All Questions
1,111 questions
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A sinking feeling
Does adding water to an object that is in water make it sink.... or is it the lack of air in the object that makes it sink?
If I had a hollow sphere that as a whole floated on water. I divided that ...
- 1
-1
votes
1
answer
50
views
Should density be considered in the role of the strength of a gravitational field?
Ok, here me out. Black holes are usually formed from the compression of the mas released during the death of a supermassive star, however they have the same mass as the star (or even less). Yet when ...
-3
votes
1
answer
84
views
Which equation does $j=ρv$ refer to?
I found this statement in my physics book, however I am unable to recall/ understand which equation this is, moreover the variables are not defined (I don't know what $v$ refers to). The relation $j=...
4
votes
0
answers
50
views
How to construct the density matrix for the BCS wavefunction: $2 \times 2$ or $4 \times 4$?
Question Body:
In BCS theory, the ground state wavefunction for superconductivity is often written as:$
\Psi_{\text{BCS}} = \prod_{k} \left( u_k |00\rangle_k + v_k |11\rangle_k \right),
$
where$ |00\...
- 319
0
votes
2
answers
43
views
Density distribution in flexible material when squeezed
A full rubber ball, a foam mattress: when we push against them a bit locally, the overall form stays the same, except around where we push, but the volume is reduced, so there must be density ...
- 761
0
votes
0
answers
20
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How can I calculate the density of moving air
I am experimenting to find a relation between the Coefficient of Power of a Wind Turbine and its Swept Area. To do that, I need to find the change in the air density before and after the air passes ...
2
votes
1
answer
43
views
Evolution of Matter Density Under Newtonian Gravity
I want to examine the physics of gravity. This subject has two parts: how the gravitational field influences the behavior of matter, and how matter determines the gravitational field. I want to ...
5
votes
3
answers
307
views
How would I calculate the gravity at a given depth on a planet with two distinct layers with different densities?
If you assume a planet to be one density then gravity just scales linearly from the centre to the surface, and you can use the surface gravity equation.
How would you go about working out the gravity ...
- 153
1
vote
2
answers
98
views
If I drop a rock on the Sun, does it fall right to the center? [duplicate]
Hydrostatic equilibrium is what determines the diameter of the Sun. The internal gas pressure inside the sun equalizes with the pressure from Gravity on the outside. This is what determines the size ...
- 4,861
1
vote
3
answers
133
views
Identify the hollow sphere without making a sound! [closed]
Recently I came across this problem,
Can we use the principle of floating and sinking here?
Both the spheres have the same mass.
So,
$\rho\cdot V=\text{constant}$
$\implies \rho \propto \frac{1}{V}$
...
- 422
3
votes
1
answer
127
views
Radial distribution of ideal gas in a cylinder
today I have a computational doubt and a theoric one.
Starting with the theoric question, suppose I have a generic ideal gas in a cylinder and each particle is subject to a potential dependent on the ...
- 115
0
votes
1
answer
53
views
Why is linear charge density $dq/dl$ and not $q/l$?
If linear charge density is charge per unit length then shouldn't it be $q/l$. Why is it $dq/dl$ instead? Wouldn't that mean it is only being calculated for a small element and not the whole length?
0
votes
4
answers
147
views
Are solids with the same density as air possible?
I was recently considering how to keep a super cheap ultrasonic range sensor safe from balls (or blocks, or whatever) going down ramps (or otherwise moving at high speeds) in a physics lab. It struck ...
1
vote
4
answers
152
views
Does a hollow sphere and solid sphere (of same outer radius) have different volumes?
I've seen volume of a hollow sphere mostly defined (in books) as volume of its equivalent solid sphere minus the volume of the hollow region/cavity. But often a solid object of some material (density)...
- 11
1
vote
0
answers
44
views
How dense exactly is a vacuum in space (because of quantum fluctuations)?
You hear all the time that quantum fluctuations take up a given amount of energy in a vacuum, and this is how experiments such as The Casimir Effect gets the results that they do.
Have we actually ...
- 119
1
vote
1
answer
43
views
Oil and water configuration in a column
liquids of different densities arrange themselves in layers if they are immiscible: oil and water for a common instance. Let us consider a very long but narrow column filled with water and oil. I want ...
0
votes
1
answer
65
views
How to determine density of a granular material?
If you have a box full of granular material like sand grains and you know the diameter (possibly average) of the particles and the packing fraction (or volume fraction) with which they fill the space (...
- 1,433
0
votes
1
answer
37
views
What is the relative density contrast?
I was reading a paper talking about the KBC Void, a local underdensity in the Universe which we are also part of.
The authors calculated: "[...] the observed relative density contrast δ ≡ 1 − ρ/...
- 183
0
votes
0
answers
30
views
The criterion about maximally localized Wannier function (WF)?
I heard that as the value of "num_iter(tag in wannier 90)" is higher, spread of Wannier function (=WF) is gradually lower in wannier 90.
If so, is this procedure that minimize the spread of ...
0
votes
0
answers
25
views
What is meant by optical density?
I’m trying to research about optical density for a school task, but the definitions online are confusing me.
From what I’ve gotten, optical density is:
how much the intensity of light is increased or ...
2
votes
2
answers
99
views
If air was the same density as water, but still a gas, could you swim in it?
I'm not talking about normal physics here, as it would be extremely hard to get such a situation to arise, this is more a hypothetical question.
If air was the same density as water, while still ...
- 23
0
votes
1
answer
156
views
Friedmann Equation and a contracting universe
Consider a universe with a nonzero curvature and matter.
One can write the Friedmann Equation in this universe as such:
$$\frac{H(t)^2}{H_0^2} = \frac{\Omega_0}{a^3}+\frac{1-\Omega_0}{a^2}$$
Where $H(...
- 453
1
vote
2
answers
92
views
Densities at a Point
I was recently reading Purcell’s text on electrodynamics. He generalizes Coulomb’s Law by introducing charge densities. However, what does the density at a point represent? (Since Purcell took charge ...
0
votes
1
answer
76
views
How to find critical density?
In Cosmology critical density is defined as the minimum density for a flat universe to keep expanding, by Friedmann Equation:
${\left({\frac {\dot {a}}{a}}\right)^{2}={\frac {8\pi G}{3}}\rho -{\frac {...
- 453
1
vote
1
answer
171
views
How to understand critical density?
In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
- 453
0
votes
2
answers
88
views
Why are planets denser as you approach the center?
Gauss' law says that the net electric force inside a hollow, uniform, not rotating sphere is zero. Since gravity is also proportional to the inverse square of the distance, I assume this should apply ...
- 545
1
vote
1
answer
68
views
Can the observable mass within the observable universe be estimated without using $G$?
The estimation of the baryonic mass of the observable universe can be done through its density parameter and the critical density, which depends on the gravitational constant $G$.
Is there a way to ...
1
vote
0
answers
39
views
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 ...
1
vote
0
answers
27
views
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 ...
- 2,165
3
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0
answers
27
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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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1
vote
1
answer
85
views
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 ...
- 187
0
votes
1
answer
69
views
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 ...
2
votes
3
answers
216
views
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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0
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51
views
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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15
votes
5
answers
3k
views
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 ...
- 317
0
votes
1
answer
78
views
What does this equation for density mean?
What does this equation for density mean?
$$\rho = \lim_{\Delta V\to\varepsilon^3} \ \frac{\Delta m}{\Delta V}$$
- 21
0
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0
answers
31
views
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 ...
- 11.1k
0
votes
2
answers
164
views
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
$$(-\...
- 171
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1
answer
60
views
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 ...
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3
votes
1
answer
99
views
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?
- 147
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votes
3
answers
103
views
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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4
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7
answers
383
views
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 ...
- 531
3
votes
2
answers
548
views
Is it possible, by monitoring the brightness of stars, to find a “copy of the Earth + Moon” near them?
More than a dozen Earth-like planets have been discovered around nearby stars based on observations of changes in the brightness of their sun as they pass across its disk (transit events). If an Earth-...
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1
vote
2
answers
44
views
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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0
votes
2
answers
97
views
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 ...
0
votes
0
answers
39
views
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 ...
- 137
1
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0
answers
96
views
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 ...
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1
vote
1
answer
128
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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 ...
- 177
1
vote
0
answers
40
views
Confusion over definition of phase space density
This is from page 58 of Kardar's "Statistical Physics of Particles":
Therefore, there must be a very large number of microstates corresponding to the same macrostate $M$.
This many-to-one ...
- 180
2
votes
2
answers
137
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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 ...
- 21