2.2 Pressure, temperature, and rms speed  (Page 7/18)

 Page 7 / 18

Another important application of partial pressure is vapor pressure    , which is the partial pressure of a vapor at which it is in equilibrium with the liquid (or solid, in the case of sublimation) phase of the same substance. At any temperature, the partial pressure of the water in the air cannot exceed the vapor pressure of the water at that temperature, because whenever the partial pressure reaches the vapor pressure, water condenses out of the air. Dew is an example of this condensation. The temperature at which condensation occurs for a sample of air is called the dew point . It is easily measured by slowly cooling a metal ball; the dew point is the temperature at which condensation first appears on the ball.

The vapor pressures of water at some temperatures of interest for meteorology are given in [link] .

Vapor pressure of water at various temperatures
T $\left(\text{°}\text{C}\right)$ Vapor Pressure (Pa)
0 610.5
3 757.9
5 872.3
8 1073
10 1228
13 1497
15 1705
18 2063
20 2338
23 2809
25 3167
30 4243
35 5623
40 7376

The relative humidity (R.H.) at a temperature T is defined by

$\text{R.H.}=\frac{\text{Partial pressure of water vapor at}\phantom{\rule{0.2em}{0ex}}T}{\text{Vapor pressure of water at}\phantom{\rule{0.2em}{0ex}}T}\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}100%.$

A relative humidity of $100%$ means that the partial pressure of water is equal to the vapor pressure; in other words, the air is saturated with water.

Calculating relative humidity

What is the relative humidity when the air temperature is $25\phantom{\rule{0.2em}{0ex}}\text{ºC}$ and the dew point is $15\phantom{\rule{0.2em}{0ex}}\text{ºC}$ ?

Strategy

We simply look up the vapor pressure at the given temperature and that at the dew point and find the ratio.

Solution

$\text{R.H.}=\frac{\text{Partial pressure of water vapor at}\phantom{\rule{0.2em}{0ex}}15\phantom{\rule{0.2em}{0ex}}\text{°C}}{\text{Partial pressure of water vapor at}\phantom{\rule{0.2em}{0ex}}25\phantom{\rule{0.2em}{0ex}}\text{°C}}\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}100%=\frac{1705\phantom{\rule{0.2em}{0ex}}\text{Pa}}{3167\phantom{\rule{0.2em}{0ex}}\text{Pa}}\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}100%=53.8%.$

Significance

R.H. is important to our comfort. The value of $53.8%$ is within the range of $40%\phantom{\rule{0.2em}{0ex}}\text{to}\phantom{\rule{0.2em}{0ex}}60%$ recommended for comfort indoors.

As noted in the chapter on temperature and heat, the temperature seldom falls below the dew point, because when it reaches the dew point or frost point, water condenses and releases a relatively large amount of latent heat of vaporization.

Mean free path and mean free time

We now consider collisions explicitly. The usual first step (which is all we’ll take) is to calculate the mean free path    , $\lambda ,$ the average distance a molecule travels between collisions with other molecules, and the mean free time $\tau$ , the average time between the collisions of a molecule. If we assume all the molecules are spheres with a radius r , then a molecule will collide with another if their centers are within a distance 2 r of each other. For a given particle, we say that the area of a circle with that radius, $4\pi {r}^{2}$ , is the “cross-section” for collisions. As the particle moves, it traces a cylinder with that cross-sectional area. The mean free path is the length $\lambda$ such that the expected number of other molecules in a cylinder of length $\lambda$ and cross-section $4\pi {r}^{2}$ is 1. If we temporarily ignore the motion of the molecules other than the one we’re looking at, the expected number is the number density of molecules, N / V , times the volume, and the volume is $4\pi {r}^{2}\lambda$ , so we have $\left(N\text{/}V\right)4\pi {r}^{2}\lambda =1,$ or

$\lambda =\frac{V}{4\pi {r}^{2}N}.$

Taking the motion of all the molecules into account makes the calculation much harder, but the only change is a factor of $\sqrt{2}.$ The result is

what are waves
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities
Discuss how would orient a planar surface of area A in a uniform electric field of magnitude E0 to obtain (a) the maximum flux and (b) the minimum flux through the area.
I'm just doing the first 3 with this message. but thankyou for the time your obviously intending to support us with. viva la accumulation
Marcel
Find the net capacitance of the combination of series and parallel capacitors shown belo
what is ohm?
calculate ideal gas pressure of 0.300mol,v=2L T=40°c
what is principle of superposition
what are questions that are likely to come out during exam
what is electricity
watt is electricity.
electricity ka full definition with formula
Jyoti
If a point charge is released from rest in a uniform electric field will it follow a field line? Will it do so if the electric field is not uniform?
Maxwell's stress tensor is
Yes
doris
neither vector nor scalar
Anil
if 6.0×10^13 electrons are placed on a metal sphere of charge 9.0micro Coulombs, what is the net charge on the sphere
18.51micro Coulombs
ASHOK
Is it possible to find the magnetic field of a circular loop at the centre by using ampere's law?
Is it possible to find the magnetic field of a circular loop at it's centre?
yes
Brother
The density of a gas of relative molecular mass 28 at a certain temperature is 0.90 K kgmcube.The root mean square speed of the gas molecules at that temperature is 602ms.Assuming that the rate of diffusion of a gas in inversely proportional to the square root of its density,calculate the density of
A hot liquid at 80degree Celsius is added to 600g of the same liquid originally at 10 degree Celsius. when the mixture reaches 30 degree Celsius, what will be the total mass of the liquid?
Under which topic
doris
what is electrostatics
Study of charges which are at rest
himanshu