How Everything Works
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Air Conditioners
Page 1 of 2 (16 Questions and Answers)

600. What are the most important energy-efficient household appliances? How do their efficiencies compare with those of standard appliances? — LM, Klong Luang, Pathumthani, Thailand
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I can think of three important energy-efficient household electric devices: (1) heat pumps, (2) electric discharge lamps (including fluorescent lamps), and (3) microwave ovens.

A heat pump is a device that transfers heat against its natural direction of flow. If you use one to heat your home, the heat pump uses electricity to transfer heat from the colder outside air to the hotter inside air, so that the inside air becomes even hotter and the outside air becomes even colder. The electricity that the heat pump uses also becomes thermal energy inside your home. Since both the electric energy and the thermal energy pumped from the air outside end up inside your home, a heat pump provides more heat than a simple space heater can provide with the same electricity. The energy efficiency of a heat pump decreases as the temperature difference between inside and outside becomes greater, but it typically provides 4 or more times as much heat to your home as a normal electric space heater would provide with the same amount of electricity. Incidentally, when the heat pump is reversed, so that it pumps heat out of your home, it is then an air conditioner.

Electric discharge lamps are between 2 and 5 times as energy efficient as normal incandescent light bulbs. The hot filament of an incandescent lamp delivers only about 10% of its electric power as visible light. In contrast, a fluorescent lamp delivers about 25% of its electric power as visible light and some gas discharge lamps (particularly low-pressure sodium vapor) deliver as much as 50% of their electric powers as visible light.

A microwave oven transfers about 50% of its electric power directly into the water molecules of the food that you are cooking. Cooking occurs quickly and because the cooking chamber doesn't get hot, there is no power wasted in heating the oven itself or the room surrounding the oven. Depending on how large an object you are cooking, a microwave oven probably uses between 5 and 20 percent of the electricity it would take you to cook the same food in a standard oven.


755. How does a refrigerator work? - SK
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A refrigerator uses a material called a "working fluid" to transfer heat from the food inside the refrigerator to the air around the refrigerator. This working fluid moves through the refrigerator's three main components—the compressor, the condenser, and the evaporator—over and over again, in a continuous cycle. I'll begin as the fluid enters the refrigerator's compressor, which is usually located on the bottom of the refrigerator where it's exposed to the room air. The working fluid enters the compressor as a low-pressure gas at roughly room temperature. The compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since squeezing a gas involves physical work (a force exerted on an object as that object moves in the direction of the force), the compressor transfers energy to the working fluid and that fluid becomes hotter as a result.. The working fluid leaves the compressor as a high-pressure gas that's well above room temperature. The working fluid then enters the condenser, which is typically a snake-like pipe on the back of the refrigerator. Since the fluid is hotter than the room air, heat flows out of the fluid and into the room air. The fluid then begins to condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy also flows as heat into the room air.

The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like pipe that's wrapped around the freezing compartment (in a non-frostfree refrigerator) or hidden in the back of the food compartment (in a frostfree refrigerator). When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor consumed electric energy during this process and that energy has become thermal energy in the room air.


756. If heat rises, how come snow accumulates on mountains? Why is it colder up there instead of down here? — HG, Grand Prairie, TX
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On a local scale, hot air does rise through cold air. That's because when hot air and cold air are at the same temperatures, the hot air has fewer air molecules per liter than the cold air and so each liter of hot air is lighter than each liter of cold air. In short, hot air is less dense than cold air and it floats upward in cold air. But when hot air rises a long way through the atmosphere, something begins to happen to the hot air. It cools off! That's because the air pressure decreases with altitude. The air pressure that's around us on the ground is only present because the air down here must support the air overhead. The air down here must push upward on the air overhead and it does this by developing a high pressure. But as you move upward in the atmosphere, there's less air overhead and therefore less air pressure around you.

So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy—the hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the air at lower altitudes, such as the air in valleys.

Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises! This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect explains the special "Katabatic" winds that blow warm and dry out of the mountains—including the Santa Ana winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in Argentina.


821. I have read recently that achieving absolute zero is impossible. Why is this the case? What will happen to objects at this temperature (i.e., solid, liquid, and gas)? — BC, Ottawa, Ontario
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Absolute zero can't be reached for the same reason that any perfect order is impossible. It's just too unlikely to ever happen. For an object to reach absolute zero, every single bit of thermal energy and every aspect of disorder must leave the object. If the object is a crystalline material, then its crystal structure must become absolutely perfect. This sort of perfection is essentially impossible. Reducing the temperature of an object towards absolute zero requires great effort and ends up creating a great disorder elsewhere. The closer the approach to absolute zero, the more disorder is created elsewhere. To reach absolute zero, you'd have to create infinite disorder elsewhere. For something to think about, imagine trying to make you lawn absolute perfect. The more perfect you tried to make it, the more gardeners you'd need and the more food, money, and services would be consumed. The lawn would grow more and more perfect but everything else would grow more disordered. And still you would never have a truly perfect lawn.

920. How does ammonia refrigeration work?
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There are actually two answers to this question. First, like the more modern chlorofluorocarbon (Freon) and hydrofluorocarbon refrigerants, ammonia (NH3 converts easily from a gas to a liquid near room temperature. If you squeeze ammonia to high density, it will release heat and convert to a liquid. If you let it expand to low density, it will absorb heat and convert to a gas. A compressor-based ammonia refrigeration unit makes use of that easy convertibility. First, it uses a compressor to squeeze the ammonia gas outside the refrigerator. The hot dense ammonia gas that leaves the compressor enters a condenser, where it releases heat to its surroundings and condenses to a cool ammonia liquid. This liquid enters the refrigerator and passes into an evaporator, where it's allowed to expand into a gas and it absorbs heat from its surroundings. The gas then returns outside the refrigerator to repeat this cycle again and again.

But there is a second type of ammonia refrigerator that makes use of an absorption cycle—ammonia dissolves extremely well in cool water but not so well in hot water. In an absorption cycle refrigerator, a concentrated solution of ammonia in water is heated in a boiler until most of the ammonia is driven out of the water as a high-pressure gas. This hot, dense ammonia gas then enters a condenser, where it gives up heat to its surroundings and becomes a cooler liquid. The liquid ammonia then enters a low-pressure evaporator, where it evaporates into a cold gas. This evaporation process draws heat out the evaporator and refrigerates everything nearby. Finally, the ammonia gas must be returned to the boiler to begin the process again. That return step makes use of the absorption process, in which the ammonia gas is allowed to dissolve in relatively pure, cool water. The gas dissolves easily in this water and thus maintains the low pressure needed for evaporation to continue in the evaporator. The now concentrated ammonia solution flows to the boiler where the ammonia is driven back out of the water and everything repeats.


993. Does the creation of life and the theory of evolution violate the laws of thermodynamics? — BY, Liverpool, NY
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While the laws of thermodynamics forbid an overall increase in the order of the universe and while life is an example of significant order, the laws of thermodynamics don't forbid some parts of the universe from becoming more orderly at the expense of other parts of the universe becoming less orderly. Living organisms are consumers of order and exporters of disorder—they derive their order by creating disorder elsewhere. You eat highly ordered chemicals in your food and you eliminate those chemicals in much more disordered forms latter on. You also emit heat, the most disordered form of energy. Thus thermodynamics has no problem with the ongoing existence of life; it simply requires that living organisms consume order and we are doing just that at a furious pace.

As for the creation of life, that could have been a random event and thermodynamics permits random events. Improbable events do occur—people win the lottery, lightning strikes twice, two snowflakes are occasionally alike—and the creation of life could have been one of those unlikely but not impossible events. Once the simplest organism had assembled itself by chance, it could then begin the process of consuming order and exporting disorder.


994. How do fruit machines work? Do they operate on a fixed mathematical model which governs payouts using probability or are they totally random? — TS, Norfolk, UK
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I assume that you are referring to the gambling machines that spin several wheels when you pull a lever and that pay you amounts that depend on the patterns of symbols that show on the faces of the wheels when they stop. While the final arrangement of symbols that appear on such a machine when it stops is entirely random, the patterns that pay and the amounts they pay are calculated to ensure a slight financial advantage for the house. The mathematics of probability is well developed for such gambling machines and it's relatively simple to determine what fraction of your money you should expect to lose if you play the game for a very long time. If you do play long enough to sample the full statistics of the game, you are certain to lose money. It's only if you play briefly that you can take advantage of statistical fluctuations to leave with more money than you had when you started.

1104. How does an air conditioner work? — RL
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An air conditioner uses a condensable working fluid—a chemical that easily converts from a gas to a liquid and vice versa—to transfer heat from the air inside of a home to the outside air. This process involves three major components and at least one fan. The three major components are a compressor, a condenser, and an evaporator. The compressor and condenser are usually located on the outside air portion of the air conditioner while the evaporator is located on the inside air portion. The working fluid passes through the insides of these three components in order, over and over again, so I'll start examining what happens to the working fluid as it enters the compressor.

The working fluid arrives at the compressor as a cool, low pressure gas. The compressor squeezes this working fluid, packing its molecules more tightly together so that their density and pressure increase. The squeezing process also does work on the working fluid, increasing its energy and therefore its temperature. The working fluid leaves the compressor as a hot, high-pressure gas and flows into the condenser. The condenser has metal fins all around it that assist the working fluid in transferring heat to the surrounding outdoor air. As this transfer takes place, the closely spaced molecules of the working fluid begin to stick to one another, releasing additional thermal energy into the surrounding air and causing the working fluid to transform into a liquid. By the time the working fluid leaves the condenser, its temperature has almost dropped back down to the outdoor temperature but it is now a liquid rather than a gas.

This high pressure liquid then flows into the evaporator through a narrow orifice. This orifice allows the liquid's pressure to drop so that it begins to evaporate into a gas. As it evaporates, it extracts heat from the air around the evaporator because that heat is needed to separate the molecules of the working fluid. Like the condenser, the evaporator has metal fins to assist it in exchanging thermal energy with the surrounding air. By the time the working fluid leaves the evaporator, it is a cool, low-pressure gas. It then returns to the compressor to begin its trip all over again.

Overall, the working fluid releases heat into the outside air and absorbs heat from the inside air. The direction of heat transfer, from a cooler region to a hotter region, is the reverse of normal and requires an input of ordered energy so that it doesn't violate the second law of thermodynamics (the disorder of an isolated system can never decrease). This ordered energy is used to operate the compressor and is converted into thermal energy in the process. This additional disordered thermal energy enters the outside air and makes up for the additional order that's given to the indoor air as that air is cooled.


1166. You stated that thermodynamics overwhelms just about everything sooner or later. Could you explain why? — MT, San Antonio, Texas
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Thermodynamics is a statistical science that deals with systems that are so complicated or vast that they can't be followed in complete detail. It makes predictions of behavior based on probability theory and while some of its laws predict probable outcomes rather than certain outcomes, a sufficiently probably event is effectively a certain event. For example, I can say with near certainty that if you play the lottery 50 times, you won't win the jackpot 50 times. I can't be truly certain of that fact, but the likelihood of my prediction being correct is pretty good.

In a sense, probability is destiny. Thermodynamics observes that vast systems tend to evolve toward the mostly likely configurations. To understand this process, consider what happens when you mix hot and cold water. The most likely final configuration for the mixed water is for it to reach a uniform temperature about half way in between the two original temperatures. While it's possible for the water to end up extremely hot in one place and extremely cold in another, that outcome is extremely unlikely. It's so unlikely that it never happens.

So in what sense does thermodynamics overwhelm things? The world is filled with relatively ordered arrangements and these ordered arrangements are unlikely by themselves (how they came to be ordered in the first place is another matter for another questions). If you take a crystal vase and drop it on the floor, it's going to evolve toward a more likely arrangement of atoms and dropping it a second time isn't going to return it toward its original unlikely state. In short, ordered systems naturally drift toward disorder when given a chance. How quickly they drift depends on their situation. A coffee cup will remain a nicely ordered object for thousands or millions of years if you don't disturb it. But in a hot environment, or one that is chemically aggressive, it may not last very long.

One last thought: how do living organisms maintain their order in the face of this tendency to disorder? They do it by consuming order and exporting disorder—they eat ordered foods and release disordered wastes to their surroundings.


1172. If living organisms maintain their order by exporting disorder to their environment, do they create more disorder than the order they maintain? — CC
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Living organisms create more disorder in their surroundings than they create order in themselves. Overall the disorder of the combined system—organisms and environment—increases. This result is an unavoidable consequence of the second law of thermodynamics, which notes that the entropy (disorder) of an isolated system can never decrease. While it is possible in principle for a living organism to export disorder so efficiently that the overall disorder remains unchanged, that perfection is never achieved. Instead, living organisms export far more disorder than is required for them to maintain order in themselves. As a result, living organisms are net producers of disorder.

In that respect, people are much more vigorous producers of disorder than most other living organisms. People seek order not only in their bodies, but also in the objects around them and they achieve this ordering by consuming order in their environment—fossil fuels, minerals, pure water—at a furious pace and producing disorder in its place—burned gases, garbage, polluted water. Fortunately, sunlight is a tremendous source of order for our earth and it undoes some of the disordering caused by living organisms. However, we are consuming much of the order that sunlight stored on earth over millions of years in only a few generations. At this pace, we're destined to have troubles with the disorder we're creating. Many of the environmental issues that face us today can be viewed from this order/disorder perspective: we have to learn how to create less disorder.


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