Second Law of Thermodynamics

The second law of thermodynamics says that it is impossible to make a machine that functions by completely converting heat, drawn from some body or object at a given temperature, into work (in the sense of physics, where a force moves something over a distance). This is one of the most important constraints of physics. A lot of heat is generated by friction, for example; this is wasteful, for more effort is required in the presence of friction than without it. What the second law of thermodynamics says is that some of this loss is not reversible.

Consider pushing a cart across a level street, an example discussed in the earlier sidebar, “The First Law of Thermodynam- ics.” Pushing a cart involves work, but unlike lifting a weight, there is little or no energy “stored” in the position of the cart—it cannot return across the street on its own or do work in the process. Friction and air resistance generates heat, and although the energy of motion that produces this heat is not destroyed, the conversion is not fully reversible. Some of this thermal energy can be used to do work, but not all. No matter how the attempt is made or what machine or technology is em- ployed, it is not possible to recapture all of the thermal energy and do work with it.

Getting around the second law of thermodynamics would be terrifi c because then it would be possible to do such things as moving heat from a cold body to a hot one without any energy input. Air-conditioning would be free. It is not, however, and can never be, because plenty of careful experiments involving heat and work confi rm that the world adheres to the second law of thermodynamics.

There are several formulations of the second law of thermodynamics. Chapter 5 describes one that is different but equiva- lent to the above.

under low pressure, it evaporates and becomes a gas, which then expands. This cools the fluid in the same way that a breath blown through compressed lips feels cool—although the air comes from the body, which is warm, as the air leaves the compressed lips, it expands. The molecules of the gas lose energy as they expand because they push away the surrounding air molecules, thus doing

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work on them. According to the first law of thermodynamics, some of their energy is lost. As a result, the temperature falls.

The evaporator is located inside. As the working fluid cools, it absorbs heat from the room. This lowers the temperature of the room. But the fluid must deposit this heat outside or the job is not finished.

The working fluid, which is now a warm, low-pressure gas, circulates through pipes and enters a compressor. The compressor does what its name suggests: it compresses the gas by application of pressure. This requires work (and therefore energy) because it forces the fluid to go into a small space.

The compression rapidly increases the temperature of the working fluid. There is a law, called the ideal gas law, that neatly describes this process with an equation relating volume, pressure, and temperature of a gas. The temperature increase can also be thought of in the following way. During the compression, the walls of the chamber holding the gaseous atoms and molecules move inward (similar to a bad horror movie in which the walls start to

The cooling process transfers heat with a working fluid flowing through the pipe. On the inside, the fluid picks up heat by evaporation (a phase transition from liquid to gas) and expansion. After compression, the fluid loses this heat to the outside by condensing (a phase transition from gas to liquid).

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cave in). The atoms and molecules are always in motion, and when they bounce off the inward moving wall, they gain a little more speed from the impact. (If the wall was moving outward, they would lose a little speed.) Temperature depends on atomic and molecular speed, and the fluid gets warmer.

Now the gas is hot, and it circulates to the condenser—a long, thin metal tube, like the evaporator, except the condenser is located outside. If the working fluid is hotter than the outside air, then heat flows out of it though the metal tube (which is an effective heat conductor). Thermal energy escapes, and the working fluid condenses into a liquid. When the fluid emerges from the condenser, it has changed from a hot, high-pressure gas to a warm liquid and has transferred heat from the inside to the outside. From here the liquid returns to evaporator, and the cycle begins again.

Heat flows in its natural direction both inside and outside the room during the operation of the air conditioner. But a little manipulation of the working fluid made it cooler than the room temperature (so heat flows in) and warmer than the outside temperature (so heat flows out). This required compression of the working fluid, which is the part of the process that needs energy, usually in the form of electricity to operate the machine.

The working fluid could be water, but in general it is not. For

a long time after people first developed air conditioners, the fluid was a substance called a chlorofluorocarbon (CFC), which con- tains chlorine atoms. This substance is an excellent choice because it makes phase transitions between gas and liquid over a wide range of temperatures. It is also cheap and does not corrode the air conditioner’s pipes and tubes. The problem is that CFCs sometimes release chlorine atoms, which enter the atmosphere and attack the ozone layer. The ozone layer is essential for the protection of life on the planet because it absorbs harmful ultraviolet radiation from the Sun. Recently, CFCs have been replaced by hydrofluoro- carbons, which do not contain chlorine. These substances do not work as well as chlorofluorocarbons, but they seem to be safer for the ozone layer. (However, they happen to be greenhouse gases, as mentioned in chapter 1.)

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Refrigerators perform the same kind of operation as air conditioners, except they do not cool the room or a house but rather the small area of the refrigerator’s interior, where food and other perishables are kept. As in air conditioners, a circulating fluid in the refrigerator extracts heat from the interior and deposits it elsewhere. In the case of refrigerators, “elsewhere” means not outside the room or house but rather outside the refrigerator; generally, this means the heat goes into the room.

A hand placed behind the refrigerator will discover the heat flowing from the working fluid as it circulates in the condenser coils. (The coils are the grill-like objects at the back of the refrigerator.) Because the refrigerator transfers heat into the room, it is not a good idea to try to use it as an air conditioner by leav- ing the door open. Although the frigid air from the refrigerator’s interior will temporarily cool a person standing next to the open door, the room will not get any cooler. When the door is open, the refrigerator will continue to run, taking heat from inside the refrigerator—which, with the door open, is the entire room—and depositing it back into the room. This is no way to run an air conditioner!

Second Law of Thermodynamics