Gas Properties: Density, Velocity, and Collisions
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Questions and Answers

What is the relationship between thermodynamic pressure and momentum flux described in the text?

  • Thermodynamic pressure is the square of the momentum flux.
  • Thermodynamic pressure is inversely proportional to the average of the momentum flux components.
  • Thermodynamic pressure is equivalent to the square root of the average of the momentum flux components.
  • Thermodynamic pressure is equivalent to the cartesian average of the momentum flux. (correct)

In the simplified model described in the text, where particles move in six directions, what fraction of $mn\bar{u}^2$ represents the momentum flux $I_{x,x}^{(u)}$?

  • 1/2
  • 1/12
  • 1/6 (correct)
  • 1/3

What parameter is essential for calculating pressure in the system?

  • The mean velocity of the particles. (correct)
  • The size of the cubic volume.
  • The mass of the cubic volume.
  • The total number of particles in the system.

In the simplified cubic model, how does the momentum transferred to the wall compare to the initial momentum of the particles before the impact?

<p>The transferred momentum is twice the initial momentum. (B)</p> Signup and view all the answers

What is temperature described as in the text?

<p>A measure of the kinetic energy due to the random motions of particles. (B)</p> Signup and view all the answers

How does the collision cross section, $\sigma_{1,2}$, relate to the radii of the colliding particles, $r_1$ and $r_2$?

<p>$\sigma_{1,2} = \pi (r_1 + r_2)^2$ (C)</p> Signup and view all the answers

Which of the following best describes the approximation made for particle movement when calculating the average relative speed, $\bar{c}_{1,2}$?

<p>Particles move in six equally probable directions: ±x, ±y, ±z. (B)</p> Signup and view all the answers

Given the mean random particle velocity $\bar{c} = \sqrt{\frac{8kT}{\pi m}}$, how does temperature affect collision frequency, assuming other factors remain constant?

<p>Collision frequency is proportional to the square root of the temperature. (B)</p> Signup and view all the answers

If two gases have different molecular masses, $m_1$ and $m_2$, and are at different temperatures, $T_1$ and $T_2$, how is the average relative speed $c_{1,2}$ calculated?

<p>$c_{1,2} = \sqrt{\frac{8k}{\pi}} \sqrt{\frac{T_1}{m_1} + \frac{T_2}{m_2}}$ (C)</p> Signup and view all the answers

What is the collision frequency $\Gamma_{1,2}$ if the average relative speed is doubled and the number density $n_2$ of the second species is halved, assuming the cross-section $\sigma_{1,2}$ remains constant?

<p>The collision frequency remains unchanged. (A)</p> Signup and view all the answers

In statistical physics, if a volume element contains $10^6$ particles, what is the approximate variation in the number of particles, expressed as a percentage?

<p>0.1% (A)</p> Signup and view all the answers

What does the notation ~u(~r, t) represent in the context of gas velocities?

<p>The macroscopic flow (bulk) velocity at a specific point and time. (B)</p> Signup and view all the answers

How is the thermal motion velocity (~c) related to the individual particle velocity (~v) and the macroscopic flow velocity (~u)?

<p>$~c = ~v - ~u$ (A)</p> Signup and view all the answers

Which macroscopic property of a gas is defined as the force exerted per unit area?

<p>Pressure (A)</p> Signup and view all the answers

What is the physical interpretation of collision frequency ($\nu$) in a gas?

<p>The number of collisions per unit time. (D)</p> Signup and view all the answers

If gas 1 has radius $r_1$ and gas 2 has radius $r_2$, what is the effective radius (r) used to simplify calculations when considering collisions between the gases?

<p>$r = r_1 + r_2$ (D)</p> Signup and view all the answers

What condition must a volume element dV satisfy when determining particle number density?

<p>dV must be large enough to contain many particles but small enough compared to macroscopic density variations. (C)</p> Signup and view all the answers

A gas contains particles with individual velocities around 500 m/s. There is a steady wind blowing at 50 m/s. What is the approximate magnitude of the thermal velocities of the gas particles?

<p>500 m/s (A)</p> Signup and view all the answers

If the particle number density ( n ) in a gas is increased, what happens to the estimated mean distance ( d ) between particles?

<p>( d ) decreases proportionally to ( \sqrt[3]{n} ). (C)</p> Signup and view all the answers

Consider a cubic volume element with side length ( a ) containing ( N ) uniformly spaced particles. If the side length ( a ) is doubled while keeping the particle spacing ( d ) constant, how does ( N ) change?

<p>( N ) increases by a factor of 8. (B)</p> Signup and view all the answers

Consider a scenario where the collision frequency in a gas increases. What is the likely effect on the mean free path of particles in the gas, assuming constant temperature and particle density?

<p>The mean free path will decrease. (D)</p> Signup and view all the answers

In a mixed gas, if the relative velocity between two types of particles increases, what is the expected effect on the collision frequency, assuming all other factors remain constant?

<p>The collision frequency will increase. (C)</p> Signup and view all the answers

In the context of gas physics, why is understanding microscopic properties important?

<p>To accurately describe phenomena like diffusion. (A)</p> Signup and view all the answers

A container of gas has a particle number density of $10^{18} m^{-3}$. Estimate the mean distance between particles.

<p>$10^{-6} m$ (C)</p> Signup and view all the answers

What is the effect on gas diffusion if the particle number density significantly increases, assuming temperature remains constant?

<p>Diffusion rate decreases due to more frequent particle collisions. (C)</p> Signup and view all the answers

Why is it important for the volume element ( dV ) to not be as large as macroscopic density variations when calculating particle number density?

<p>To accurately represent the density at a specific point. (D)</p> Signup and view all the answers

Given the formula for the combined temperature $T_{1,2}$, which of the following scenarios would result in $T_{1,2}$ being approximately equal to $T_1$?

<p>$m_2$ is much greater than $m_1$ (D)</p> Signup and view all the answers

In the context of mean free path calculation, how does increasing the number density ($n_2$) of particles affect the mean free path ($l_{1,2}$)?

<p>Decreases $l_{1,2}$ linearly (B)</p> Signup and view all the answers

For a system where all particles are identical, how does the mean free path ($l_{1,1}$) change if the effective cross-sectional area ($σ_{1,1}$) of the particles doubles, assuming the number density ($n$) remains constant?

<p>$l_{1,1}$ is halved (C)</p> Signup and view all the answers

What is the correct formula for calculating flux?

<p>$Φ = n \times u$ (A)</p> Signup and view all the answers

Which factor directly affects the pressure exerted by a gas on the walls of a container?

<p>The average kinetic energy of the particles (D)</p> Signup and view all the answers

Consider a scenario where gas A has a higher molecular mass than gas B, but both are at the same temperature. How will their average particle speeds ($c_A$ and $c_B$, respectively) compare?

<p>$c_A &lt; c_B$ (B)</p> Signup and view all the answers

In a mixture of two gases, what would likely lead to the combined collision rate ($Γ_{1,2}$) being approximately equal to the collision rate of gas 1 with itself ($Γ_{1,1}$)?

<p>The number density of gas 1 is much higher than gas 2. (B)</p> Signup and view all the answers

How does the net momentum transferred by a particle flux to a wall relate to the pressure exerted on that wall?

<p>Pressure is directly proportional to the net momentum transferred. (A)</p> Signup and view all the answers

Flashcards

Gas Macroscopic Properties

Macroscopic properties used to describe gases, such as pressure, temperature, and mass density.

Connecting Macro & Micro Properties

Relates macroscopic properties to the microscopic behavior of gas particles, essential for understanding phenomena like diffusion.

Particle Number Density, n(~r, t)

The number of particles (N) per unit volume (V) at a specific point in space and time.

Volume Element Size (dV)

The volume (dV) must contain many particles but be smaller than the scale of macroscopic density variations.

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Mean Distance Between Particles (d)

Estimate of the average distance between particles in a gas.

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Formula for Mean Distance (d)

d ⇡ 1/n^(1/3). Relates the mean distance to particle number density.

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Volume Element Requirement

The volume element, dV, must be much larger than the cube of the mean distance between particles (d^3).

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Microscopic Properties

Microscopic properties are necessary to describe phenomenon such as diffusion.

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Flow Velocity (~u)

The overall bulk motion of a gas, representing the average velocity of its particles at a specific location and time.

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Thermal Motion (~c)

The motion of gas particles after the bulk flow (wind) is removed; reflects random movement due to temperature.

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Collision Frequency (⌫)

The average number of collisions per unit of time for a particle in a gas.

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Mean Free Path

The average distance a particle travels between collisions with other particles.

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Particle Number Variation

Statistical physics indicates that the variation in the number of particles (N) within a volume is inversely proportional to the square root of N.

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Macroscopic Variation Scale

Macroscopic variations in gases occur over large scales, typically on the order of kilometers (kms), such as scale heights.

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Flow Velocity Definition

The flow velocity is defined as the average of the individual particle velocities at a given location and time: ~u(~r, t) = <~v>~r,t

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Thermal Motion Calculation

Thermal motion is calculated by subtracting the flow velocity from the individual particle velocities: ~c = ~v - ~u.

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Collision Frequency (⌫1,2)

The average number of collisions per unit time between species 1 and 2.

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Collision Cross Section (𝜎1,2)

Effective area for collisions between two particles. Represented as the area of a circle.

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Collision Cross Section Formula

The cross section is the area encompassing both particles sizes added together.

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Average Relative Speed (~c1,2)

A simplification accounting for the average relative speed between particles of different species.

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Mean Random Particle Velocity (c̄ )

Relates the average speed of particles to temperature and mass. Used in calculating collision frequency.

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I(u)x,x

Momentum flux notation, equivalent to mnu^2x.

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Thermodynamic Pressure (p)

The average of momentum flux in x, y, and z directions.

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Velocity Distribution

Knowing this allows pressure calculation.

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I(u)x,x Simplified

1/6 * m * n * ū^2. Represents momentum transfer in the x-direction.

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Temperature

Proportional to kinetic energy from random particle motion.

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Combined Mass (m1,2)

m1,2 = (m1 + m2)

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Combined Temperature (T1,2)

T1,2 = (m2T1 + m1T2) / (m1 + m2)

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Collision Rate (⌫1,2)

⌫1,2 = s1,2 * n2 * √(8kT1,2 / ⇡m1,2)

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Mean Free Path (l1,2)

Average distance a particle travels between collisions.

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Mean Free Path Formula

l1,2 = c¯1 / ⌫1,2

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Flux

Net amount of a scalar quantity transported per unit area per unit time.

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Particles Passing Through Area

= ux * t * A * n

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Pressure

Momentum transferred to walls by impacting particles.

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Study Notes

  • Gases are described by macroscopic physical properties like pressure, temperature, and mass density.
  • Understanding how macroscopic properties connect to microscopic properties is important for describing phenomena like diffusion.

Particle Number Density

  • Particle number density at any point with spatial and temporal coordinates (r, t) is given by n(r,t) = lim (ΔN/ΔV)
  • dV must have many particles but cannot be so large as to encompass macroscopic density variations
  • The mean distance (d) between particles is estimated by d ≈ 1/√n

Gas Velocities

  • Gas velocities can be described by individual particle velocities (v), macroscopic flow velocity (u), and random thermal motion velocities (c).
  • Flow velocity is the overall bulk motion of the gas; u(r,t) = r,t
  • Thermal motion is the motion left after the bulk component is removed; c = v - u
  • The average of thermal motion is zero: = = - = 0

Collision Frequency

  • Collision frequency (ν) is defined as the number of collisions per unit time.

Mean Free Path

  • In a gas of particle type 1 with radius r₁ and particle i with velocity C1,i in type 2 gas with radius r₂ and velocity C2,j, simplification to relative velocities for collision problems is possible.
  • Relative velocities: C1,2 =< |C1 – C2| >
  • Simplify geometry by considering particle 1 with radius r = r1 + r2 in a sea of stationary particle 2's, now point particles.
  • Volume of particles encountered in time interval Δt by particle 1: V1,2 = π(r1 + r2)²C1,2n∆t
  • Collision frequency: V1,201,201,2N2/Δt
  • The cross section is defined as σ1,2 = π(r1 + r2)²
  • Approximating that particles move in one of 6 directions (±x, ±y, ±z): 1,2 =< |2 - 1 | >or 4,2 = ≈ C211+(2)2

Flux

  • Flux definition: the net amount of a scalar quantity transported per unit area and per unit time.
  • Applicable to photons, particles, heat, charge, or a vector component.
  • Number of particles passing an area A: # = ux Δt An

Pressure

  • Pressure relates to the momentum transferred to the walls of a pressure vessel by impacting particles.
  • Net momentum in the x direction depends on net particle velocity, number of particles, and mass.
  • Notation for momentum flux: (u) = mnux²
  • Thermodynamic pressure is the average of the momentum flux: p = ⅓(...)
  • With 1/6 of particles moving in ±x direction, the momentum flux impinging on the wall of a cubic volume is I(u) = ⅙mnu²
  • Transferred momentum is twice the above value because velocity reverses post-impact.
  • p = ⅓mnu²

Temperature

  • Temperature measures the kinetic energy due to random particle motions.

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Description

Explore the macroscopic properties of gases, including pressure, temperature, and density. Understand particle number density, gas velocities (individual, macroscopic flow, and thermal motion), and collision frequency. Learn how these properties relate to gas behavior.

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