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  • Explain the relationship between vapor pressure of water and the capacity of air to hold water vapor.
  • Explain the relationship between relative humidity and partial pressure of water vapor in the air.
  • Calculate vapor density using vapor pressure.
  • Calculate humidity and dew point.
Dew drops like these, on a banana leaf photographed just after sunrise, form when the air temperature drops to or below the dew point. At the dew point, the rate at which water molecules join together is greater than the rate at which they separate, and some of the water condenses to form droplets. (credit: Aaron Escobar, Flickr)

The expression “it’s not the heat, it’s the humidity” makes a valid point. We keep cool in hot weather by evaporating sweat from our skin and water from our breathing passages. Because evaporation is inhibited by high humidity, we feel hotter at a given temperature when the humidity is high. Low humidity, on the other hand, can cause discomfort from excessive drying of mucous membranes and can lead to an increased risk of respiratory infections.

When we say humidity, we really mean relative humidity    . Relative humidity tells us how much water vapor is in the air compared with the maximum possible. At its maximum, denoted as saturation    , the relative humidity is 100%, and evaporation is inhibited. The amount of water vapor in the air depends on temperature. For example, relative humidity rises in the evening, as air temperature declines, sometimes reaching the dew point    . At the dew point temperature, relative humidity is 100%, and fog may result from the condensation of water droplets if they are small enough to stay in suspension. Conversely, if you wish to dry something (perhaps your hair), it is more effective to blow hot air over it rather than cold air, because, among other things, the increase in temperature increases the energy of the molecules, so the rate of evaporation increases.

The amount of water vapor in the air depends on the vapor pressure of water. The liquid and solid phases are continuously giving off vapor because some of the molecules have high enough speeds to enter the gas phase; see [link] (a). If a lid is placed over the container, as in [link] (b), evaporation continues, increasing the pressure, until sufficient vapor has built up for condensation to balance evaporation. Then equilibrium has been achieved, and the vapor pressure is equal to the partial pressure of water in the container. Vapor pressure increases with temperature because molecular speeds are higher as temperature increases. [link] gives representative values of water vapor pressure over a range of temperatures.

Two containers, each filled two-thirds with water. One is open to the atmosphere and the other is sealed at the top. The water molecules are depicted as circles with vector arrows of different lengths and directions to indicate velocity. In the sealed container the density of molecules in the air above the water is greater than in the unsealed container. In the sealed container, water is condensing along the walls and top of the upper part of the container.
(a) Because of the distribution of speeds and kinetic energies, some water molecules can break away to the vapor phase even at temperatures below the ordinary boiling point. (b) If the container is sealed, evaporation will continue until there is enough vapor density for the condensation rate to equal the evaporation rate. This vapor density and the partial pressure it creates are the saturation values. They increase with temperature and are independent of the presence of other gases, such as air. They depend only on the vapor pressure of water.

Questions & Answers

When a toilet is flushed or a sink is drained, the water (and other material) begins to rotate about the drain on the way down. Assuming no initial rotation and a flow initially directly straight toward the drain, explain what causes the rotation and which direction it has in the northern hemisphere.
Collin Reply
find the change in entropy of a 2.00 kg block of gold at 1063^0C when it meets to become liquid gold at 1063^0C
precious Reply
if you are asked to make a very sensitive thermometer which of the following fluids would you choose
precious
between mercy and gasoline
precious
it good to use mercury because mercury does not wet glass and it does not evaporate easily
Desmond
0
firdaus
SFAR Sifar SIFAT -<SIFST
firdaus
how many particles are in 2 moles of chromium
Mario Reply
if so use the normal formula number of atom= number of particle/Avogadro's number
Aki
n= np/avogadtos constant. therefore n= 24/ 6.022×10²³
albert
24÷6.022×10²³
albert
@Albert is wrong
Aki
when you cross multiple it should give you Number of particles= mole*Avogadro's number X=2m*6.022*10^²³ X=1.20*10²⁴g
Aki
1.204×10^-22
Maame
please what is final velocity and initial velocity
Nonso Reply
don't know
Ekene
what do you want to become in future
Ekene
Y ar u asking pls
Nonso
I think initial velocity is the velocity that the mobile starts with at the start time (t=0s) but I don't think I heard abt final velocity
Malak
Malak where are you now I need to learn more from you
Ekene
initial velocity is the velocity an object possess at it intial position or is the starting velocity, while a final velocity is the velocity an object or body possess at it final stage or at the end of it motion
Mubarak
Bohr is kimia, of toksid, cloud, tree have cloud, tree, river but small from toksid fish or another.
firdaus Reply
Heavy, heavy kehidupan susah, kekayaan, berlambak, bergumpul. Dikenali.
firdaus Reply
Gravitional, Gravitional mean kehidup seseorang. Kehidupan bumi, kehidupan muka bumi, kehidupan dalam longitude, kehidupan dalam momentom, kehidupan dalam mongitude. Kehidupan dalam Pelajaran, mean Pelajar kolej.
firdaus
Nonconservative. Sains belajar
firdaus
hello
LFX
what are the types of kinetics
pawi Reply
what is torque pls
Sunday Reply
An aluminum rod of length 1.8cm at 0°C is heated to produced a difference in length of 0.007cm. Calculate the temperature to which it is heated. Take the linear expansively aluminun as 2.3×10^-5 K^-1.
cladious Reply
apply the formula... linear expansivity... everything you need are in the question already
Aki
Guys pls who understand equilibrium of forces mostly the calculation aspect
Faith
so can u explain how to solve the calculations
Faith
CcmT 0°0°M°T 4°4°0°0 4400°c Right Calculate 4400c Don't have rules with him
firdaus
please can someone explain how coulombs law is used to determine electric force
Favour Reply
u know coulomb's law and electric force are related in a formula which is E=f/q where E is equals to electric field intensity and f is equals to the force while-q is equals to the charge
Juilet
and I guess no the formula for electric field intensity
Juilet
and I guess you know the formula for electric field intensity
Juilet
exactly what i want to say
Haruna
Field mean like don't have we give have.. Yap... That oil Airplan. Dshell n petrol..
firdaus
what is momentum
Chinonso Reply
Momentum mean bacteria... OR CARE MONEY FROM STOLED.. OR CARE WE PLACE OR HOME FROM STOLED OR CARE WE BODY FROM STOLED
firdaus
Moment
firdaus
let him explain the statement well
diewgatdet Reply
Hello guys I'm new here
diewgatdet
hello
Godwin
Statement is resit, temu bual.
firdaus
the specimens of different materials can have same if their dimensions don't match a) resistance b) resistivity c) both d) statement is ambiguous
shoukat Reply
please explain the question
shoukat
b
diewgatdet
Bacteria.
firdaus
Specimen mean super human.. Power hin
firdaus
, yap like that lag
firdaus
please someone should help me with the Archimedes principle
Savior Reply
Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. Archimedes' principle is a law of physics fundamental to fluid mechanics.
Destiny
Have you heard of quantum fluctuations?
Adam
Or quanta
Adam
Hkekuasa human
firdaus
hello
LFX
It states that the body of a substance is partially wholly, when initially imirssed, lower in to a fluid liquid or gas, the expression upthrust which is equal to the body of the fluid displaced. But bouncy is the upward force in which a body suppressed upon a submerged body.
albert
it states that when a body is fully or partially immersed in a fluid, it experiences an upward force known as upthrust which is equal to the weight of fluid displaced.
Maame
Understanding the periodic table
David Reply
Mean kapsul don't have.. We do... Black kapsul
firdaus
We use tree. Any.. To be black
firdaus
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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