What is Effusion: Definition and 38 Discussions

In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a pinhole and the escape of the gas is due to the pressure difference between the container and the exterior. Under these conditions, essentially all molecules which arrive at the hole continue and pass through the hole, since collisions between molecules in the region of the hole are negligible. Conversely, when the diameter is larger than the mean free path of the gas, flow obeys the Sampson flow law.
In medical terminology, an effusion refers to accumulation of fluid in an anatomic space, usually without loculation. Specific examples include subdural, mastoid, pericardial and pleural effusions.

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  1. rogdal

    Deriving the kinetic energy flux in an effusion process

    I could not find any derivations in the litterature, except for the expected value of the energy flux expression itself: $$\overline{\Phi_{effusion,\epsilon}} = \overline{\dot{N_{ef}}}\overline{\epsilon_{ef}}=\frac{3Nl}{2A}\sqrt{\frac{(k_BT)^3}{2\pi m}}$$ I've started off by calculating the...
  2. P

    Gas effusing through hole, working out time dependence

    Consider instead a thermally insulated container of volume V with a small hole of area A, containing a gas with molecular mass m. At time t = 0, the density is ##n_0## and temperature is ##T_0##. As gas effuses out through a small hole, both density and temperature inside the container will...
  3. Viona

    I Diffusion & Effusion: What Is It?

    I understand that diffusion is the movement of molecules from a region where concentration is high to one where it is lower. This happens due to the molecular agitation which leads to frequent collisions and hence scattering of molecules. But I do not know what is effusion. Any help? Thanks
  4. P

    How Long Does It Take for Pressure to Increase in a Punctured Container?

    Homework Statement A thin-walled container of volume 10−3 m3 is evacuated to a pressure of 10−7 mbar. The vessel is surrounded by air at 1 bar and 290 K. If a small hole of area 10−17 m2 is made in the wall of the container, how long does it take the pressure inside to rise to 10−6 mbar...
  5. J

    Effusion and setting up the calculus problem

    Homework Statement Gas escapes from a leaky cylinder so that the rate of decrease of pressure in the cylinder is proportional to P - Patm where P is the pressure in the cylinder and Patm is atmospheric pressure (1.0 atm). The initial pressure in the cylinder is 1.1 x 103 atm. If the rate of...
  6. Pouyan

    Effusion and isotopic composition

    1. In a gas UF6 (uranium hexafluoride) are uranium atoms of both the fissionable uranium isotope 235U , and 238U. To enrich the fissile isotope can let a gas UF6 with natural isotopic composition (0.7% 235U, 99.3% 238U) undergo effusion process. The process is then repeated in many steps in the...
  7. I

    Effusion in Uranium Enrichment: Calculating Concentration and Number of Repeats

    Homework Statement Naturally occurring uranium contains 0.7% ##\;^{235}U## and 99.3% ##\; ^{238}U## . To enrich it one uses a method based on repeated effusion. a) Consider a step in this process. In a container, which is divided in two parts by a porous plug, one part contains natural uranium...
  8. Robsta

    Knudson Effusion between two gasses

    Homework Statement Consider two chambers of equal volume separated by an insulating wall and containing an ideal gas, maintained at temperatures T1 = 225K and T2 = 400K. Initially the two chambers are connected by a long tube whose diameter is much larger than the mean free path in either...
  9. mukul

    Adiabatic effusion and violation of energy conservation

    when the gas effuse out of an insulated container, its velocity is given by \sqrt{\frac{2\gamma RT}{(\gamma-1)M}} (irodov's problem 2.42) This leads to the result that every mole of leaking gas has an energy of \frac{\gamma RT}{(\gamma-1)} whereas we know the energy contained in 1 mole of gas...
  10. P

    Understand Effusion of a Gas: Speed & Angle Distribution

    Homework Statement Show that the number of molecules hitting unit area of a surface per second with speeds between v and v+dv and angle between θ and θ+dθ to the normal is dΦ=0.5vnf(v)dvsinθcosθdθ where f(v) is the distribution of molecular speeds and n is the number density. Hence calculate...
  11. K

    Effusion of gas from higher to lower concentration under same pressure

    If we have a hole in a container with pressure equal to the pressure outside it and the concentration of gas inside the container is more than that outside will the gas effuse out?
  12. U

    Effusion of particles from one box to another - pressure calculation

    Homework Statement Suppose we have a container divided into equal halves. Right half is fixed at temperature ##T##, volume ##V_2##. Initially it has pressure ##P_0##, a hole of area ##A## is opened between them. Part (a): Show that the pressure in the right half is: p =...
  13. A

    How many effusion stages required to increase the 235U to 4%

    Homework Statement Uranium has two common isotopes with atomic masses of 238 and 235. One way to separate these isotopes is to combine the uranium with fluorine to make uranium hexafluoride gas, UF6, and then to let it effuse through a small hole. The natural abundance of the 235U isotope is...
  14. U

    Effusion through hole of box

    Homework Statement Part (a): Explain what A and f means. Describe an experiment that shows this relation is true. Part (b):Find the temperature of vessel 1. Part (c): Why is temperature of vessel initially the same as vessel 1? Calculate the final temperature of vessel 2. Find the time taken...
  15. U

    Effusion and speed of molecules

    Homework Statement Part(a): Find rate of incidence of gas molecules. Part(b): Find v1 and v2. Part (c): Find mean free path for faster molecules. Part (d): Find mean free path of 10m/s molecules. Homework Equations The Attempt at a Solution Part (a) Letting ##\phi## be flux, and ##\Delta...
  16. U

    What is the Equilibrium Temperature of Gas between Boxes?

    Homework Statement Part(a): explain the meaning of f(v) and A. Part(b): Gas from surrounding air effuse into a box; find the temperature in the box. Part (c): Explain why initial temperature of gas escaping from first box into second is the same as temperature in first box. Part...
  17. U

    Effusion of molecules into a Box through a hole

    Homework Statement A container of volume 10−3 m3 has a pressure of 10−7 mbar. Surrounded by air at 1 bar and 290 K, how long does it take the pressure inside to rise to 10−6 mbar when air effuses through a small hole of area 10−17 m2? Homework Equations The Attempt at a Solution Flux ø =...
  18. xatu

    Kinetic Theory of Gases: Effusion and Collisions

    Question: In the following link I'm concerned with only pages 1 and 2. I'm wondering if there is a typo or an error of some kind. I feel like there is some inconsistency to what the author refers to as \upsilon, \theta, and \phi. The only angle shown in the diagram is \upsilon, which is the...
  19. C

    Effusion Question about Uranium enrichening

    Homework Statement A gas with equal number densities of uranium hexafluoride (UF6) molecules containing two isotopes of uranium, atomic masses 235amu and 238amu, is passed through a porous membrane containing very small holes. Use Graham's Law of Effusion to calculate: How many successive...
  20. E

    Statistical Mechanics: Phase Diagrams and Effusion

    Homework Statement There are two problems: The first problem is to calculate the slope of a phase-equilibrium line of a substance given its properties (Helium, Water, etc.) as the temperature approaches a certain value (in this case, T approaches zero). The second problem is the effusion of...
  21. J

    Rate of effusion dependance on pressure

    In my class rate of effusion was told to be directly proportional to the mean speed of gas molecules which is intuitive and i understand. RMS speed of molecules is √3RT/M So rate of effusion is inversely proportional to sqaure root of molar mass But i don't understand how rate of effusion is...
  22. G

    Calculating Gas Particle Effusion Rate Through Two Holes

    Homework Statement A gas effuses into a vacuum through a small hole of area A. The particles are then collimated by passing through a very small circular hole of radius a, in a screen a distance d from the first hole. Show that the rate at which particles emerge from the second hole is...
  23. B

    Effusion Physics Problem: Calculating Final Temperature of Gas in a Vessel

    Homework Statement A vessel contains gas at temp T (monatomic). Maxwellian distribution of speeds. Molecules diffuse via small hole into vacuum. Box opened for short time and catches some molecules. Calc. final temp of gas in box. Homework Equations The Attempt at a Solution...
  24. B

    Effusion Equation Help | Find Expressions for v1 and v2

    Homework Statement Maxwellian gas effuses through a small hole to form a beam. After a certain distance from the hole, the beam hits a screen. Let v1 be the most probable speed of atoms that during a fixed time interval, hit the screen. Let v2 be the most probable speed of atoms situated at...
  25. S

    Thermodynamics Rate of Effusion

    Homework Statement Effusion is the slow leakage of atoms through a small hole in a container. We'll consider a box at pressure P, with volume V and temperature T. The hole will have the area A, and is in one of the six walls of the container. a) At what rate do atoms (assume mass M) pass...
  26. B

    Effusion differential equation from Newtonian mechanics

    Homework Statement If you poke a hole in a container full of gas: the gas will start leaking out. In this problem, you will make a rough estimate of the rate at which gas escapes through a hole: effusion. (This assumes the hole is sufficiently small). Consider such a hole of area "A". The...
  27. A

    Rates of Effusion: Homework Questions & Solutions

    Homework Statement So this isn't just one problem but it is a question as to why it is done one way for two instances and then the opposite way for a third problem. Here it goes. 1) An unknown gas effuses at a rate that is .462 times that of nitrogen gas (at the same temperature)...
  28. N

    Gas Effusion Rate Problem: He, Cl2, H2, Kr, Rn

    The Problem A sample of N2 gas is contaminated with a gas. It is found that that contaminant effuses at 0.25 times the rate of N2. What is the contaminating gas? Choices are: He, Cl2, H2, Kr, Rn Homework Equations Rate x/Rate N2 = (square root of:) Mass N2/Mass x Where Rate x...
  29. S

    Thermal effusion problem - typical qualifier

    Homework Statement Part a: A thin walled vessel of volume V, kept at constant temperature T, contains a gas which slowly leaks through a small hole of area A. The outside pressure is negligible (assume zero), so no leakage back into the vessel is possible. Find an expression for the pressure...
  30. Z

    Effusion of Gas from a Balloon

    Homework Statement Suppose we make a circular hole 2 millimeter in diameter in a balloon. Calculate the initial rate at which oxygen escapes through the hole (at 0° C), in number of oxygen atoms leaving the balloon per second. Homework Equations 1/4 n*A*v The Attempt at a Solution...
  31. H

    Chemistry Effusion Homework: Calculate Molar Mass of Unknown Gas

    Homework Statement One percent of a measured amount of chlorine gas escapes through a tiny hole in 33 sec. One percent of a unknown gas escapes under the same conditions in 75.2 sec. Calculate the molar mass of the unknown gas in g/mol. I can't seem to figure out how to do this so if some...
  32. P

    Effusion Gas Leakage: Estimating the Rate of Escape Through a Hole

    Homework Statement If you poke a hole in a container full of gas, the gass will start leaking out. In this problem, you weill make a rough estimate of the rate at which gas escapes through a hole. If we now take away this small part of the wall of the container, the molecules that would have...
  33. N

    Just checking answer for effusion

    hi guys , i did this chemistry question and me and my friends got very different answers.. so i want to know wats the right answer... a professor fills a balloon with equal molar amounts of hydrogen and oxygen . the demo for that day gets postponed to the next day and over the night 45%...
  34. M

    Gas Leak (Effusion): Solving Differential Equation

    I'm working on a question where gas is leaking out of a container from a small pinhole. I have a differential equation dN/dt = (Constants)*N. I solved this to get an exponential. The exponent is that group of constants, which are A/2V*(kT/m)^1/2. Shouldn't the exponent have something to do...
  35. K

    Diffraction diffusion effusion differences

    Can somebody give me for mind refreshment the differences between those process : diffraction diffusion effusion ?
  36. W

    Why the molecules still moving to create the diffusion and the effusion?

    i know that the molecules in the "freedom" form that have charges equal 0. but i wonder why the molecules still moving to create the diffusion and the effusion? some one please explain to me. thanks:!)
  37. L

    Calculating Molar Mass of an Unknown Gas from Effusion Rate

    effusion rates...please help! Could someone please explain to me how to do this question... The rate of effusion of a particular gas was measured to be 24.0 mL/min. Under the same conditions the rate of effusion of pure methane gas, (CH4) is 47.8 mL/min. What is the molar mass of the unknown gas?
  38. J

    Laws of Effusion Explained: Textbooks Don't Help

    Will someone please explain to me the laws of effusion my textbooks suck.