Questions tagged [ideal-gas]

A gas that behaves as randomly moving, non-interacting molecules. This allows a simplified equation of state.

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In ideal gas $\partial_V(PdV)_T=P$

First $\partial_V(PdV)_T=\partial_V(PV)_T=P$ However, if write it by chain rule and $P=nRT/V$ $\partial_V(PdV)_T=P\partial_V(dV)_T+\partial_V(P)_TdV=P\cdot 1+\partial_V(P)_TdV=P+P=2P$. What's wrong ...
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163 views

Is RMS velocity of gas particles always larger than the average velocity?

Is the root-mean-square velocity of gas particles always faster than the average velocity? I've done a couple of calculations with both and that seems to be the case; however can you make a formal ...
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103 views

Computing entropy for two ideal gas systems

I'm trying to show the processes in the following rows are different. On the left column there are two boxes of $12$ particles each. Both boxes are divided in three imaginary and equal rooms with $...
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993 views

Earth-like Atmosphere Breaks Second Law of Thermodynamics due to Conduction?

Consider a neutral gas around an Earth-like atmosphere with density, pressure and temperature given by $\rho,\ p$ and $T$ respectively. To keep the system a closed system assume that the atmosphere is ...
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107 views

Piston cylinder assembly with separate compartments [closed]

Consider an ideal gas in a piston cylinder assembly, initially divided into 3 compartments by impermeable diathermal membranes. The compartments initially have the same mass and temperatures, but ...
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57 views

Ideal gas law for a gas with nonuniform temperature

Consider a piston-cylinder assembly with an axial temperature distribution $T(x)$ in the gas. The entire system, however, has a uniform pressure. Can I still apply the ideal gas law $PV = nRT(x)$?
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67 views

Isn't the equation of state for ideal gas valid in the free expansion?

I am deeply confused.... For a thermally insulated ideal gas expanding freely, I think that $PV^{\gamma}=cnst$ must hold. Through the equation $PV=nRT$, it must be that $TV^{{\gamma}-1}=\rm constant$. ...
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86 views

Forces on piston

When we heat any container containing a gas fitted with a piston, a part of the heat energy increases its internal energy and the rest does work on the piston. But when the gas does work the piston, ...
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382 views

Effect of mass on gas pressure

I know that $\mathrm PV=nRT$ and this equation does not take mass into consideration. However, $\mathrm P =Force/Area$. And weight is a force which is equal to $\mathrm mg$. Will gas pressure be ...
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Isothermal bulk modulus of an ideal gas

To calculate isothermal bulk modulus, I have two methods. Method 1: $P_1V_1=P_2V_2=K$ $B=-\dfrac{(P_2-P_1)V_1}{V_2-V_1}$ $B=-\dfrac{(\dfrac{K}{V_2}-\dfrac{K}{V_1})V_1}{V_2-V_1}$ $B=-\dfrac{\dfrac{...
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197 views

Why does frequency of collision increase as volume decreases?

Today I was presented with the following scenario: A syringe with one end sealed and with a frictionless piston is dipped into cold water. What is the change in frequency of collision(ie, more or ...
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Ideal gas assumption for the Temperature Jump Distance calculation

I want to calculate the contact conductance between two surfaces that have a thin layer of air (on the order of micrometers) between them. A quick literature review shows that the heat transfer ...
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83 views

Why does $H_2$ have $C_V$=$7/2 R$ at high temperatures, while the total number of degrees of freedom is 6?

The two hydrogen atoms have 6 degrees of freedom in total. Of them, $3$ contribute to translation, $2 $contribute to rotation and $1$ contribute to the vibration. I know that the vibrations motion is ...
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How do I calculate the height of a given that a bubble rises from the bottom of the lake to the top of the lake? [closed]

I am stuck on a physics problem which involves calculating the depth of a lake given that a bubble increases two times in volume as it rises from the bottom of a lake to the top of the lake. The ...
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264 views

Ideal gas in piston with resistor, isothermal expansion, calculate velocity

I have this problem from Resnick's Physics, 5th ed, problem 31-15, sorry I only have the Spanish version (i'll translate): A resistor R=550 ohm, connected to an external battery, is inside an ...
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74 views

Any physical basis for this pressure vs flow model across flow restriction?

Recently read a journal paper on biomedical engineering where the authors used the following expression to model the expected volumetric gas flow, $Q$ from a pressure potential $\Delta P$ : $$\Delta ...
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265 views

Negative Heat Capacity of an Ideal Gas

Sign Conventions:- Work if done by system, heat given to system and Increase in Internal Energy implies $+$ change. Otherwise $-$. Ok, consider an ideal gas in a container with perfectly conducting ...
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diffusion of gases in both directions

I am a technical specialist that works on analytical equipment. Gas chromatographs use carrier gases like helium and hydrogen that flow through long, thin glass tubes coated on the inner surface with ...
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1answer
180 views

Canonical ensemble near absolute zero

In the canonical ensemble for an ideal gas of $N$ bosons, the partition function for $T\to 0$ scales like $$Z\sim e^{-\beta\epsilon_0N},$$ when $\epsilon_0$ is the lowest (non-degenerate) single ...
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625 views

Sound pressure levels: Why doesn't blowing in someone's ear cause hearing damage?

Quoth Wolfram Alpha: 5000 Pascals ≈ 0.51 × lung air pressure that a typical adult human can exert (≈ 9800 Pa) ≈ 50 × sound pressure at the threshold of human pain (≈ 100 Pa) And yet the ...
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218 views

Egg in the bottle demo gas law identity

so, i've taught chemistry and physics for 15 years, and yet i still need help with this. the classic egg in a bottle demo i'm referring to is the one done with a lit object being placed in the bottle....
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616 views

Is the ideal gas equation valid for a flowing fluid?

The assumptions for an ideal gas are: the gas consists of a large number of molecules, which are in random motion and obey Newton's laws of motion; the volume of the molecules is negligibly small ...
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Star formation from gas? [closed]

Considering the ideal gas law , where pressure is always positive, i wonder : How can gravity turn a gas into a star? Yes gas has mass too. But a light gas obeing the ideal gas law seems ...
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105 views

Do ideal gasses really not interact with each other at all?

I'm having trouble understanding how two different ideal gasses occupying the same space affect/interact with eachother. Presumably, the answer to this question is as in the title: Ideal gasses do not ...
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37 views

Gas and liquid at different temperatures. How do I read the pressure?

I am a homebrewer. My setup holds 6 5-gallon kegs. Previously I had my CO2 inside the chest freezer I've converted into keezer. I've recently acquired a 6th keg and there is not room for both 6 kegs ...
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139 views

Work done by isothermal gas expansion followed by an isobaric expansion

A mono-atomic ideal gas of $2$ moles undergoes an isothermal expansion which makes its volume double. Then the volume of the gas is doubled again through an isobaric process. Given a starting ...
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195 views

Entropy of the ideal gas - How to derive the $S(U,V,N)?$

I'd like to derive the $S(U,V,N)$ function. In the lecture, we were using the $S=k_b \log(\Omega)$, some combinatorics, approximation, $6N$ dimensional spheres, etc. But I'd like to avoid that if it's ...
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180 views

Calculating entropy for the surroundings the logical way doesn't work

EDIT: I was missing the entropy change due to temperature change and now everything makes sense Suppose I have $n$ mol of an ideal gas in a chamber with a piston with volume $V$ at pressure $P_{1}$ ...
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54 views

Bulk viscosity in jump conditions for shock waves

One of the jump conditions for shock waves can be derived from the Navier-Stokes equation and it takes the form (see Shu, Gas dynamics, p.212) $\dfrac{d}{dx}\left(\rho u^2 + P - \frac{4}{3}\mu \frac{...
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Maximum velocity of piston pushed by a gas on adiabatic irreversible expansion [closed]

I am currently studying for my country's IPhO selection process and thermodynamics has to be one of my favorites right now. However I came across a very odd problem that is making me revise a lot of ...
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What is the difference between $\langle v^2 \rangle$ and $\langle |v|\rangle^2$?

Let us take an example: the ideal gas. I know for example that $\langle v \rangle^2$ is different than $\langle v^2 \rangle$, as $\langle v \rangle=0$ due to no preferred direction. But know if I ...
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209 views

Using the work-energy theorem in thermodynamics

If an $n$ moles of an ideal gas (one atomic) is heated at constant volume $V$ from temperature $T_1$ to temperature $T_2$, the amount of energy heat needed can be calculated as $$ Q = nC_V \Delta T $$...
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Is Avogadro's law applicable for atoms or just for molecules?

I notice that online definitions of this experimental law always say, molecules or atoms. From the Wikipedia article on Avogadro's Law: $${\frac {V_{1}}{n_{1}}}={\frac {V_{2}}{n_{2}}}$$ The ...
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476 views

How to calculate density of states for different gas models?

There are a couple examples I'm trying to understand, all in a box/square of length $L$: For an ideal gas in 2-D with $\varepsilon=\frac{\hbar^2k^2}{2m}$:$$ D(\varepsilon)=\frac{L^2m}{2\pi\hbar}\,.$$ ...
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Internal energy of an ideal gas independent of any thermodynamic variable at constant temperature

https://physics.stackexchange.com/a/199976/170242 In this answer (in the link), it is mentioned that "the internal energy of an ideal gas is independent of volume when considered as a function of ...
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106 views

Am I getting the definitions of these thermodynamics quantities right?

I started thermodynamics mostly through independent study and basically built up my own definitions of terms that appeared to fit with what was going on. They seemed to work but my question is whether ...
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Expansion work of monatomic gas [closed]

Monatomic ideal gas takes up Q heat, while its volume increases but the pressure stays the same. How many %s of the heat is for expansion work? I think it is an isobar process but how should I ...
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55 views

What's the difference between dP<0 and dP>0?

Assume a situation shown in the image1 below. The column is filled with gas. The number density is denoted as $n$, and the mass of each particle is $m$. Take the axis downward. By considering the ...
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201 views

Definition of chemical potential of an ideal gas contradicts equation of state?

My geometric intuition breaks down horribly regarding advanced concepts in thermodynamics, such as the use of thermodynamic potentials. My question here is therefore about the latter concepts (in ...
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156 views

1st law of thermodynamics mcq question [closed]

Given the following question: could someone explain how A is correct and D is wrong? I'm a little confused. Since its a closed cycle shouldnt there be no energy lost?
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ideal gas adiabatic process

I know how it is derived and here is my concern: $dU=-dA=-d(pV)$ p is not a constant here, it depends on V. So my logic says, ...
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464 views

Is entropy zero at temperature zero for ideal gas?

we know that internal energy of the system is defined in terms of temperature as $(3/2)kT$. so if temperature is zero so internal energy is zero. and that means that particle will not have much ...
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On the existence of ideal gases

I am following Statistical Mechanics course at the university, and my professor came up with this statement: Ideal gas model is nothing but a fictitious ideal system that doesn't really exist in ...
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Why is there a constant in the ideal gas law?

Why do we have constants? Consider, for example, the ideal gas law, $$PV = nRT \, . \tag{ideal gas law}$$ Sometimes I believe that the constant is there in order to make the equation work (make the ...
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Feynman Lectures I 40: Is the homogeneity of the Maxwell-Boltzmann distribution a result of, or a pre-requisite for its derivation?

I refer to hand-waving a lot in this post. That is not to say that it was in appropriate. Feynman pretty much said at the outset that his treatment of thermal physics was going to be less than ...
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87 views

Does the saturated vapor pressure depend on the external pressure or just only temperature?

So far I understand that the saturated vapor pressure is the pressure exerted by a substance in liquid (or solid) vapor equilibrium, and that this pressure depends on the temperature of the system. ...
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Difference between ideal gas law and saturated vapor pressure

Little bit confused on what each of these concepts are. So far i understand that the ideal gas law tells us how much pressure an amount of gas exerts on the walls of a container. I also understand ...
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Why the force is equal to minus the variation of the momentum for a gas particle hitting the wall?

Why when one calculates the force exerted by a gas on the wall we say that the force is equal to (for a single particle ): F = - $\Delta p$ . But to me , it should be : $F = (p$f $- p$i ) $= -p$i - $...
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53 views

Ideal gas reversible cycle

So I got this cycle on a quiz. As of my understanding, in the transformation 1-2 there is also received heat and given heat ( sorry if that's not the english exprimation, not my first language). Now I ...
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205 views

Boltzmann constant times absolute temperature

I came across a research paper with the following paragraph in a research paper. Research Paper section I am trying to fetch fluctuations from Kirchhoff's matrix and that is why I am interested in ...