What is Gas: Definition and 1000 Discussions

Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture, such as air, contains a variety of pure gases. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.
The gaseous state of matter occurs between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention.
High-density atomic gases super-cooled to very low temperatures are classified by their statistical behavior as either Bose gases or Fermi gases. For a comprehensive listing of these exotic states of matter see list of states of matter.

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  1. A

    Exploring the Possibility of a Hollow Current Sheet in a Gas Chamber

    There is a type of fusion pinch concept where instead of imploding a metallic liner filled with gas fuel mixture they have a chamber with a central electrode and outer electrode , filling the chamber with gas and then a arc discharge is initiated and this discharge drives the gas like a piston...
  2. I

    I Rotational Orientation of Monatomic Gas: Angular Momentum Effects

    In other words, is there a rotational orientation of each atom in a monatomic gas (and corresponding rotational speed conserving angular momentum) that affects collisions, or does a substance need to have at least 2 atom particles to have the orientation/rotational ability to have particle...
  3. A

    I Modeling the concentration of gas constituents in a Force Field

    Say there is a gas made up of two gas molecules: Molecule A and Molecule B. Molecule A has a mass: ma and mole fraction: na. Molecule B has a mass: mb and mole fraction: nb. The gas is at thermal equilibrium and has a constant temperature throughout itself (T) everywhere. It is placed in a...
  4. Eko

    Understanding Gas Dynamics in Piston-Crank Slider System

    I need help understanding what happens to the gas that is being worked by the piston, in the crank slider. My goal is to produce linear motion on an object up to a certain height and I'm wondering since the slider moves back during the rotation of the crank what happens to the object I'm trying...
  5. F

    How To Convert Kvs-Value For Water To Kvs For Gas?

    Hello, for an instrument I have given the Kvs-Value for water and I want to convert it to gas. Can someone help or explain the calculation? Of course I have the formulas for liquid and for gas KVs-calculation. How do I convert it? Thanks for the help Falko
  6. BenDover

    Oxygen gas produced by a Solid element + sodium hydroxide solution?

    Hello. I am wondering; which solid at room temperature elements produce oxygen gas when put into a solution of sodium hydroxide? Thanks
  7. B

    B Does gas expanding in a container cause displacement of the container?

    If particles are trapped in one half of a massless container by a barrier, and the barrier is removed so that the particles are allowed to expand to fill the whole container, it seems the center of mass would be displaced to the center of the container, whereas before it was located at the...
  8. B

    B Loschmidt's paradox and free gas expansion

    Following up on a previous discussion: https://www.physicsforums.com/threads/conservation-of-momentum-in-a-closed-system.1009693/post-6570341 An uncontained system of particles interacting only by elastic collision is the same as a gas undergoing free expansion. If, for simplicity, the...
  9. F

    What Is the Best Method to Calculate Gas Flow Through an Orifice?

    Summary:: How to find gas flow through an orifice that links two vessels at different pressures. Two vessels containing the same gas are linked through an orifice with diameter ##d##. The pressure inside vessel 1 is ##P_1## and the pressure inside vessel 2 is ##P_2##, and let's have...
  10. C

    I Ideal working gas for a Stirling engine

    I was reading around Stirling engines and there's something I am not sure about, can't see a logical explanation. So the ideal gases discussed are hydrogen and helium on account of their higher thermal conductivities. But I am unclear on what motivation there would be to use hydrogen which has...
  11. stuartsjg

    I Compressible choked gas flow through an orifice -- Excel formula

    Hello, I normally get these things working but I am a bit stuck as i don't feel I am getting sensible answers... The problem is simple (!): Q: Model the flow through an orifice with an upstream/supply pressure of 301BarA where the downstream pressure is in the range 1BarA to 301BarA. The...
  12. Melbourne Guy

    Good gas to trace lasers in a vacuum

    I'm excitedly close to the end of proofing my latest novel, and as happens, tweaking of passages occurs. In this instance, a fleet of warships travel together in a collective warp space bubble that has inconsistent gravity, especially around the edges. To facilitate shuttle transfers between...
  13. L

    I Fermi Gas Model: Energetic Degeneration & the Pauli Exclusion Principle

    In the fermi gas model, there is assumption that there is a 3D potential well, but there is "energetic degeneration" for each three index "nx, ny, nz". Now the problem is with that image, if there is degeration, for some level En there may be 10 distinctive state with same energy, so there is 20...
  14. B

    Verifying properties of Van der Waals Gas

    So a and b were pretty straightforward. Got stuck on part c. The question says they approximated Van der Waals in first order in a and b. So I started with that by rewriting Van der Waals eqn as ## p = \frac { N \tau } { V - Nb } - \frac {N^2a} {V^2} ## and I then Taylor approximated ## \frac...
  15. P

    A Temperature of gas in a cluster

    The following link states that: "since kT ~ mpv2. A typical velocity dispersion 700 km/s implies T ~ 6 x 107 K from this source alone." How did they get 6*10^7K ? When I try this, using mp = 1.67*10^-27 , k =1.38*10^-23 I end up getting 84 as a final answer, nowhere near 6*10^7. Can anyone...
  16. cianfa72

    I About Reversible vs Irreversible Gas Compression and Expansion Work

    Hi, reading the interesting Reversible vs Irreversible Gas Compression and Expansion Work insight by @Chestermiller I would like to ask for clarification on some points. In the second bullet at the beginning my understanding is as follows: consider an ideal gas contained in a cylinder...
  17. M

    B CO2 Emissions, Gas Vs Electric

    I read that a kg of gas provides about 13kWh (47.5MJ) of energy and that burning a kg of gas emits about 3.15 kg of CO2. So presumably a gas-powered engine emits 3.15/13 or about .242kg of CO2 per kWh of work, right? According to the EIA, the US electric grid emits about 0.85lb or 0.386kg of...
  18. C

    Calculate force to open a door with a gas spring

  19. F

    Internal Energy of an Ideal Gas

    a) We use the definition of heat transfer in a gas at constant volume: Q = n*C_v*delta_T = (0.01 mol)(12.47 J/mol*K)(40 K) = 4.99 J b) We use the definition of heat transfer in a gas at constant pressure: Q = n*C_p*delta_T = (0.01 mol)(12.47 J/mol*K)(40 K) = 8.31 J c) In both processes delta_U...
  20. B

    The Pressure-Entropy Relationship for a Photon Gas

    This is from Problem 7.45 of Thermal Physics by Daniel Schroeder.
  21. questionmonkey123

    Constant pressure process (piston) on gas violates Newtons 2nd law?

    I often see this set up in thermodynamic problems and need clarification on how Newton's Laws are involved for the piston: Gas inside a piston cylinder (1) is heated expanding the gas and raising the piston (initially at rest) to a height (2) in a constant pressure quasi-equilibrium process...
  22. B

    Changes in pressure, temp, & entropy of ideal gas in atmosphere

    So for a collection of particles each with mass m, the pressure beneath them, ##p(z)## should be higher than the pressure above them ##p(z + \Delta z)##. This is a change in force per unit area (force per unit volume I suppose) times a volume to equate with the gravitational force $$ \frac...
  23. D

    B Irreversible Process to Move Gas from State 1 to State 2

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  24. S

    How does metering device in an A/C cause flash gas?

    When refrigerant enters the metering device of an air-conditioner, the refrigerant enters the metering device as 100% liquid. When the refrigerant in a straight air-conditioner goes through the metering device, the metering device lowers the pressure of the refrigerant a great deal. The...
  25. ikihi

    I Compressing gas with piston: reasons for temperature increase?

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  26. burian

    I Entropy after removing partition separating gas into two compartments

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  27. H

    Exploring Pressure Variations of CO2 Gas at 200°C

    A container of 0.5l containing 6 moles of CO2 are heated to 200 ° C on is interested in explaining the pressure variations between the pressure of a perfect gas and the actual value measured on a pressure gauge. The coefficients of the real gas a=3.59 [atm l2/mol2-] b=0.0427 [l /mol] indicate...
  28. J

    I Does a photon travel freely through the CMB photon gas?

    A photon often travels billions of years (Gyr) through the CMB photon gas (410 photons per cubic centimeter) to reach us. Does it travel freely? Let’s share our thoughts about this. For discussion purpose, let’s assume the photon has a wavelength of 500 nm, close to the peak of the solar...
  29. L

    Finding the increase in entropy of the universe in gas expansion

    a) ##P_f=\frac{nRT_f}{V_f}=\frac{nR\frac{T_i}{2}}{2V_0}=\frac{1}{4}\frac{nRT_i}{V_0}=\frac{1}{4}P_i## b) ##Q=\Delta U=nC_V \Delta T=n\frac{5}{2}R(-\frac{T_i}{2})=-\frac{5}{4}nRT_i=-\frac{5}{4}P_i V_0## (##L=0## since the gas expands in a vacuum;Now, (a) and (b) are both correct but not (c), for...
  30. A

    Sound waves inside an inert gas fusion reactor

    Hey, its that under educated guy again, I hope the mighty big brains can spare me my feelings. Anyways, I saw this toy that levitated a ball of water using nothing but sound waves. So naturally I tried to apply this to a plasma and fusion. Using a inert gas reactor, or a fusor for that matter or...
  31. warhammer

    Entropy Change & Heat Transferred to a Gas

    By using the given relationship that S=a/T --(1) along with the equation ∫ (delta Q rev)/T=∫dS -- (2) I found out that my answer for the value of Q is mc*ln (T2/T1)*a upon equating (1) & (2). But the solution is instead given as Q=a*ln*(T1/T2). I would be grateful if someone would point out...
  32. Chestermiller

    I Expansion or Compression Work by Gas ##=\int{P_{ext}dV}##

    In private discussions, another member @burian and I have been discussing why, in compression and expansion of a gas, the work can be calculated from ##W=\int{P_{ext}dV}##. We thought it would be of value to open the discussion to the full membership. Burian last asked about: When the...
  33. Istiak

    Calculate the volume change with gas temperature for this piston in a cylinder

    Solution attempt : Option : I am sure that my work is wrong. But, I must add solution attempt in PF that's why I just added that. How can I solve the problem?
  34. G

    I Heat capacity for a real gas using the ideal gas (zero pressure) equation

    Summary:: Heat capacity for real gas with ideal gas (zero pressure) equation I'm looking at this problem and I'm stuck. I usually question everything but this problem is confusing me. I don't know how they've made the jump from reduced properties (from generalized Cp charts(?)) to...
  35. S

    Why does air have to be compressed first in gas turbines?

    In all gas turbines air is first compressed using a compressor and then head added in a combustor and finally the hot pressurized air is expanded in nozzles to convert air contained energy to kinetic energy which can be harvested and transferred in the form of thrust or shaft rotation. My...
  36. G

    RPM & Gas Mileage: Effects on Overdrive Vehicles

    I was reading a post about the effects of RPM on gas mileage, It was from 2007 so I couldn't reply to it. RPM does make a difference on gas mileage when it comes to overdrives. I had an '82 Buick Riviera with a small block engine with an overdrive and a quadrajet that got 26 MPG on the freeway...
  37. P

    What % reduction in gas flow results from changing to LPG?

    There are 70 jets of 1/16" diameter along the tube and I neeed to create flames 1" high. I know the calorific value of natural gas is much less than LPG (39 MJ/m3 Vs 93 MJ/m3) . I am not sure what assumptions to make from there in order to compare the predicted natural gas to the LPG volume...
  38. P

    Investigating the Sound Reflector of a Gas Inlet Tube

    My theory is that the gas inlet end, which is supposed to act as a sound reflector may not be doing a great job because it is plastic and has a hole cut in the centre for the gas inlet port. But by that reasoning, the flames at the end most distant from it (the sound inlet end) should be the...
  39. Zahid Iftikhar

    Time of Impact of Gas Molecules on the Inner Wall of a Pressure Vessel

    In derivation of relation for pressure of Gas ,(see pic below) the time for rate of change of momentum of molecules should be the time for which the wall of the vessel and the molecules kept interacting with each other, not the time between successive collisions. As circumscribed below, my...
  40. Limebat

    I Does combusted gas travel faster in a vacuum tube?

    Hey all, So if a gas is combusted, would it travel faster through a vacuum tube or a regular ole' tube? I would assume the vacuum tube, as there are less particulates collisions in the way of the fast-moving gas molecules. Yet this also implies pressure on the outside of the thin barrier _ 2...
  41. Z

    DeltaG and DeltaA calculation for heating a gas at constant volume

    Summary:: Gibbs and Helmholtz energies calculations for heating an ideal gas at constant volume I am solving a problem involving an ideal gas that undergoes several chained changes of state. One of the steps asks to calculate the change in Gibbs Energy (DeltaG) and Helmholtz energy (Delta A)...
  42. T

    A Properties of Degenerate Electron Gas

    Basically the thread title. For some background, I'm trying to model laser-material interactions, where I'm assuming that the laser is interacting with a free electron gas (copper). To model the interaction, I need to determine the properties of the electron gas, such as the heat capacity...
  43. Limebat

    I What happens when gas is combusted while moving?

    If a combustible gas mixture (say O2 and H2 are combusted to form H2O) are already moving before combustion, what would happen? Intuition tells me the previous velocity (say the gas was moving 150 feet per second) would just add to the velocity achieved when standing still and then combusted...
  44. Kaguro

    Understanding the Differences Between Bose and Classical Gases

    In classical statistics, we derived the partition function of an ideal gas. Then using the MB statistics and the definition of the partition function, we wrote: $$S = k_BlnZ_N + \beta k_B E$$, where ##Z_N## is the N-particle partition function. Here ##Z_N=Z^N## This led to the Gibb's paradox...
  45. F

    Instantaneous Gas Compression: temperature increase?

    If I a have a gas confined in a certain initial volume Vin at a certain pressure Pin and at a certain temperature Tin, and istantaneously compress it down to a final volume Vfin < Vin, how do I calculate the increase in temperature? Assume I know the exact pressure curve (P vs. V). The system...
  46. S

    Confusion about energy conversion mechanisms in gas turbines

    Hello Gents, I am very confused about the working principle of gas turbines. I understand that air is first compressed in a compressor to obtain high pressure air at the inlet of turbine, but before air is introduced to the turbine, first it has to be heated to very high temperatures through...
  47. M

    Solving Unusual Problems: Using the Polytropic Ideal Gas Equation

    My teacher likes to make really weird problems. How can I start this problem? The only thing I thought of doing is using the polytropic ideal gas equation when cp= constant. (p2/p1)^k-1/k = T2/T1 and making p1 and t1 in each case the normal state of the lungs
  48. Like Tony Stark

    Determine Joule-Kelvin coefficient for gas given equations of state

    Hi ##\mu=\frac{\alpha TV–V}{N c_P}##. So, firstly, I have to calculate ##\alpha## and ##c_P##. So ##\alpha=\frac{1}{V} \frac{\partial V}{\partial T}## at constant ##P##. I can write ##U=PV##, then I replace it in the equation of ##T##, solve for ##V## and then I differentiate with respect to...
  49. P

    Thermodynamics problem (ideal gas law, kinetic theory, processes, etc.)

    It is a long problem, but it is simple to understand. I am having trouble with part A. My attempt: Pressure outside > pressure inside container. pV = constant (isothermal). At equilibrium, all gases are at atmospheric pressure. Because it is quasi-static, the pressures of both compartments are...
  50. Domuks

    How do I find average kinetic energy if I don't have mass of the gas?

    So always in my problems i had mass (M) but now i don't and it seems impossible to solve this problem if I don't have mass I think i am missing something. I was looking for similar problems in my book and internet and didn't find any.
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