ is constant vector, $r$ is magnitude of the position ...
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Black holes are not infinitely dense [closed]

If a black hole is infinitely dense, then therefore its terminal velocity in a determined field of gravity would be greater than the speed of light which is impossible, therefore a blackhole is not ...
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Do materials sort in order of density in space the way they do near Earth?

Obviously, if you drop materials into water near the surface of the Earth, they sort according to densities, with the least dense materials near the surface, and the most dense materials near the ...
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Force required to pull a floating object under water relative to mass and volume

Let's say you have multiple objects with the same density floating on top of the water but they have different volumes and masses. Does it take any more force to pull the different objects under the ...
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Accelerated system + container . Tension to be calculated

So there is this question which is as follows the tension in a string holding a solid block below the surface of a liquid is $T$, as shown. It is given that the density of the solid is more than that ...
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Why do rotten eggs float in water?

My mom told me that one can check whether an egg is rotten by sinking it in a glass of water. If it floats, then it is rotten. I didn't find any explanation for this phenomenon. If anyone knows one, ...

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