Questions tagged [gas]

This tag is for questions relating to "gas", one of the four fundamental states of matter (the others being solid, liquid, and plasma). Gases follow certain laws known as the gas laws. These laws tell us about the behavior of gases i.e., the values and relations of temperature, pressure and volume etc.

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How can gas be cooled to liquid by expansion?

My friend works at a gas company and told me that in order to cool gas to low temperatures to form a liquid, they compress the gas and then cool it to say -10 degrees C, then let it expand and it gets ...
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Time taken to discharge a gas tank from an orifice

I have a pressurized tank with a total volume of 100L filled with nitrogen gas. The tank has an orifice of 20mm which vents out to atmosphere. The initial pressure in the tank = 100Mpa, and final ...
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Using venturi tube for gas sample dilution?

Say I have a proposed setup as per the following illustration (Fig 1.). A pipe carries an unknown mixture of compressible gas from left to right under a vacuum (black arrows). By introducing a ...
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Do pressure gradients affect diffusion speed for gases dissolved in liquids (scuba offgassing question)

This question comes from a scuba forum where we are discussing the technical scuba technique of breathing 100% O2 during stops at shallow depths to speed up the offgassing of nitrogen (N2) from body ...
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Ghost modes in Lattice-Boltzmann Method (LBM)

I've read a lot of documents on Lattice-Boltzmann Method (LBM) and "ghost modes" are commonly referred to. It is explained that they are unphysical modes that can pollute the numerical ...
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How does transition from supersonic to subsonic flow occur?

I was working with orifices that were restricting flow and had to do a few calculation to verify some measurments (and to see if stuff was within tolerances). So I stumbled upon the calculations our ...
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3 votes
5 answers
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Why is the value of the heat capacity ratio $\gamma$ never less than 1?

For an monoatomic gas, the value of the heat capacity ratio $\gamma$ is 1.67 For diatomic gases, the value of γ (gamma) is 1.40 For many atomic gases, the value of γ (gamma) is 1.33 It's never less ...
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Why do elliptical galaxies not have large proportion of blue stars?

In an article it was given that elliptical galaxies are also formed when a spiral galaxy experiences continuous star burst thus depleting it's gas. So if this is correct than why do elliptical ...
1 vote
1 answer
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Maxwell Boltzmann distribution of speed [closed]

I am currently reading 'Concepts in Thermal Physics' and in its chapter 5 it is written that fraction of molecules traveling with speed between $v$ and $v+\mathrm{d}v$ corresponds to a spherical shell ...
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Pressure problem [closed]

I would like to solve the following problem by equating the pressure of the gas to the pressure due the weights, the force and atmosphere summed. How can I do that?
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Deriving an expression for specific heat from the first law of thermodynamics

I am self-studying Elements of Gasdynamics by Liepmann and Roshko. The authors define the specific heats of a gas at constant volume and at constant pressure as $$ c_v=\left(\frac{\partial q}{\partial ...
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Work on system with deformable boundaries

Background I am self-studying the book Elements of Gasdynamics by H.W. Liepmann and A. Roshko. In Chapter 1: Concepts from Thermodynamics, the authors discuss the First Law of thermodynamics and a way ...
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Equilibrium conditions established in the following scenario

If I was to create a setup as demonstrated below, where I have seperated a container into two halves of equal volumes, and in both halves, I half placed a different gas such that the final pressure ...
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3 answers
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Are we measuring static, dynamic, or total pressure?

Our lab is using a gas driven shock tube and are using piezoelectric sensors to measure the pressure that the shock wave produces. There are three sensors placed near the end of the shock tube, ...
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Sealed container with water with air bubble at the top — Basic hydrostatic principles fail? What is the pressure distribution?

From hydrostatics, it is said that when you have a liquid with no moving parts, the pressure does not vary on any horizontal slice of the liquid. The pressure only varies with depth. Thinking about ...
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The physics of variations on the Tesla Turbine

Is the tendecy of Tesla tubines to have low effiencies at low rpm caused by the very geometry of the disks? If, so, would a surface with the same area, but smaller radius, allow higher efficencies at ...
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0 votes
2 answers
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Does it become harder to compress a gas beyond a certain point?

While I was reading a book, I came across the following: Real gases show deviations from ideal gas law because molecules interact with each other. At high pressure molecules interact with each other. ...
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Simple Waves In Context of Riemann Invariants for One Dimensional Gas Flow governed by Euler Equations

I'm struggling to understand why in a simple wave, where one Riemann invariant is constant, why the characteristic curves along which the other Riemann invariant remains constant can't intersect? My ...
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Query about average velocity of gas molecule

When we can say that gas molecule average velocity is 0 and in which case it is not possible?
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1 answer
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Boyle’s law is applied for (almost) vacuum or not

Seems like a strange question, I want to know if Boyle’s law is applied for (almost)vacuum or not. Imagine you have a very very small sac which is filled completely with a fluid, and there is no air ...
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1 answer
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How to measure cylinder volume by water weight

I'm trying to determine the volume of a cylinder ( 25 cc approximately) in a precise way. First I'm measure the weight of the empty cylinder. Then, I'm add water to the cylinder until the water ...
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1 vote
1 answer
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Does the molecules in a gas approach a common speed over time?

and if they do - how does that actually work, if we look at the individual molecules 'bouncing around and about'? To my untrained eye, it appears it would require speedier molecules to collide with ...
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Van der Waals Equation, pressure, and the physical location of phase transitions

Returning to the Van der Waals Equation, I am stumbling over a few things which have never been quite clear to me; specifically the pressure. What I understand is that we are essentially modelling a ...
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Do gases speed up as they leave the barrel of a gun? If so why?

I am attempting to understand the exact physics behind the recoil in guns. I made an original post on reddit here specifically asking why gases exiting a barrel after the bullet leaves causes a sudden ...
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Henry's Law - When scuba diving, why is breathing pure $\rm O_2$ not the same as going to outer space?

In Short I had a discussion with my dive instructor the other day. He said that for "decompression dives" you can cut down decompression time to 1/3 if you switch to pure $\rm O_2$ at 6m. ...
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2 answers
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Hydrogen in soap bubble

I read somewhere that if hydrogen gas is passed through soap solution bubbles containing hydrogen are formed. My question is how these bubbles are stable! Hydrogen has much less density and mass than ...
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1 answer
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High Pressure Gas Scenario

Imagine a concept scenario in which there is a gas initially at standard atmospheric pressure contained in a standard mild steel cylindrical container of 10cm diameter and 1cm thickness. The pressure ...
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Doubt with partial pressures

I have a conceptual doubt about partial pressures. Suppose you have a closed system composed of a gas mixture, say Argon and Helium. If they behave according to Ideal Gas Law, then the molar fraction ...
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Interdifussion (binary diffusion) of Gases Boundary Condition Question

I was trying to solve this 1D unsteady state diffusion problem of two gases being mixed in a finite length of a tube over time (each end is blocked and impermeable). $$ \frac{\partial{C}}{\partial{t}}...
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1 answer
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Can noble gases emit infrared?

So from my understanding infrared radiation can be emitted due to rotation and stretches in a intramolecular bond and by the emission of an electron from the Paschen Series. Considering noble gases ...
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1 answer
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Calculating the required volume of Helium to put in a weather balloon [closed]

Question I am working on a project where we will launch a weather balloon with some scientific instruments tied beneath the balloon. We fill the balloon with helium from tanks of compressed helium. ...
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1 vote
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Determining the Boyle temperature (non-numerical) [closed]

I have been given the following equation of state (in relation to ideal gas laws): $$PV_{m} = RT \left( \ 1+\frac{8}{57}\left(\frac{P}{P_{c}}\right)\left(\frac{T}{T_{c}}\right)\left[1-4\left(\frac{T_{...
1 vote
1 answer
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Why do Greenhouse Gases absorb heavily in certain wavelengths? [closed]

What molecular properties make greenhouse gases absorb and reemit primarily IR radiation? That is, why are CO2, H2O and NO2 all greenhouse gases (GHGs), but others (such as helium and neon) aren't? ...
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1 answer
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Pressure in a gas as it becomes a plasma

Let's say you have 1 mole of hydrogen, and you heat it to the point that the H-H bond breaks and you get free hydrogen atoms, and then fully ionized hydrogen ions + electrons. When H2 becomes 2H (...
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Independent variables in thermodynamics

When we are dealing with a gaseous thermodynamic system, in books it's written that state of the system can be described by only two independent variables from the three $(p,V,T ) $. But it's not ...
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1 answer
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Van der Waals derivation: How is the pressure term exactly derived?

The van der Waals equation is: $$ \left(p + \frac{n^2 a}{V^2}\right)\left(V-nb\right) = nRT $$ I understand the volume term, it basically reduces the avalaible volume because the particles have volume ...
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Which electron shells make up the electron gas?

I want to get to know the concept of electronic gas. The first question is what does an electron gas consist of? If it consists for example of s or d electrons, I have a contradiction since the ...
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1 vote
3 answers
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Temperature of gas in accelerating container

This is a question whose different versions have been asked here a couple of times, but I don't find a clear answer. Does the temperature of a gas increase on accelerating the container? What ...
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If the inclination of a cloud of oxygen in outer space is to diffuse, then how do nebulae form?

If a cruise ship-size object in outer space were surrounded by a spherical cloud of oxygen, and there were no other bodies exerting significant gravitational force in the vicinity, would the cloud of ...
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1 answer
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How comes, that my book states that internal energy is dependent on volume?

Currently I am reading in the 11 edition of Physical Chemistry, Atkins in the chapter 1st law of thermodynamics. A few pages ago I learned the following: Makes sense for now, as far as I know it ...
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Cubic nature of van der Waals' gas equation [duplicate]

$V_m^3-\dfrac{RT+bP}{P}V_m^2+\dfrac{a}{P}V_m-\dfrac{ab}{P}=0$ So, At $T<T_c$ the above equation has three real roots say $V_1,V_2,V_3$, my doubt is what does this mean physically because for some ...
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Analogy between Curie's Law and the Ideal Gas Law

I've been trying to derive Curie's Law from the Ideal Gas Law for a while, since numerous sites on the internet state that they are analogous. However, if we apply the equivalences - $P \iff B$; $V \...
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1 answer
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Partition Function Question

I've been looking at calculating the Internal energy of a non-isothermic van de Waals gas, and in doing so have been researching the free energy and hence the partition function necessary to calculate ...
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4 votes
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Why is there no temperature difference in the Joule expansion experiment?

The whole system is adiabatic, and no heat exchange can take place. If the volume of the gas now doubles, it should actually cool down. That's why I don't understand $dT=0$
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Does Gas constant depend on molecular weight?

I came across the following question recently Calculate the difference between two specific heat of 1 g of helium gas at NTP. Molecular weight of helium = 4 and J = $4.186×10^7$ erg $cal^{-1}$ The ...
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Can a small amount of super-compressed gas cause much cooling upon rapid expansion and contact? [closed]

Generally, a small amount of mass in the range of $0.001$ - $0.01$ kg, even if chilled to tens of degrees below absolute zero in a hypothetical case to test extremes, even with 5000 specific heat and ...
2 votes
1 answer
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What exactly is the specific heat of a gas?

When I read that specific heat of Steam $(100 °C)$ is $2010 J/kg \cdot °C$ what exactly am I reading? What does specific heat of a gas essentially mean when it is not mentioned whether it was measured ...
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Can we have constant volume, and pressure in a system?

I was given a system where heat was added to a mass of Nitrogen gas in a canister at constant pressure? And given the volume of the canister as 1L, I assumed that the gas would expand to fill the ...
2 votes
1 answer
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How to Model the Forces within Inflatable Tube Man

I have a CPP setup for a cloth simulation. Currently it takes in forces for wind simulation and gravity and these forces act on individual "point masses". I'm trying to figure out how to ...
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1 answer
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Internal Energy of Van de Waals Gas when non-isothermic

I have been looking online and at previous posts for a way of calculating the internal energy of a gas or the work done on gas due to a changing volume. The problem that I keep encountering is that ...
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