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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5answers
179 views

What would happen to a room full of neutrons?

Suppose you had a room full of neutrons at standard pressure and temperature. Would it be just like a room full of hydrogen?
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101 views

Combined gas equation in adiabatic process

We know that $PV^{\gamma}=constant $. There are also 2 more equations which relates $P$ & $T$ and $V$ & $R$. Is there any equation which relates all these 3 things like ideal gas equation? ...
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98 views

Weird and unintuitive results from thermodynamics/hydrostatics for an isothermal atmosphere and possible explanation(?)

It is known from hydrostatics that for a fluid in equilibrium in a gravitational field, $$\frac{dP}{dz} = -ρg$$ Let us from now on suppose the atmosphere is isothermal and has temperature $T$. We ...
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1answer
52 views

Is it possible to estimate the average time until entropy decreases a certain amount?

We all know the classical Joule free expansion experiment for an ideal gas: we trap an ideal gas inside half of a cylinder. Then, we open the door and the gas expands to the whole cylinder. After ...
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3answers
813 views

What is the meaning of $PV^\gamma=$ constant in an adiabatic process?

In an adiabatic process, heat transfer doesn't occur and hence $\Delta U=-W$ (an increase in internal energy due to work done on the system or a decrease in internal energy due to work done on the ...
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0answers
31 views

Difficulty in calculating effusion rate using the kinetic theory of gases and statistical mechanics

I have tried multiple sources and methods, but my attempts at a proof of the number of particles leaving a gas using statistical mechanics keep finding the same wrong result. I have tried to read ...
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1answer
98 views

Confusion while deriving kinetic-molecular theory of gases

How many of the molecules will collide within a given area A of the wall normal to the x-direction in some fixed time interval Δt ? On average half the molecules have a positive x-...
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3answers
289 views

Atmospheric pressure, density and temperature variation with altitude

I'm trying to understand how one can calculate pressure, density and temperature of the atmosphere as a function of altitude. My assumptions are mostly sourced from https://en.wikipedia.org/wiki/...
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1answer
128 views

Ideal gas law in combustion engine analysis

I'm studying this derivation of the Diesel engine theoretical efficiency: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/diesel.html Here the ideal gas law is used in many occasions. Why is it ...
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1answer
101 views

Why is potential energy zero in this calculation of partition function?

In section 7.2 of Rief's "Fundamentals of Statistical and Thermal physics". While calculating the partition function for ideal gas he writes: $$ \begin{array}{l} \displaystyle{Z' = \int{ \...
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1answer
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As I inflate a balloon at a constant temperature does the pressure of gases inside it on the balloon increase?

I know it's a dumb question but I am having a little misunderstanding with Boyle's Law. Shouldn't pressure be inversely proportional with Volume , or this is meant for the pressure outside of gases ...
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49 views

Temperature increase of a gas in a period of time [closed]

Pardon my simple question for you physics geniuses but: I'm attempting to simulate hot air balloons for a game. I have a gas of known volume, temperature, pressure and density inside a confined space....
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42 views

Is translational/rotational KE conserved when diatomic molecules collide?

When two diatomic molecules of a gas collide, is the collision elastic or inelastic? Is rotational KE also conserved/not conserved? My guess is that translational KE is conserved while rotational KE ...
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3answers
120 views

If we push a syringe with gas in it In space , would its temperature increase permanently , Violating Boyles Law?

On Earth If we push a gas syringe very fast Inwards , work is done on the gas by W = PV which increases the Kinetic energy of molecules in it and since Temperature is Average Kinetic Energy ...
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2answers
39 views

Does Temperature decrease on increasing No of mol of a gas in container ?

ideal gas Law says that PV/T = nR So does lets say you increase n and you Keep P and V constant should'nt the temperature decrease?
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1answer
143 views

Confusion between molar gas constant and boltzmann constant?

So after reading reading through internet I came to the conclusion that : For gas of any element PV/T = Constant of proportionality and that constant of proportionality can be expressed in ...
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1answer
63 views

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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2answers
186 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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1answer
105 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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1answer
1k 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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1answer
131 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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1answer
63 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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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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100 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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1answer
475 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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251 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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2answers
89 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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3answers
294 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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1answer
78 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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2answers
306 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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34 views

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
193 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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2answers
921 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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2answers
231 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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1answer
711 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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164 views

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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3answers
107 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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1answer
38 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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145 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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1answer
209 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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1answer
199 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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1answer
59 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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1answer
84 views

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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3answers
142 views

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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2answers
235 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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5answers
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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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2answers
553 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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2answers
1k views

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 ...