Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid. Evaporation is also part of the water cycle.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to 'escape' (evaporate).

Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room temperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquids ''are'' evaporating. It is just that the process is much slower and thus significantly less visible.

Evaporation is an essential part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor, this vapor can then rise up and form clouds.

Theory

For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces. Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid, thus, decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat cools the human body. Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts to evaporation are heat, humidity, and air movement.

On a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface can be seen.

Evaporative equilibrium

If evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium, the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius-Clapeyron relation:

: $\backslash ln\; \backslash left(\; \backslash frac\{\; P\_2\; \}\{\; P\_1\; \}\; \backslash right)\; =\; -\; \backslash frac\{\; \backslash Delta\; H\_\{\; vap\; \}\; \}\{\; R\; \}\; \backslash left(\; \backslash frac\{\; 1\; \}\{\; T\_2\; \}\; -\; \backslash frac\{\; 1\; \}\{\; T\_1\; \}\; \backslash right)$

where ''P''₁, ''P''₂ are the vapor pressures at temperatures ''T''₁, ''T''₂ respectively, Δ''H''_vap is the enthalpy of vaporization, and ''R'' is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.

The ability for a molecule of a liquid to evaporate is based largely on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.

Factors influencing the rate of evaporation

;Concentration of the substance evaporating in the air: If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly. ;Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating. ;Concentration of other substances in the liquid (impurities): If the liquid contains other substances, it will have a lower capacity for evaporation. ;Flow rate of air: This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer. ;Inter-molecular forces: The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by the enthalpy of vaporization. ;Pressure: Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves. ;Surface area: A substance that has a larger surface area will evaporate faster, as there are more surface molecules that are able to escape. ;Temperature of the substance: If the substance is hotter, then its molecules have a higher average kinetic energy, and evaporation will be faster. ;Density: The higher the density the slower a liquid evaporates.

In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over per year.

Applications

When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low humidity, heat (from the sun), and wind. In a clothes dryer, hot air is blown through the clothes, allowing water to evaporate very rapidly.

The botijo, a traditional Spanish porous clay container designed to cool the contained water by evaporation.

Combustion vaporization

Fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat (energy) can also be received by radiation from any hot refractory wall of the combustion chamber.

Film deposition

Thin films may be deposited by evaporating a substance and condensing it onto a substrate.

See also

Atmometer (evaporimeter)

Condensation

Crystallisation

Desalination

Distillation

Drying

Eddy covariance flux (aka eddy correlation, eddy flux)

Hydrology (agriculture)

Latent heat flux

Evaporator

Evapotranspiration

Flash evaporation

Heat of vaporization

Latent heat

Pan evaporation

Transpiration

References

Has an especially detailed discussion of film deposition by evaporation.

Category:Atmospheric thermodynamics Category:Basic meteorological concepts and phenomena Category:Materials science Category:Phase changes Category:Thin film deposition

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Julius Sumner Miller (May 17, 1909April 14, 1987) was an American physicist and television personality. He is best known for his work on children's television programs in North America and Australia.

Off-screen

Julius Sumner Miller was born in Billerica, Massachusetts the youngest of nine children. His father was Latvian, his Lithuanian mother spoke 12 languages.

Miller graduated with a Master in Physics from Boston University in 1933 but due to the Depression worked as a servant for the next two years during which time he married Alice Brown; they were to have no children, but he was to go on to reach millions of children through his popular science programs.

In 1937 he obtained employment in the Physics Department of Dillard University, a private, black liberal arts college in New Orleans. He was to go on to the Physics Department of the Michigan College of Mining and Technology (1949–1951) and the Physics Department at El Camino College in Torrance, California (1952–1974), to maximum student enrollments due to his great popularity.

From 1963 to 1986 he was the visiting lecturer for the Physics Department of the University of Sydney and from 1965 to 1985, the US Air Force Academy.

A student of Albert Einstein, he was instantly recognizable by his casual hair and horn-rimmed spectacles.

Television

Miller began hosting his educational program, "Why Is It So?" in 1959 on KNXT Channel 2 in Los Angeles. From 1962 to 1964, Miller was Disney's "Professor Wonderful" on new introductions, filmed at Disneyland, to the syndicated reruns of ''The Mickey Mouse Club''. During the same period, he appeared on a semi-regular basis, performing physics experiments, on Steve Allen's late night TV show in Hollywood, syndicated by Group W.

Miller's first television appearance in Australia was on ''Bob Sanders People'' in 1963. In an improvised physics demonstration he attempted to drive a straw through a raw potato. A paper straw does not have the strength but if you pinch the end, the trapped air acts as a piston, easily piercing the potato. For the first time in his career he could not get it to work and loudly exclaimed "''Australian straws ain't worth a damn''". The next morning, Miller arrived at his Sydney University laboratory to find one million drinking straws on the floor with a telegram reading "''You might find one of these fitting your requirements''". He later stated "''I sat amongst the straws with straws stuck in my hair and ears. But clearly I had made a mistake. I should have said: "Australian potatoes ain’t worth a damn," and I’d have cornered the potato market!''"

Shortly after, he was offered a job presenting science for ABC Television. When asked how much money he wanted he replied he never asked, he listened to an offer then "multiplied it by a factor between two and ten". Due to budget constraints the offer was withdrawn but later an agreement was reached for Miller to host his own science based TV series. ''Why Is It So?'' (his eventual stock phrase), was broadcast from 1963 to 1986 and became an instant hit, the 1960s program became ''Demonstrations in Physics'' (also called ''Science Demonstrations'' when it was aired on American Public television). He introduced each episode with the line: "How do you do, ladies and gentlemen, and boys and girls ''[sometimes adding "and teachers"]''. I am Julius Sumner Miller, and physics is my business."

"My first TV series on demonstrations in physics - titled ''Why Is It So'' were now seen and heard over the land. The mail was massive. The academics were a special triumph for me. They charged me with being superficial and trivial. If I had done what they wanted my programs would be as dull as their classes! I knew my purpose well and clear: to show how Nature behaves without cluttering its beauty with abtruse mathematics. Why cloud the charm of a Chladni plate with a Bessel function?"

Miller's on-air popularity was due to an enthusiasm not normally associated with serious science. Shows would be liberally sprinkled with phrases such as "''He who is not stirred by the beauty of it is already dead!''" and he also liked to trick the audience. A common ploy would be to hold up an empty glass and ask guests to confirm it was empty....then chide them for not noticing it was full of air. Before each demonstration he would usually ask for a show of hands to indicate which of several results they expected. Often he would then add "''hands up those who don’t care''".

In 1966 questions from his show with an answer to the previous day’s question were published as "Millergrams" for ''The Australian'' newspaper. 112 of these questions were published as a book "''Millergrams; Some Enchanting Questions for Enquiring Minds''". Further books were published in 1967: "''The Second Book of Millergrams: Some More Enchanting Questions for Enquiring Minds''" and 1988: "''Why is it So?: The Very Best Millergrams of Professor Julius Sumner Miller''".

Sample Millergram:Q32: A juggler comes to a foot-bridge of rather flimsy design. He has in hand four balls. The safe load is no more than the juggler himself and one ball. Can he get across the bridge by juggling the balls, always having at most one ball in the hand (and three in the air)?A: No. A falling ball exerts a force on the hand greater than its own weight.Rather, a 'thrown' ball exerts greater force than a 'held' one. That is, the additional force equal and opposite to that imparted to a flung ball, in addition to the juggler's mass, would exceed the bridge's tolerance (the bridge can tolerate a juggler and held ball, but not the additional downward force associated with forcing a ball 'up').

Miller appeared as "The Professor" in the Canadian series ''The Hilarious House of Frightenstein'' (1971), in a 2- to 3-minute segment each episode where he demonstrated physics experiments and explained the principles involved.

Miller was also an occasional guest in the 1970s on ''The Tonight Show Starring Johnny Carson'' in the United States.

Commercials

During the 1980s Miller appeared in a famous series of Australian television commercials for Cadbury chocolate, using his stock phrase "Why is it so?", demonstrating a simple scientific principle, and describing how each block of chocolate "embraces substantial nourishment and enjoyment," and contained "a glass and a half of full-cream dairy milk."

While in Australia, Miller also appeared in ads for non-stick saucepans and Ampol Petroleum which included demonstrations of real principles of physics, albeit briefly. The ads were sufficiently popular to be played for some years after his death.

Foundations

Miller died on April 14, 1987, of leukemia in San Jose, California. Professor Miller's wife, Alice Brown Miller, wanted to perpetuate the memory and achievements of her husband, and so conceived the idea of the Julius Sumner Miller Foundation, which was established in 1998. Miller willed his body to the University of Southern California's School of Dentistry.

Through an offer by Cadbury-Schweppes Pty Ltd, the Cadbury-Julius Sumner Miller Scholarship for Academic Excellence was set up to provide undergraduate scholarships in the School of Physics at the University of Sydney.

In 1993 the Australian Science Foundation for Physics established the Julius Sumner Miller Fellowship in his memory. The fellowship is currently held by Karl Kruszelnicki, best known as "Doctor Karl" for his appearances on Australian radio and television as a science commentator and author.

Bibliography

''Millergrams; Some Enchanting Questions for Enquiring Minds'', Ure Smith, 1966

''The Second Book of Millergrams: Some More Enchanting Questions for Enquiring Minds'', Ure Smith, 1967

''Quiz Questions in Physics'', Horwitz-Martin, Australia 1967

''Why It Is So'', ABC books, 1971 ISBN 0642972966

''Why It Is So: Heat and Temperature'', ABC books, 1973 ISBN 0642974969

''Why It Is So: Sound and Electricity & Magnetism'', ABC books, 1973 ISBN 0642975841

''Why It Is So: Mechanics, Heat & Temperature, Sound and Electricity'', ABC books, 1978 ISBN 064297523X

''Enchanting Questions for Enquiring Minds'', Currey/O'Neil, 1982 ISBN 0-85902-280-3

''Why is it so?: the very best Millergrams of Professor Julius Sumner Miller'', Australian Red Cross Society, Ringwood, Vic; Penguin Books, 1988

''The Days of My Life: an autobiography'', Macmillan, 1989. ISBN 0-333-50337-6

References

External links

Julius Sumner Miller Foundation

Australian Julius Sumner Miller tribute page

Julius Sumner Miller page at a Hilarious House of Frightenstein tribute site

Julius Sumner Miller at TV.com

Why is it so? site at Australia's ABC.net.au featuring original episodes

http://www.physics.usyd.edu.au/foundation/Outreach/DRK/jsm.pdf Julius Sumner Miller 1909 - 1987, Physics Department, University of Sydney.

Category:1909 births Category:1987 deaths Category:American educators Category:American physicists Category:American television personalities Category:Boston University alumni Category:People from Billerica, Massachusetts Category:Deaths from leukemia Category:Cancer deaths in California

Ken Caldeira is an atmospheric scientist who works at the Carnegie Institution for Science's Department of Global Ecology. He researches ocean acidification, climate effects of trees, intentional climate modification, and interactions in the global Carbon cycle/climate system. He also acted as an inventor for Intellectual Ventures, a Seattle-based invention and patent company headed up by Nathan Myhrvold.

Caldeira's work was featured in a November 2006 article in ''The New Yorker'', entitled "The Darkening Sea." In 2007, he contributed two op-ed pieces on the subject of global warming to ''The New York Times''. He was named a "Hero Scientist of 2008" by New Scientist magazine.

In response to the controversy caused by Superfreakonomics over Caldeira's view on geoengineering, Caldeira rejected the suggestion that he said ″Carbon dioxide is not the right villain″. He responded by posting on his website, "Carbon dioxide is the right villain...insofar as inanimate objects can be villains." He said that while the other statements attributed to him by authors Steven Levitt and Stephen Dubner are ″based in fact″, the casual reader could come up with a misimpression of what he (Caldeira) believes.

Biography

In the 1980s, Caldeira worked as a software developer. He received his Ph.D in Atmospheric Sciences in 1991 from the New York University Department of Applied Science. From 1991 to 1993, Caldeira worked at Penn State University as a post-doctoral researcher. He then worked as an Environmental Scientist and Physicist at Lawrence Livermore National Laboratory until 2005, when he began his current position at the Carnegie Institution Department of Global Ecology.

References

External links

Carnegie Institution Department of Global Ecology

Ken Caldeira and his Carnegie lab

SourceWatch profile

Category:Living people Category:Atmospheric scientists Category:New York University alumni Category:Lawrence Livermore National Laboratory