# 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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### Kinetic theory: Computing the flux density

The problem is the following: Set up: A sealed box contains a collisionless classical gas of particles, each of mass $m$. The box is coupled to a thermostat such that interactions between particles ...
16 views

### What probability denisity function would we consider if we wanted to initialise the component-wise velocities of a gas rather than the magnitudes?

Some context: I'm trying to make a molecular dynamics simulation in python and want to initialise the velocities of all particles according to some temperature T. The Maxwell-Boltzmann distribution (...
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### Why is cold air drier than warm air?

I know this has been asked before but i could not understand the replies. If someone has a simpler answer please do so. My simple Geog lesson just said that warm air molecules are further apart ...
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### What is the heat needed for an isothermal expansion of gas and why do physics and chemistry yield different answers?

One mole of a certain ideal gas is contained under a weightless piston of a vertical cylinder at a temperature $T$. The face of the piston opens into the atmosphere. What is the heat supplied in the ...
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### Why hot air cools off when it rises and expands? [closed]

I know that when air is heated it will have a high temperature that leads to high kinetic energy of particles,high pressure, low density. But, why when hot air rises and expand they get cold ? ...
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### Air Pressure & Atmospheric Pressure

I know that when air is heated it expand, and thus it makes density decrease, pressure increase, and temperature increase. But, why hot air cools off when it expands and rise ? Please answer with a ...
28 views

### How to calculate temperature change of a decompressing gas canister?

I am currently trying to find the temperature drop of a decompressing gas canister for rocketry club to make sure that it doesn't get too cold to affect nearby components like electronics. The ...
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### Energy transfer from Engine exhaust gas to hot water

I am looking at waste heat recovery from gas fuelled engine exhaust. I am using a data sheet for a Jenbacher J620 engine-. I assume the engine exhaust flows out through some kind of waste heat ...
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### Kinetic theory of gases

In the kinetic theory of gases I know that the temperature of an ideal monatomic gas is proportional to the average kinetic energy of its atoms. If I give energy to the system through irradiation, ...
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### Ideal Gas Equation Assumptions

The two assumptions are that the gases are points of mass that move, they have no volume and that there is no interaction between other molecules. But even the points of mass can collide with each ...
1k views

### Why does the ideal gas law exactly match the van't Hoff law for osmotic pressure?

The van't Hoff law for osmotic pressure $\Pi$ is $$\Pi V=nRT$$ which looks similar to the ideal gas law $$PV = nRT.$$ Why is this? Also, in biology textbooks, the van't Hoff law is usually instead ...
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### Finding the critical $P,T$ and $V$ in VDW / Berthelot equation

I need to find the critical pressure, temperature, and volume from the Berthelot equation: $$\left(P+\frac{a}{vT}\right)(v-b)=RT$$ in terms of the parameters: $a,b,R$ . The problem is pretty similar ...
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### How many degrees of freedom does the air have?

Very simple question that I am overthinking... But how many degrees of freedom does the air have? Assuming let's say the air is confined in a rigid box.
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### If liquid and gas are both chaotic states of matter, what's the difference between them on the molecular level?

I'm a laywoman in physics and recently found myself pondering about the matter reflected in the title of this post. To make my question more precise from the mathematical standpoint, let's suppose ...
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### What is the fundamental mechanism(s) that allow energy to be stored in a compressed gas?

I thought I could simply Google this question and get a straight, simple answer. Everything but. Is it simply that the mean velocities of the molecules are increased (so molecular kinetic energy) as ...
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### Do gases become more opaque as they are cooled or compressed?

So recently I was thinking gases (at least colourless ones) are more transparent in their gaseous state than liquid state. And as we talk of continuity in liquid and gaseous state (fluids) is it ...
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### Why does breakdown voltage increase with pressure in gas insulators

There are two main theories which I have been reading into Townsends avalanche effect and Paschen's curve. I understand that as electrons move from negative to positive electrode they collide with gas ...
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### Two versions of Diffusion coefficient

I found two versions of the Diffusion coefficient, first: $$D=\frac{\pi \lambda }{8}\overline{c}$$ Where $\overline{c}$ ist the particles mean thermal velocity and $\lambda$ the particles mean free ...
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### Linear or quadratic damping term in Euler's equation?

In the Euler equation with frictional damping $$\begin{cases} \rho_t + (\rho u)_x = 0 \\ (\rho u)_t + (\rho u^2 + P(\rho))_x = \color{red}{-\alpha \rho u} \end{cases}$$ sometimes I see a different ...
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### Time period of piston performing SHM

Suppose I have a closed piston-cylinder system which contains an ideal gas,and it is being compressed adiabatically by the piston which has a vertical orientation,and in doing so,the piston is ...
25 views

### Thermodynamics: work done on a gas

How much work would be done on a gas contained in a metallic container at high pressure if it is leaked slowly into the atmosphere?
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### Is static electricity an “electrical discharge”?

I understand that any electrical discharge such as corona discharge is the scattering of electrical charge through a medium; most likely a gas ("discharging charges"). As the air is a gas, would it ...
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### Time between collisions and collision time

I’m having a hard time really getting how the relationship between pressure and square mean velocity of a gas is derived for one single reason...seems like I don’t really know what impulse is. From ...
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### Finding entropy of Nitrogen at low pressures

I'm trying to calculate the entropy of gaseous Nitrogen (assumed to be ideal) at a particular state in a thermodynamic cycle. I know the pressure, temperature, and specific volume, but all property ...
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### Bifurcation of Van Der Waal equation for real gases

Van Der Waal's equation leads to a cubic equation in v of the form $$Pv^3-(bP+RT)v^2+av-ab=0$$ This equation has 3 roots for $T<T_C$ and one root for $T>T_C$ I understand why region ABCDE is ...
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### Would connecting two pressurized gas tanks together exchange gases?

While the answer to the title is inevitably "Yes", allow me to clarify. I have no background in physics, so please bare with me and I'll be happy to add any clarifications requested in the comments. ...
27 views

### Pressure on wall by gas molecules

Pressure by gas molecules of an ideal gas on the walls is P = nRT/V But by derivations P=1/3 nmv², where v is velocity. Are they equal, if yes, then it means that we can deduce volume of free path ...
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### What is the air pressure on a magic wall that reflects all incident molecules perpendicularly to itself?

I am a total laywoman in physics and humbly hope that physics experts of this SE can help me find the answer tо my question. A friend of mine and I had an argument about that question, and I am very ...
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### What pressure should be used when calculating Work in gas expansion?

The formula for the work done by a gas is $W = \int_i^f P dV$. What pressure should be used here? I'm asking because I thought you use the gas pressure at every instant of an expansion. For an ideal ...
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### In the evaporation process, will the temperature of both liquid and vapor molecules decrease?

The theory says that the molecules that become vapor "take" some energy from the surrounding molecules (the latent heat of evaporation). As a consequence, the temperature of the liquid falls a bit. I'...
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### Is there a difference between two molecules, of the same substance, one in liquid phase and the other in gaseous phase?

What are the main differences? Volume, shape, or what?
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### Why are sound waves longitudinal even though they are mechanical energy?

Waves on a string, ripples on a pond are transverse waves generated by mechanical energy and in the simplest form oscillations. Sound also is a form of energy and basically oscillations. What makes ...