# Questions tagged [density]

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

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### Is there a limit to the amount/number of photons that can be measured/detected in one single point in space?

I've heard that there is no limit to photon density. So if we have a bunch of photons going in the same direction, or with some going in opposite directions but with arbitrarily low energy(so that a ...
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### Why does R. Feynman states that in a low-density gaseous enviornment, the pressure exerted by the molecules is proportional to density?

I was reading Dr. Feynman's Physics Lectures Vol 1 and in the first chapter about the atomic theory, he gives the example of a piston-type container where he explains the proportionality of force and ...
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### Understanding the Singularity of Black Holes [duplicate]

Could someone please explain how we know the singularity has an infinite density and zero size/volume? From reading around online black holes have a defined radius (the event horizon) but this isn't ...
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1 vote
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### Densest substance under STP?

The densest pure element is said to be osmium at $22.59~\rm{g/cm}^3$ (with iridium a close second). Wondering for a long time if pure osmium is also the densest substance - or could there be an alloy ...
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### Change in density of wave on a string

Suppose a string of linear mass density $µ_1$ which is tied at one end to another string of density $µ_2$. A wave pulse is sent from end of the string of density $µ_1$. This wave when reaches tied ...
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### Feeling of coldness in heights

We know that due to buoyancy the cold air sinks and warm air floats above it due to it being less dense than cold air. Then why do we feel cold as we go to greater heights/hill stations and feel hot ...
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### Why do some objects tend to sink after some time in water even if they float at the start?

I have observed this phenomenon in swimming pools: I have seen many dead insects floating on the surface, but after some time some they tend to sink down without any external influence. Why does this ...
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### About the floatation of bodies on a surface

A body which is partially floating in water is in equilibrium due to the balancing of the buoyant force and the weight of the body. Now what will happen to a body floating on the surface of the liquid ...
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### Why don't we get infinite energy from a continous emission spectrum?

If the spectrum emitted contains all kinds of photons of all kinds of wavelengths, doesn't that mean it should have infinite number of photons of very small energies? For example, 4000 angstrom to ...
1 vote
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### Reading of weighing machine [closed]

Consider the following situation: The following ball is released and just after that reading of weighing machine is noted. Now, it is given that Mass of container is $M$ and mass of liquid is $m$ and ...
1 vote
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### Do we use $p=ρgh$ to calculate the pressure caused by any materials or just by liquids?

If instead of a liquid in those shapes the shape was a rigid body made up of iron or any other thing, Can I still use the same ration $p=ρgh$ to calculate pressure at some height $h$?
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### Why does the milk spurt out of the packet when a packet filled with milk is punctured with a sharp object? What is the physics behind it?

I want to know the theory of pressure applied in this experiment. Is it the pressure of milk applied on the packet that the spurting happens, or the pressure of the sharp object or the pressure of ...
1 vote
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### Density of substances slightly above and below the freezing point [closed]

I would find a table useful where I can look up the density of various substances just above / just below their freezing point in order to find out how the density changes as the phase changes. But I ...
1 vote
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### Convert reflection spectrum to optical density

I have the spectrum 380-780 nm at 10 nm intervals of a nearly grey patch. I need to input its optical density into a calibrating programme. This is what a densitometer would do - but I don't have one ...
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### Why does refractive index increase with concentration of the medium?

I learnt that lights decrease velocity in a medium during absorbance and emission of its energy in the charged particles in the medium. From the Beer's law, I read that absorbance is directly ...
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### Newton's law in terms of pressure [closed]

Why can force be written as f = -dpda where p is pressure and a is acceleration yielding the equation $\rho a= \nabla p$ where $\rho$ is density?
1 vote
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### Speed of sound and density [duplicate]

The speed of sound in a medium is inversely proportional to the density of the medium. Why?
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### What algebraic manipulations make this two equations equivalent?

I have a very basic mathematical backround. I kind of understand the concept of a partial derivative but i dont know what algebraic manipulations make this two equations equivalent (the ones on 2A.4) ...
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### How to compute the Reynolds number of a fluid without knowing the density?

I am running a simulation with LAMMPS involving a colloid suspended in a fluid. Simple shear is applied creating flow. My question is, how can I calculate the Reynolds number of the fluid given I don'...
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### What was Laplace's argument?

In this Wikipedia article about Van der Waals gas equation there is a very nice derivation of the law in the conventional part. But in the last steps it is stated that pressure reduced on the walls of ...
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### What would happen if the atmosphere were compressed until it has the density of water?

What would happen if the atmosphere were compressed until it has the density of water? If the atmosphere were compressed to the density of water, it would be covered by a layer of height, say $h$. ... 588 views

### Why does a chimney sometimes create a "draft"?

Sometimes, while lighting a fire in a wood stove in a basement, the smoke does not exit through the chimney like it normally does. Rather, a large amount of the smoke seems to get "pushed back&...
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### The impact of electron density on photoelectric effect

I'm a second year radtion therapy student and i'm looking at the attentuation of KV energy beams. I'm wondering why for the photoelectric effect to occur does there need to be a high amount of 'inner'/...
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### Are number of molecules per unit volume constant (say in water)?

I have a very basic doubt. I knew that $$\rho = \frac{m}{V}$$ And since mass is constant so volume may decrease or increase depending upon density. But suppose I have water in a beaker and I read it ...
1 vote
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### Adding water into a container with water and an iron block inside [closed]

My question is directed towards the concept of buoyant force but to show it I have to examplify it with this A cubic block of iron of density 7.8g/cm³ and side L, floats in a container with water of ... 89 views

### Can ice ever sink in the water? [closed]

It is generally taught in schools that ice is less dense than water, which is why ice is seen to float on the water. But isn't ice heavier compared to water at a temperature quite above 4 degrees ...
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### How to solve Poisson's equation for the potential?

I'm working with a spherical symmetric system and have found it to have mass density $$\rho=\frac{1}{4\pi r^2} \frac{dM(r)}{dr}.$$ Poisson's equation says $\nabla^2 \Omega(r)$ = $4\pi G \rho(r)$. I'...
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### How do viscosity and density of fluids correlate?

Many viscous fluids seem to also have a higher density. Water seems to be an exception especially when looking at its solid state, but for most other examples, especially of high viscosity fluids, it ...
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### DFT Hartree Potential with Periodic Boundary Conditions

Suppose we have a periodic crystal. Let $\rho(r)$ be the electronic density, and let $a$ be the lattice vectors. Due to the periodicity, the Hartree potential can be written as \begin{align} V_H(r)&...
1 vote
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### Smoothing a density field in Python [closed]

I want to smooth a density field. It's represented as a contour plot. The data were given from a 3d array. A smoothed field should be periodic but what I take does not have periodicity. The code I use ...
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### Deriving Formula for Archimedes' Principle [duplicate]

Let B be the buoyant force acting on an object and W be the weight of the fluid it displaces. I can't derive the formula for Archimedes' Principle: B = -W I have found plenty of derivations for B = W ...
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### Correlation of surface tension and density of fluids

When we plot the surface tension $\gamma$ against the density $\rho$ of different fluids, there seems to be an approximately linear (not proportional) correlation. I wonder if it's possible to explain ...
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### Why is the kinetic energy of a fluid given as an integral?

The kinetic energy of a fluid occupying a region $\Omega \subset \mathbb{R}^3$ is given by $$T = \frac{1}{2}\int_\Omega |v(x)|^2 dx.$$ I am looking for some physical intuition on where the above comes ...
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### Peculiar velocities, density gradients and geodesic motion [closed]

I am studying cosmology and I have learnt that the galaxies in the universe can be represented by a pressureless fluid. Therefore, they have a geodesic motion (the so called matter frame). When ...
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### Is anti-matter less dense than a vacuum?

Fill a balloon with Helium and it floats (it is buoyant in air). 'Fill' a balloon with a vacuum (not withstanding the difficulty of a structurally stable void) and it is more buoyant as it is less ...
1 vote
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### What exactly is linear mass density?

Consider a thin rod of length $l$ and linear mass density $\lambda = 2x$ where $x$ is position from origin (let this be the left most edge of the rod). Now my question is, if let's say I consider a ...
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### Is the continuity equation used to **define** the current density?

Recall the continuity equation: $$\frac{\partial}{\partial t}\rho+\boldsymbol{\nabla\cdot J}=0$$ Given $\rho$, there is obviously not a unique solution $\boldsymbol{J}$, but I guess one could choose ...
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### How do I define the density of system?

Lets' say I have a solid system made with N atoms in a box with a volume of V as shown below. In this case, the number density of my system is simply $\frac{N}{Volume\; of \; occupied\;3D\; space}$. ...
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### What does the change in volumetric rate with respect to change in volume mean for an expanding fluid mean?

What does $$\frac{d}{dV}\left(\frac{dV}{dt}\right)$$ mean, and how does it apply to an expanding fluid? $V$ is volume, and $t$ is time. I used the divergence theorem to relate the volumetric rate ...
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### Find the density of liquid Y

While solving the question, I had some confusion, that we know net pressure at equal depths must be equal in fluids so if we try to equate the pressures at depth of 8cm (instead of the conventional 10 ...
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### Problem with an alternative explanation of the tea leaf paradox

Just recently I learnt about Einstein's and Thomsom's explanation of the tea leaf paradox. In my mind I always thought that the leaves move to the center when stirring the tea due to the fact that ...
1 vote
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### Oil and gravity

If we put oil in a cup and then we put water in the same cup, the oil will rise to the top. But isn't gravity supposed to stop that. Like if the oil was at the bottom isn't it supposed to stay there ...
1 vote
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### Is there any empirical relation between hardness and density?

Inspired from: Why is diamond not the most dense substance? The answer says that there is no general relation between hardness and density but we do have a connection between packing/bonding of atoms ...
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