College PhysicsChapter 15 THERMODYNAMICSPowerPoint Image Slideshow
A steam engine uses heat transfer to do work. Tourists regularly ride this narrow-gauge steam engine train near the San Juan Skyway in Durango, Colorado,part ofthe National Scenic Byways Program. (credit: Dennis Adams)
This boiling tea kettle represents energy in motion. The water in the kettle is turning to water vapor because heat is being transferred from the stove to thekettle. Asthe entire system gets hotter, work is done—from the evaporation of the water to the whistling of the kettle. (credit: Gina Hamilton)
Beginning with the Industrial Revolution, humans have harnessed power through the use of the first law of thermodynamics, before we even understoodit completely. This photo, of a steam engine at theTurbiniaWorks, dates from 1911, a mere 61 years after the first explicit statement of the first law of thermodynamicsby RudolphClausius. (credit: public domain; author unknown)
Heat transfer to the gas in a cylinder increases the internal energy of the gas, creating higher pressure and temperature.The force exerted on the movable cylinder does work as the gas expands. Gas pressure and temperature decrease when it expands, indicating that the gas’s internal energy has been decreased by doing work.Heat transfer to the environment further reduces pressure in the gas so that the piston can be more easily returned to its starting position.
APVdiagram in which pressure varies as well as volume. The work done for each interval is its average pressure times the change in volume, or thearea underthe curve over that interval. Thus the total area under the curve equals the total work done.Workmust be done on the system to follow the reverse path. Thisis interpretedas a negative area under the curve.
Thework done in going from A to C depends on path. The work is greater for the path ABC than for the path ADC, because the former is at higherpressure. Inboth cases, the work done is the area under the path. This area is greater for path ABC.Thetotal work done in the cyclical process ABCDA is the area inside theloop, sincethe negative area below CD subtracts out, leaving just the area inside the rectangle. (The values given for the pressures and the change in volume are intended foruse inthe example below.)Thearea inside any closed loop is the work done in the cyclical process. If the loop is traversed in a clockwise direction,Wis positive—it iswork doneon the outside environment. If the loop is traveled in a counter-clockwise direction,Wis negative—it is work that is done to the system.
Theupper curve is an isothermal process ( ΔT= 0 ), whereas the lower curve is an adiabatic process (Q= 0 ). Both start from the same pointA, butthe isothermal process does more work than the adiabatic because heat transfer into the gas takes place to keep its temperature constant. This keeps thepressure higherall along the isothermal path than along the adiabatic path, producing more work. The adiabatic path thus ends up with a lower pressure and temperature atpoint C, even though the final volume is the same as for the isothermal process.Thecycle ABCA produces a net work output.
These ice floes melt during the Arctic summer. Some of them refreeze in the winter, but the second law of thermodynamics predicts that it would beextremely unlikelyfor the water molecules contained in these particular floes to reform the distinctive alligator-like shape they formed when the picture was taken in the summer of 2009. (credit: Patrick Kelley, U.S. Coast Guard, U.S. Geological Survey)
Examples of one-way processes in nature.Heattransfer occurs spontaneously from hot to cold and not from cold to hot.Thebrakes of this car convertits kineticenergy to heat transfer to the environment. The reverse process is impossible.Theburst of gas let into this vacuum chamber quickly expands to uniformly fillevery partof the chamber. The random motions of the gas molecules will never return them to the corner.
In the four-stroke internal combustion gasoline engine, heat transfer into work takes place in the cyclical process shown here. The piston is connected toa rotatingcrankshaft, which both takes work out of and does work on the gas in the cylinder. (a) Air is mixed with fuel during the intake stroke. (b) During the compressionstroke, theair-fuel mixture is rapidly compressed in a nearly adiabatic process, as the piston rises with the valves closed. Work is done on the gas. (c) The power stroke hastwo distinctparts. First, the air-fuel mixture is ignited, converting chemical potential energy into thermal energy almost instantaneously, which leads to a great increase inpressure. Thenthe piston descends, and the gas does work by exerting a force through a distance in a nearlyadiabatic process. (d) The exhaust stroke expels the hot gas toprepare theengine for another cycle, starting again with the intake stroke.
This Otto cycle produces a greater work output than the one inFigure 15.19, because the starting temperature of path CD is higher and the startingtemperature ofpath AB is lower. The area inside the loop is greater, corresponding to greater net work output.
This novelty toy, known as the drinking bird, is an example of Carnot’s engine. It contains methylene chloride (mixed with a dye) in the abdomen, which boils ata verylow temperature—about 100ºF . To operate, one gets the bird’s head wet. As the water evaporates, fluid moves up into the head, causing the bird to becometop-heavy anddip forward back into the water. This cools down the methylene chloride in the head, and it moves back into the abdomen, causing the bird to become bottom heavyand tipup. Except for a very small input of energy—the original head-wetting—the bird becomes a perpetual motion machine of sorts. (credit: Arabesk.nl, Wikimedia Commons)
Schematic diagram of a pressurized water nuclear reactor and the steam turbines that convert work into electrical energy. Heat exchange is usedto generatesteam, in part to avoid contamination of the generators with radioactivity. Two turbines are used because this is less expensive than operating a singlegenerator thatproduces the same amount of electrical energy. The steam is condensed to liquid before being returned to the heat exchanger, to keep exit steam pressure lowand aidthe flow of steam through the turbines (equivalent to using a lower-temperature cold reservoir). The considerable energy associated with condensation mustbe dissipatedinto the local environment; in this example, a cooling tower is used so there is no direct heat transfer to an aquatic environment. (Note that the water goingto thecooling tower does not come into contact with the steam flowing over the turbines.)
Real heat engines are less efficient than Carnot engines.Realengines use irreversible processes, reducing the heat transfer to work. Solid linesrepresent theactual process; the dashed lines are what a Carnot engine would do between the same two reservoirs.Frictionand other dissipative processes in theoutput mechanismsof a heat engine convert some of its work output into heat transfer to the environment.
Almost every home contains a refrigerator. Most people don’t realize they are also sharing their homes with a heat pump. (credit: Id1337x, Wikimedia Commons)
Heat transfer from the outside to the inside, along with work done to run the pump, takes place in the heat pump of the example above.Note that thecold temperatureproduced by the heat pump is lower than the outside temperature, so that heat transfer into the working fluid occurs. The pump’s compressor producesa temperaturegreater than the indoor temperature in order for heat transfer into the house to occur.
In hot weather, heat transfer occurs from air inside the room to air outside, cooling the room. In cool weather, heat transfer occurs from air outside to airinside, warmingthe room. This switching is achieved by reversing the direction of flow of the working fluid.
The ice in this drink is slowly melting. Eventually the liquid will reach thermal equilibrium, as predicted by the second law of thermodynamics. (credit: JonSullivan, PDPhoto.org)
Heattransfer from a hot object to a cold one is an irreversible process that produces an overall increase in entropy.Thesame final state and,thus, thesame change in entropy is achieved for the objects if reversible heat transfer processes occur between the two objects whose temperatures are the same asthe temperaturesof the corresponding objects in the irreversible process.
ACarnot engine working at between 600 K and 100 K has 4000 J of heat transfer and performs 3333 J of work.The4000 J of heat transferoccurs firstirreversibly to a 250 K reservoir and then goes into a Carnot engine. The increase in entropy caused by the heat transfer to a colder reservoir results in asmaller workoutput of 2400 J. There is a permanent loss of 933 J of energy for the purpose of doing work.
When ice melts, it becomes more disordered and less structured. The systematic arrangement of molecules in a crystal structure is replaced by a morerandom andless orderly movement of molecules without fixed locations or orientations. Its entropy increases because heat transfer occurs into it. Entropy is a measure of disorder.
Earth’s entropy may decrease in the process of intercepting a small part of the heat transfer from the Sun into deep space. Entropy for the entireprocess increasesgreatly while Earth becomes more structured with living systems and stored energy in various forms.
When you toss a coin a large number of times, heads and tails tend to come up in roughly equal numbers. Why doesn’t heads come up 100, 90, or even 80%of thetime? (credit: Jon Sullivan, PDPhoto.org)
Theordinary state of gas in a container is a disorderly, random distribution of atoms or molecules with a Maxwell-Boltzmann distribution of speeds. It isso unlikelythat these atoms or molecules would ever end up in one corner of the container that it might as well be impossible.Withenergy transfer, the gas can be forcedinto onecorner and its entropy greatly reduced. But left alone, it will spontaneously increase its entropy and return to the normal conditions, because they are immenselymore likely.
The two cyclical processes shown on thisPVdiagram start with and return the system to the conditions at point A, but they follow different paths andproduce differentamounts of work.
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