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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 ( ° C ) 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

R.H. = Partial pressure of water vapor at T Vapor pressure of water at T × 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 ºC and the dew point is 15 ºC ?


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


R.H. = Partial pressure of water vapor at 15 °C Partial pressure of water vapor at 25 °C × 100 % = 1705 Pa 3167 Pa × 100 % = 53.8 % .


R.H. is important to our comfort. The value of 53.8 % is within the range of 40 % to 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.

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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    , λ , the average distance a molecule travels between collisions with other molecules, and the mean free time τ , 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 π 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 λ such that the expected number of other molecules in a cylinder of length λ and cross-section 4 π 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 π r 2 λ , so we have ( N / V ) 4 π r 2 λ = 1 , or

λ = V 4 π 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 2 . The result is

Questions & Answers

Newton's second laws is call with
Dyutee Reply
what is mean by thermodynamics
Prasad Reply
it is study about temperature and it's equilibrium
Its the study of heat and its relation with others kind of energy
state caulombs law clearly
constand Reply
show mathematically that an electron has the greater speed than the proton when they attract each other
ezra Reply
show mathematically that an electron has the greater speed than the proton when they attract each other
@ezra & srikanta; for electrons: a=ke^2/(mr^2) and for protons: a=kp^2/(mr^2)
what is electrostatics
Hero Reply
the study of charge at rest
@Hero; the study of charges at rest is the electrostatics
okay what is electrostatic?
charge at rest
set of character...
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Minister Reply
@Minister; use equation v= sq root(2×eV/m)
what is the reaction of heat on magnet
Magnetization decreases with increase in temperature. But in case of diamagnetic substance heat has no role on magnetization.
what is a physical significant of electric dipole moment .
A dipole moment it's a mechanical electrical effect used in nature
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Malik Reply
to minimize heat
at what temperature is the degree Fahrenheit equal to degree Celsius
Grace Reply
Celsius and Faharaneith are different, never equal
find their liners express of n=a+b/T² ( plot graph n against T)
Donsmart Reply
Radio Stations often advertis "instant news,,if that meens you can hear the news the instant the radio announcer speaks it is the claim true? what approximate time interval is required for a message to travel from Cairo to Aswan by radio waves (500km) (Assume the waves Casbe detected at this range )
mahmod Reply
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Pawan Reply
Can someone please predict the trajectory of a point charge in a uniform electric field????
erlinda Reply
what is deference between strong force and coulomb force
zahid Reply
how do you convert temperature in degree Celsius to Fahrenheit
Celsius x 9/5 +32
Practice Key Terms 8

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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