Tagged Questions

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

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Pressure versus Volume system problem

A rigid air tight container of volume 1.5 m$^3$ is filled with a gas of density 8 kg/m$^3$. The container has a density meter that allows us to watch the changes in density in the gas in the ...
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2answers
157 views

Is there a physically motivated “trick” to evaluate this convolution?

I'm working on adapting some of the formalism in this paper to a system I'm working with. The part I'm interested in amounts to convolving a density profile $\rho(r)$ with a smoothing kernel ...
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1answer
104 views

What does the absence of hydrodynamic entrance effects do to the momentum conservation Equation?

I am trying to do a homework problem where we re-write the mass, momentum and energy conservation formulas for downward flow in a vertical pipe and it says "where all hydrodynamic entrance effects ...
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2answers
249 views

Fluids and density [closed]

Is the answer C? Assume that you look at three objects that all float on water, but have different shapes and sizes. Which one has the greatest density? A.The object that has the largest fraction of ...
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0answers
30 views

concept of density in gravitational lensing

I may just be being very dense (no pun intended) but i'm reading up on gravitational lensing and it seems to require a notion of density (e.g. see here) I'm working on a question involving light ...
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2answers
123 views

Thermodynamics of thermometer

Mercury is used in thermometers because it increases in length significantly due to rise in temperature, However, mercury has high density relative to water, which means more inter-molecular forces ...
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0answers
168 views

Acting on a current density operator in second quantization

I have a current density operator in second quantization in the form: $\hat{J} = -i\left(\psi^\dagger \partial_x \psi - (\partial_x \psi^\dagger) \psi \right) $ operating on some state: $|\phi ...
2
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1answer
93 views

Does salt $NaCl$ as a solid float in molten salt?

I know ice floats in water because it's crystalline structure causes $H_20$ solid to be less dense than $H_20$ liquid. Is the same true for salt because it is crystalline? If not why?
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2answers
150 views

Point masses and infinite densities

Point masses are masses who don't have volume. It is said that they are infinitly dense, but I though division by zero is undefined hence you can't define the density for a point mass because ...
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3answers
445 views

Archimedes' principle for two liquid layers

Problem: I have a cylindrical vessel of height $H$ and radius $R$. There are two liquid layers in the vessel. The first has density $D_1$ and height $h_1$, the second has density $D_2$ and height ...
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2answers
384 views

What causes liquids to have different densities?

We know that different liquids have different densities and it is mass over volume. When the mass is more, then its gravitational attraction will be higher (Compared to other lighter atomic mass ...
2
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1answer
116 views

Is there some special cutoff density after which spacetime “collapses” and forms a black hole?

With crude calculations following densities can be approximated: Given that radius of proton is $1.75×10^{−15} m$ and it's mass is $1.67 × 10^{-27}kg$, this gives density of proton to be $\dfrac ...
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0answers
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Charge distribution and potential in a 1-dimensional quasistatic system

Suppose you have an 1-dimensional system with a charge distribution $\rho(x)$ (not given) moving with an speed $v(x)$ (not given), calculate the potential $\phi(x)$ and the charge distribution ...
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1answer
273 views

How to calculate upward force of a container filled with a less dense gas than the surroundings where it is placed?

I'm trying to determine the calculation which would state the upward force of a container where the density inside the container is less then that in which it sits. Does this calculation change with ...
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2answers
188 views

In a glass of water, will an ice cube ever touch the bottom?

No matter the mass of the ice cube or the level of the water, will the ice cube ever touch the bottom of the glass or will there always be a layer of water in between?
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3answers
648 views

What is the relationship between Maxwell–Boltzmann statistics and the grand canonical ensemble?

In the grand canonical ensemble one derives the expectation value $\langle \hat n_r\rangle^{\pm}$ for fermions and bosons of sort $r$: $$ \langle \hat n_r\rangle^{\pm} \ \propto \ ...
11
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4answers
972 views

Why does this perpetuum mobile not work?(Gases and Densities) [duplicate]

I recently came up with the following concept. It is very simple, and may have been thought of before. A picture says more than a thousand words, so here is it explained in a picture: Note that ...
0
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2answers
109 views

Density of a material in different dimensions

Hope this isn't something trivially wrong, I am a beginner in classical mechanics. But anyways, say there is a cube, of side length $a \ \text{cm}$. The volume of the cube is $a^3 \ \text{cm}^3$. If ...
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2answers
1k views

When can a surface charge density exist?

In my syllabus about electromagnetism, they state: "This surface charge density will not always be present, e.g. when considering two non-conducting dielectrics such surface charge density remains ...
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1answer
295 views

Confusion about the probability cloud

What is the meaning of the electron probability cloud? I understood it to mean that the electron has a probability to be found in a certain postion before measurement, but now after reading ...
1
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0answers
85 views

Nucleon-nucleon collision density

Can someone help me derive expression for nucleon-nucleon collision density as a function of transverse plane coordinates (ultra-relativistic heavy ion collisions) using Wood-Saxon nuclear densities( ...
0
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0answers
84 views

What is the electrical field of a capacitor plate as a charge approaches

The electrical field for a charge density $\varrho(r)$ is $${\bf E}({\bf r})=\frac{1}{4\pi\varepsilon_0}\int \varrho({\bf r}')\frac{{\bf r}-{\bf r}'}{\hspace{.1cm }|{\bf r}-{\bf r}'|^3} \mathrm ...
3
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3answers
330 views

Why is gas(oline) in gas stations sold by volume (as opposed to mass)?

Fluids (including natural gasoline/petroleum) have variable volume based on the ambient temperature for the same mass of fluid. So, really, the amount of gas that you're filling your car with depends ...
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3answers
8k views

Why does ice have a lower density than water?

Can someone explain me why is ice lighter than water? As I know, all solids are usually heavier than the liquids (correct me if I am wrong).
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0answers
109 views

What are the physical limitations on energy density for a useful, safe battery, and are we anywhere near them?

Energy storage seems to be a fairly major bottle neck in technological development at the moment, especially for important things like clean-energy powered transport, but also for consumer goods. Are ...
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2answers
217 views

What structural or chemical mechanism results in Saturn having a density less than that of water?

According to NASA’s Saturn fact sheet, Saturn has a density considerably less than water and the lowest density of the major planets. What compositional and/or structural mechanisms are theorised to ...
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3answers
858 views

Is it possible that the force due to gravity varies based on density?

It took a while to find relativity and the various subtle (or unmeasurable) effects it has within our universe. Is it possible that, although Newton's gravitational force equation leaves out any ...
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1answer
256 views

How to calculate flux density in a toroidal CT that has an electrical line pass through the center?

I have seen the Biot–Savart equation, but I don't know how to apply it. Does the conductor length need to be considered finite and equal to the core length, or will the calculation be for a conductor ...
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1answer
102 views

How can I model a two dimensional and three dimensional equivalents of one dimensional delta dirac (impulse) function?

I just started to read the book 'A Brief History of Time' by Stephen Hawking. Actually When he was talking of the idea of infinite density 'thing' before big bang suddenly the mathematical function ...
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1answer
99 views

Singularity in a black hole [duplicate]

This might be a naive question, but how can an object such as a black hole singularity have infinite density but finite mass? (For example, we can approximate the mass of a black hole based on ...
0
votes
1answer
561 views

Density of Air in a balloon

A balloon with volume 2800 m³ is heated up to 60 °C. If the outer temperature is 12 °C and the air has a pressure of 960 hPa, what is the density of the air inside the balloon and on the ...
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1answer
4k views

Besides water, which substances are less dense as solids than as liquids?

You can read everywhere about water's extraordinary property of expanding when frozen, thus the reason ice floats on liquid water. What other substances do this? There are claims of mercury, silica, ...
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2answers
116 views

Possibility of determining the mass of the water without a scale?

Possibility of determining the mass of the water by knowing water volume, water temperature and atmospheric pressure. I want to know if I can determine the mass of $V=0.01\,m^3$ of water in ...
6
votes
3answers
196 views

Hollow gold bar

A scammer got a hollow gold bar and fills it with a combination of lead and air, with the same average density as gold. What's the simplest way of discovering the fraud? I know that x-rays will see ...
3
votes
2answers
331 views

Determination of mass density distribution of an object

This is a follow-up to a previous question How can you weigh your own head in an accurate way?. My purpose is not to restart the flurry of more or less humorous jokes (which are not such a bad thing ...
3
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1answer
540 views

What is electron density and its unit in plasma physics?

I can not find (through Google search engine) that what means electron density and what is its unit? and what is there relation between electron density and electrical conductivity?
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1answer
271 views

What happens when I increase the density of a stellar object so that its mass surpasses the Schwarzschild limit?

We know that every object that has mass, also has a Schwarzschild radius $r_s$: $$r_s = \frac{2Gm}{c^2}$$ With $G$ being Newton's gravitational constant, $m$ the mass of the object and $c$ the speed ...
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2answers
227 views

Help with buoyant forces and liquids (high school physics)

I am stuck on this problem. Here is the scenario Suppose you send a balloon (filled with air) from your submersible craft to the surface of the ocean, $10.8 \text{ km}$ above, with the ...
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3answers
323 views

Possible Rock Density Determination Method

I am trying to do density determinations on a large number of individual rock samples. I know the standard dry weight - wet weight methods but have an idea that might be better. I can buy a ...
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1answer
112 views

is gravitational force dependent on objects radius?

I was told that objects with the same mass but different sizes (or densities) will have a different gravitational pull. Is this true? If so, why?
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3answers
382 views

Is it possible to have a singularity with zero mass?

A singularity, by the definition I know, is a point in space with infinite of a property such as density. Density is Mass/Volume. Since the volume of a singularity is 0, then the density will thus ...
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2answers
339 views

Calculating air density lapse with altitude (specifically, pressures)

This might be a bit more of an engineering question, but I'm calculating air density drop-off with altitude, and I'm having some problems calculating the pressure (I'll run through my method). This ...
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3answers
7k views

Is there a compound denser than the densest element?

I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a 10x10x10 cm cube of iron, lead etc. ...
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1answer
115 views

Measure density with the help of buoyancy

I am trying to derive a formula to calculate the density of a irregulary shaped object. I can measure the (false) weight of the object in pure air (of known density), and the (false) weight of the ...
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1answer
595 views

Do glass panes become thicker at the bottom over time?

Is it true that a glass window, that has been placed in a wall for about 10 years or more, is thicker on the bottom than on the top? I can vaguely remember my physics teacher saying that this was ...
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1answer
148 views

Would Oscars made of pure gold bend? [duplicate]

Since gold is often considered "soft": If the Oscars were made of pure gold (instead of gold-plated britannium), would they bend (deform) over time?
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2answers
1k views

Water vs Milkshake being sucked through a straw

Consider water in a glass being sucked through a straw. The water rises up in the straw because of a pressure gradient introduced by the sucking action. Now, change the liquid from water to something ...
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1answer
119 views

Energy transfer by a rotating disk at a constant RPM

Does Energy transfer from a rotating disk at a constant RPM into ambient fluid depend on disk material density? Why?
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1answer
467 views

Does increasing the density of a solution decrease the rate of temperature change?

I did an experiment to compare whether salt water (5% concentration of salt) or fresh water of the same volume took longer to heat up to a certain temperature. We found that salt water took longer to ...
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3answers
2k views

If hydrogen and helium are lighter than air, why won't liquid hydrogen and liquid helium defy gravity?

Title says it all. If hydrogen and helium are lighter than air, why won't liquid hydrogen and liquid helium defy gravity?