Thermal Physics

[5.1] HEAT / TEMPERATURE SCALES / HEAT ENERGY

However, properly defining these terms is tricky. Formally speaking, heat is not a property of an object, but is instead a transfer of energy between objects. It clearly follows a few simple rules:

• Heat flows from a hot object to a cooler one. We use a hot plate to boil water, for example, essentially pumping heat into the water.

• Heat never flows in the reverse direction by itself. We have to do work to cool something down, for example using a refrigerator to freeze water into ice cubes.

Before the 18th century, scientists envisioned that a hot object contained a concentrated amount of a hypothetical "fluid" they named "caloric" that was the agent of heat transfer. It could be generated out of a material by mechanical work, for example rubbing sticks together to start a fire, and could be converted back into mechanical work, for example using a steam engine.

The problem with caloric was that its only identifiable characteristic was that it transferred heat. It was not visible in any way and had no other recognizable properties. What transferred heat? Caloric. What was caloric? It was what transferred heat. Physicists learned to distrust such arbitrary constructs.

It wasn't until the 19th century that physicists began to understand that heat was simply the consequence of the motion of the molecules in an object. Adding energy to the object increased the velocity and the kinetic energy of the molecules, increasing the "internal energy" or, more informally, the "thermal energy" of the object. The temperature of the object was then a measure of the thermal energy, essentially an average of the kinetic energy of all the molecules of that object.

Heat in turn became a transfer of this thermal energy. If a hot object was brought in contact with a cooler object, collisions between the energetic molecules of the hot object increased the velocity of the slower molecules of the cooler object. The fact that temperature was a measure of molecular motion established a direct link between heat and elementary classical mechanics.

One of the interesting implications of this view of temperature was that if all molecular motion in an object ceased, then there would be no way for the object to get any colder. This implied the existence of an "absolute zero" temperature.

In the Celsius scale, the freezing point of water is specified as 0 degrees Celsius, while the boiling point is specified as 100 degrees Celsius. In the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit and the boiling point is 212 degrees Fahrenheit. Conversions between the two can be performed as follows:

    degrees_Fahrenheit = ( 9/5 ) * degrees_Celsius + 32
 
    degrees_Celsius    = ( 5/9 ) * ( degrees_Fahrenheit - 32 )
 

    degrees_Kelvin  =  degrees_Celsius + 273.15
 
    degrees_Celsius =  degrees_Kelvin - 273.15
 

Incidentally, there is also a "Rankine" scale, now largely out of use, that is the same as the Fahrenheit scale, except that the 0 point is at absolute zero, equivalent to -459.67 degrees Fahrenheit.

In English units, heat is given by the "British thermal unit (BTU)", which is the amount of heat required to raise a pound of water one degree Fahrenheit. The BTU remains in use, but it is being increasingly obsoleted by the calorie. One BTU is equivalent to 252 calories or 1,055 joules.

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[5.2] HEAT & MATTER

• Suppose a gas is stored in a rigid container with solid walls, meaning its volume is constant.

  If Dexter then heats up the gas inside the container to twice its original temperature (using the absolute Kelvin scale), them the final pressure is twice the original pressure. Similarly, if Dexter cools the container to half its original temperature, the pressure falls to half of its original pressure. This is a "constant volume" process.

• Now suppose the gas is in a flexible balloon. Ignoring the tension forces that hold the balloon together, the balloon expands until the internal pressure matches that of the external atmosphere, which is constant in the balloon's immediate surroundings.

  If the gas in this balloon is warmed to twice its original temperature, the pressure inside the balloon will remain the same, but the balloon will stretch until it has twice its original volume. Similarly, if Dexter cools the gas in the balloon to half its original temperature, the balloon will shrink to half its original volume. This is a "constant pressure" process.

The relationships become more complicated if temperature, volume, and pressure are varied all at the same time, but the general effects of changes in pressure, temperature, and volume remain apparent. Incidentally, gases at typical Earthly conditions are a good approximation of ideal gases. However, at extremes of pressure or temperature gases may depart from this nice neat behavior.

One simple practical application of this property is the "bimetallic strip" used in traditional thermostats. This is a strip of metal with brass on one side and iron on the other. Since the two metals have different coefficients of thermal expansion, as the strip is heated or cooled it will bend one way or another in a predictable fashion, and the thermostat can be set so that if it bends to a particular position, it will complete an electric connection and turn on the heat or air conditioning as need be.

Some materials don't go through a liquid phase, converting directly from solid to vapor, at least under typical Earthly atmospheric pressures, a process that is known as "sublimation". Frozen carbon dioxide sublimates, which is why it is used to cold-pack parcels that have to be shipped, since such "dry ice" doesn't create puddles.

The amount of heat that must be added to a substance produce a phase change is called the "latent heat". They will vary for a given material for different phase changes, and so there are different latent heats of melting, vaporization, or when it applies, sublimation.

At normal atmospheric pressure, water cannot be heated above 100 degrees Celsius, because any additional energy simply vaporizes the water. Boiling water is actually, in a sense, a "cooling" process since it prevents the accumulation of energy. A pressure cooker is used to obtain higher water temperatures. When water condenses again, that latent heat that vaporized it is released.

The level of water vapor in the air is referred to as "humidity". There is a maximum level of humidity that air can support, which increases with temperature since water condenses back to liquid more easily at low temperatures than high. When the air can accommodate no more water vapor, it is said to be "saturated".

Humidity is usually given in terms of "relative humidity", or the ratio of actual water vapor to the saturation level. At 50% relative humidity, the air contains half the amount of water vapor that it is capable of supporting, and at 100% relative humidity the air is saturated.

The evaporation of water is used for cooling in the "evaporative cooler" or "swamp cooler", a simple household cooling device used in dry, warm climates. It consists of little more than a fan pulling air through a filter through which water is pumped. The air causes the water to evaporate, moistening and cooling the air.

It is substantially cheaper to buy and operate than a conventional air conditioner, whose operating principles will be discussed in a later section of this chapter. An evaporative cooler is ineffective in hot humid climates since the rate of evaporation slows, and in fact it may simply make a living space more humid.

Incidentally, water is an unusual substance in that it actually expands when frozen, due to the way its molecules rearrange themselves. This is a fortunate circumstance for life on Earth, since if it were not so, all ice forming on the ocean would sink to the bottom and gradually build up a reservoir of ice that would make the planet a permanent "icebox".

In the case of materials that are compressible, specific heat has to be rated in terms of whether the materials are held at constant pressure or at constant volume.

The concept of specific heat leads to the notions of "thermal mass" and "heat reservoir". For example, a lake has a thermal mass in that it will require energy and involve a certain delay to heat it up, and once heated up will act a reservoir of heat, slowly releasing it back to the environment.

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[5.3] HEAT TRANSFER

• The first is simple "conduction", where it is transferred through direct contact.

• The second is "convection", in which currents are set up in a material that transfer heat through it, like the circulating currents in a pot of soup.

• The third is "radiation", in which heat is transferred through space by an invisible form of light known as "infrared radiation". This is the heat projected by a heat lamp or a glowing electric heater. The hotter the object, the more radiation it emits. Infrared radiation will be discussed in more detail in a later chapter.

Solid and opaque objects support heat transfer primarily through conduction. Different materials have specific "thermal conductivities", or rates of heat flow through them.

For example, metals generally have high thermal conductivities. If you wear metal-rimmed sunglasses on a cold day, they tend to painfully suck the heat right out of your nose. Plastics generally have lower thermal conductivities, and it is much more comfortable to wear plastic-rimmed sunglasses on a cold day.

Materials with low thermal conductivities are, naturally, used as thermal insulators for homes and buildings. Gases generally have low thermal conductivities, and most forms of home insulation are basically designed as "gas traps". The most vivid example of this principle is sheet plastic foam insulation, which is filled with bubbles of gas.

Incidentally, the insulating capability of commercial insulation is specified by an "R-value", which is inversely related to the thermal conductivity. The higher the R-value, the better the insulator. For example, thick fiberglass insulation has an R-value of almost 20, while double-paned glass has an R-value of a little over 1.

For a high degree of thermal insulation, a "Dewar flask" or "Thermos bottle" is used. A lab-quality Dewar flask consists of a double-walled vessel made of Pyrex glass with silvered surfaces and the space between the two walls evacuated. A vacuum has no thermal conductivity and of course it can't support convection, and the silvered surfaces reflect radiation and so limit loss by that avenue. The neck and cap of the flask end up providing most of the thermal conductivity of the scheme, and so are generally made as small as possible.

Extreme cooling requires a "double Dewar" scheme, with one Dewar flask contained in a second, and the space between the two filled with a cryogenic fluid such as liquid nitrogen (with a boiling point of 77 degrees Kelvin) or, for really cold applications, liquid helium (with a boiling point of 4 degrees Kelvin).

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[5.4] THE LAWS OF THERMODYNAMICS

Thermodynamics is an abstract field even by the standards of physics. To understand them, a few formal definitions must be set down. The most important is that of a "thermodynamic system", which is defined as a domain bounded in space where heat can flow across the boundaries in either direction.

The properties of a thermodynamic system at any one time define its "state". The most important properties are the "thermodynamic variables" of temperature, pressure, and volume, but there are other variables, such as density, specific heat, and the coefficient of thermal expansion.

If the properties of a thermodynamic system do not change over time, and if there are no changes in its configuration and no net transfers of heat across the boundary of the system, the system is said to be in "thermal equilibrium". If the thermodynamic system moves from one state of thermal equilibrium to another, a "thermodynamic process" is said to have taken place.

A thermodynamic process can be "reversible" or "irreversible". A reversible process can be run in one direction, and then be reversed to return to the same state that it started from with no net change in system energy. An irreversible process can be run in one direction, but will require a net input of system energy to reverse it.

The Zeroth Law is basically a definition of the term "temperature". It states that if two thermodynamic systems are in thermal equilibrium with a third thermodynamic system, then the first two systems are in thermal equilibrium with each other. They will all share the same "temperature".

The First Law of Thermodynamics is simply a rearrangement of the law of conservation of energy into thermodynamic terms. It states that the amount of heat transferred into a system, plus the amount of work done on the system, must result in a corresponding increase in the thermal energy of the system.

The First Law further implies that if any work is done by the system, it must be by draining the internal energy of the system. A system where work can be done without draining the internal energy of the system is referred to as a "perpetual motion machine of the first kind". The First Law rules out such machines.

                    heat_transfer
    entropy  =  --------------------
                absolute_temperature
 

The significance of the specific form of this definition will be discussed in the following section. Entropy can actually be expressed in other ways, and the definition of the term has to be carefully considered for any given scenario.

The Second Law states that the entropy of a "isolated" or "closed" system, meaning one where there is no transfer of energy across its boundaries, can never decrease. It may remain the same, or it may increase. A closed system where the entropy decreases is referred to as a "perpetual motion machine of the second kind". The Second Law rules them out.

These statistical methods yield the thermodynamic variables, and amount to a scheme where the simple classical mechanics of individual particles are translated to the thermodynamics of a system. In this light, temperature is a measure of the average kinetic energy, or essentially the motion, of the molecules of a system. A temperature increase means that the average kinetic energy has increased.

Similarly, heat transfer between two thermodynamic systems is caused by collisions between individual molecules of the systems. The collisions continue until on the average the net energy passing across the boundary between the two systems is zero, meaning the two systems have achieved thermodynamic equilibrium.

The First Law of Thermodynamics, then, exactly corresponds to the classical law of conservation of energy. The molecular nature of the Second Law of Thermodynamics is a little more subtle, and basically expresses entropy as the measure of the "probability" of a system. Given a large number of molecules in a thermodynamic system, probability dictates that the molecules will be moving in many different directions (high entropy) than in one direction (low entropy).

The Second Law essentially sets the direction of time in our Universe as being in the direction of increasing entropy. From the molecular physics point of view, there is no reason that the motion of all the molecules in a brick that has been dropped to the ground could not point straight up, causing the brick to fly back spontaneously into the air. However, this configuration is vanishingly improbable. It simply doesn't happen, it is an irreversible process.

Similarly, there is no reason on the microscopic scale that air molecules could not transfer net heat into a brick, making it warmer, but this is improbable as well. A hot brick will release its heat into cooler air, but cooler air will not warm up a brick. Heat always transfers from a warm object to a cooler one, never the reverse. In fact, this is actually just another way to state the Second Law.

The same remarks apply to dissolving salt in water; it's easy to do, but then extracting the salt from the water requires work. It is easy to dissolve, mix, or disperse materials, but it is hard to bring them together and sort them again.

The Second Law specifies that entropy for a closed system must increase over time. The initial creation of life from the organization of simple molecules into more complicated ones does imply a decrease in entropy, but only for the molecules themselves, and not necessarily for the complete thermodynamic system in which they exist.

Lab experiments have been performed in which more complicated molecules have been spontaneously self-assembled out of simple molecules through electric sparks. The Second Law is not violated because energy inputs were used to promote the reactions that created the molecules. Lighting bolts could have done the same in the primeval Earth.

As far as the evolution of more elaborate organisms from simpler organisms, the Second Law has nothing specific to say about the matter at all. Nobody has been able to come up with any particularly meaningful scheme to quantify the complexity of biological systems, or in other words assign a useful value of the complexity of a roundworm versus a human being, much less come up with such a scheme that could assign a value of entropy to such different levels of organization.

There is no thermodynamic reason that biological systems may not acquire more elaborate levels of organization over time. All the Second Law says is that they will waste a lot of energy doing it. Whether Darwin's theory of is valid or not, the Second Law neither proves nor disproves it.

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[5.5] HEAT ENGINES & THE SECOND LAW

In an "ideal" heat engine, all the heat is converted to work. However, the 19th-century French engineer Nicolas Leonard Sadi Carnot demonstrated that due to the Second Law, there are always losses. In a work published in 1824, Carnot showed that the efficiency of an engine is proportional to the temperature difference between the input and output, and to obtain 100% efficiency the difference would have to be infinite.

For example, a power turbine uses steam obtained from a boiler heated by coal or oil to drive the turbine, with the exhaust of the turbine consisting of steam that has cooled in the process. Ideally, the heat flowing out of the process is equivalent to the heat flowing into the process minus the work done:

    heat_out  =  heat_in - work
 

    work  = heat_in - heat_out
 

                     work         heat_in  -  heat_out           heat_out
    efficiency  =  ---------  =  ----------------------  =  1 - ----------
                    heat_in             heat_in                  heat_in
 

Such engines operate on "thermodynamic cycles", or steps of thermodynamic processes that operate in a circular fashion. The Carnot engine cycles through four phases of operation, as shown at the left side of the illustration below. The changes in pressure and volume for each phase are shown in the "PV (pressure-volume) chart", also known as an "indicator diagram", at the right side of the illustration.

In more detail, the cycle works as follows:

• [1] Input of heat "Qh" at high temperature "Th" results in constant-temperature (isothermal) expansion, raising the piston.

• [2] Heat input is removed, and gas continues to expand, raising the piston further. Heat does not escape from the system during this phase of the cycle, or in other words this phase is an "adiabatic" process.

• [3] Loss of heat "Ql" at a low temperature "Tl" causes the gas to compress in an isothermal fashion.

• [4] Heat no longer flows out of the cylinder but the gas continues to compress as an adiabatic process. The engine then starts over at phase 1 again.

The amount of work W performed by the Carnot engine is given by the area enclosed by the PV curve. This work is equivalent to the input heat Qh less the output heat Ql. As above, this means the efficiency of the Carnot engine is:

    efficiency  =  1 - ( Ql / Qh )
 

    efficiency  =  1 - ( Tl / Th )
 

Another implication of the Carnot engine is that it requires that heat be drained out of the cylinder for it to work. If the heat were not drained, pushing the piston back in would require as much work as was produced when it was pushed out, and the net work delivered by the engine would be zero. This illustrates why it is impossible to build a heat engine that is 100% efficient.

However, the basic revelations of the Carnot engine hold true: efficiency is roughly proportional to the temperature difference between engine inlet and outlet, and waste heat must be produced by the engine for it to work.

The fact that engine efficiency is directly proportional to the temperature difference gives an insight into the definition of entropy as heat transfer at a given absolute temperature. As the absolute temperature falls for a heat transfer process, the ability of the heat to do useful work, its "quality", falls or "degrades" as well, or in other words its entropy increases. A quantity of heat transferred at a high temperature has low entropy; the same quantity of heat transferred at a low temperature has high entropy.

Over time, in a closed system the transfers of heat occur at lower and lower temperatures, reducing the ability of the system to do work for both artificial engines and natural processes. This implies that the entire Universe is running down slowly, towards an ultimate "heat death" in the distant future.

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[5.6] PRACTICAL ENGINES

The Stirling cycle engine's indifference to fuel is its main advantage, and it is some use in undeveloped countries where access to fuels is limited. Its disadvantage is that it has a poor "power to weight ratio (PWR)", and all other engines in common use can provide the same power for much less weight.

The illustration below is an example of a simple (and unrealistic) implementation of a Stirling cycle engine. It consists of two cylinders, each with their own piston, linked through a gearbox. In this example, steam is fed around the first "hot" cylinder as a heat source, while cooling water is fed around the other "cold" cylinder as a heat sink. The "heads" of the two cylinders are linked by a heat-storage element known as a "regenerator".

The operation of the engine's gearbox is a little odd, since it will bring one piston to a dead stop while moving the other piston or allowing it to move.

The gearbox steps the engine through a four-part cycle as follows:

• [1] While the piston in the cold cylinder is stopped at the top of its travel, heat input into the hot cylinder causes the piston there to move downward. This is the power stroke, an isothermal expansion process.

• [2] The piston in the hot cylinder is then moved to the top of its travel while the piston in the cold cylinder is moved to the bottom of its travel. The volume of the working gas in the engine remains the same, while heat from the gas is stored in the regenerator. This is a constant-volume cooling process.

• [3] The piston in the hot cylinder is stopped at the top of its travel while the piston in the cold cylinder is moved up the cylinder to half its full travel. This compresses the working gas, but the temperature remains constant because heat is being drawn off by the cold water. This is an isothermal compression process.

• [4] Now the piston in the hot cylinder is moved downward to half its full travel, while the piston in the cold cylinder is moved to the top of its travel. This is a constant-volume process, in which heat from the regenerator is fed back into the gas flowing into the hot cylinder. The cycle is now ready to begin all over again.

In practice, a Stirling-cycle engine is built in a subtler fashion, with the two pistons nested inside the same cylinder and a simpler gearing system designed so that a piston may slow down but won't ever actually stop, except for the instant between reversals of direction. This construction "rounds off" the edges of the PV diagram, but otherwise the operation is the same. Details are beyond the scope of this document.

Operation of the Otto cycle is conceptually simple:

• [1] In the first part of the cycle, the "intake stroke", the intake valve opens and the piston moves down to its lowest position. This draws in a gasoline-air mixture at (constant) atmospheric pressure.

• [2] In the "compression stroke", the intake valve is closed and the piston moves to its top position, compressing the fuel-air mixture. This is done quickly enough so that this part of the cycle is effectively adiabatic.

• [3] In the "power stroke", an electric spark plug ignites the compressed fuel-air mixture, driving the piston to its lowest position.

• [4] In the "exhaust stroke", the exhaust valve opens and the piston moves up, exhausting the combustion products of the fuel-air mixture at (constant) atmospheric pressure.

Analysis of the Otto cycle engine shows that its efficiency is mostly dependent on the "compression ratio", that is, the ratio of maximum compression of the fuel-air mixture to atmospheric pressure. The greater the compression ratio, the more efficient the engine.

However, the Otto cycle engine is limited on the level of compression it can obtain, since at high compressions the temperatures and pressures will cause the fuel-air mixture to ignite spontaneously before the piston reaches the top of its travel. This phenomenon is known as "engine knock".

The "Diesel cycle" engine avoids this problem. It has the same general four-stroke operational cycle as the Otto cycle engine, but uses a fuel injection system to spurt fuel directly into the cylinder at the end of the compression stroke, permitting higher compression ratios. The Diesel engine uses heavier fuels than the Otto cycle engine. It does not use spark plugs, instead using "glow plugs" that are heated and ignite the fuel-air mixture when the compression reaches the proper level.

A second variation on the Otto cycle is the "two-stroke" engine, in which the intake and expansion cycles are combined, as are the compression and exhaust cycles. This means that the two-stroke engine has, in the limit, twice as much power for a given RPM than a four-stroke engine and is correspondingly lighter. The problem with two-stroke engines is that they unsurprisingly burn very dirty, and so have been generally banned by air-pollution regulations. However, experimental two-stroke engines have been built that use electronic control systems to meet air-pollution regulations.

Turbine engines are generally known as "Brayton cycle" engines. A turbojet is an "open" Brayton cycle engine, in that it operates on a continuous flow basis rather than in a loop, as follows:

• [1] Intake air is first compressed by a "fan" assembly. This is an adiabatic process.

• [2] Fuel is then mixed with the compressed air and burned in the "combustion chambers" of the engine. This is a constant-pressure process.

• [3] The hot gases generated in the combustion chamber flow out the back to provide reaction thrust, and also spin a "turbine" to run the compressor.

A turbojet is a little different from the other engines considered in this section because it is mainly intended to generate thrust. Turbine engines used to power ground vehicles or "turboprop" engines used to spin an aircraft propeller mainly generate torque.

"Closed" cycle turbine engines are used in powerplants and on large vessels. In such engines, the intake air or, more often, steam, is compressed, and then heated by internal combustion or, more often, an external source of heat. The steam then drives a turbine to provide power, and is finally cooled to be routed around to the intake of the compressor again.

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[5.7] REFRIGERATION SYSTEMS

In practice, a simple "refrigeration system" consists of a long sealed pipe arranged in a loop. Half of this loop is placed inside the refrigerator while the other half is placed in the environment outside the refrigerator, where the heat from the inside will be dumped. Both halves of the loop of pipe are turned back and forth with a series of hairpin turns to make them more compact, forming what are somewhat misleadingly referred to as "coils", though they don't coil back on themselves.

The half of loop inside the refrigerator is called the "evaporator coil" or just "evaporator", while the part outside the refrigerator is called the "condenser coil" or just "condenser". There are of course two connections between the evaporator and condenser. One connection is linked by an electric compressor, while the other is linked by a "flow restrictor" or "expansion valve".

The entire loop is filled with a "working fluid" or "cooling fluid" that changes easily from liquid to gas. The compressor drives the cooling fluid from the evaporator into the condenser, while the expansion valve throttles the flow of the cooling fluid from the condenser back into the evaporator.

The cooling fluid evaporates as it goes from the condenser through the expansion valve into the evaporator, and becomes cooler due both to its change of state and its expansion in volume. The cool gas in the evaporator soaks up heat from inside the refrigerator. The gas is then condensed and liquified when the compressor drives it from the evaporator back into the condenser. The warm fluid releases heat to the environment, and circulates back to the expansion valve to begin the cycle again.

For more aggressive cooling, a "two-stage" refrigeration system can be used, consisting of two such refrigeration loops in cascade.

This led to the introduction of a new class of cooling fluids, a set of compounds based on chlorine and fluorine known as "chlorinated fluorocarbons (CFCs)". They looked like an absolutely perfect solution to the problem, since they not only had nearly ideal thermodynamic properties, but they were almost completely inert, non-toxic, and relatively cheap.

In the 1980s, however, atmospheric researchers began to discover that trace amounts of CFCs were accumulating in the upper atmosphere, where they were broken down by solar radiation. The chlorine in the molecules was able to cause the breakdown of "ozone", the O3 molecule, which covers the Earth in a high-altitude layer that is opaque to harmful radiation.

While ozone breakdown was only observed in the cold air above Antarctica, the possibility that increasing accumulations of CFCs could entirely wipe out the Earth's ozone layer, destroying the shield that protects the planet from high-energy radiation, was frightening enough to lead to international agreements to phase out the use of CFCs in refrigeration and air conditioning systems, as well as in their common use in plastic foam insulation.

There is a popular misconception that CFCs were banned because they were toxic. Actually, the truth was that they were too inert for their own good. Once they were released into the environment, they didn't degrade until they floated to high altitudes where they could do the most harm.

CFCs are now being replaced by "hydrofluorocarbons (HFCs)" that don't contain chlorine, and by "hydrochlorofluorocarbons (HCFCs)" that do contain chlorine but break down before they reach high altitudes. Neither of these compounds are quite as efficient as cooling fluids or insulators as CFCs, however, so the conversion to HFCs and HCFCs had not been straightforward.

This is the basic principle of the "heat pump", a device in which the flow of cooling fluid can be reversed to either cool or heat a room. Heat pumps are efficient compared to most other forms of heating, but only in fairly moderate climates where the temperature changes are not too drastic.

As noted in the section on heat transfer, a warm object will emit infrared radiation. An infrared camera can obtain an image of an object just from its infrared radiation, even if there is no other light source available. The military is particularly fond of such infrared camera systems, which they call "forward looking infrared (FLIR)" imagers.

The military also uses "heat-seeking" missiles, such as the well-known "Sidewinder" missile, that will "home in" on a target aircraft from its infrared radiation using a heat-seeking sensor. However, both FLIRs and heat-seeking sensors suffer from an inherent limitation: they can't sense a target that is cooler than they are, because their own infrared radiation drowns out any radiation from the target.

This means that it is preferable to cool the FLIR or heat-seeking sensor to improve its sensitivity. Early military FLIRs used in the 1960s had bulky refrigeration systems and had to be carried in fairly large aircraft. Early versions of the Sidewinder missile didn't have any cooling system, and so they could only be targeted on an aircraft's hot exhaust.

In the 1970s, improved versions of the Sidewinder were introduced that could be fitted before a mission with a can of compressed nitrogen gas, with the gas leaking out a pinhole to provide cooling. These improved "all aspect" Sidewinders could lock onto an aircraft target from any angle and no longer had to be targeted on an aircraft's exhaust.

However, this scheme meant that the cooling system only worked for a relatively short time. The introduction of compact, relatively low-cost Stirling cryocoolers in the 1990s provided a better solution, since it can provide cooling for the sensor for as long as power is available. It is also much less bulky than a traditional refrigeration system and so is also very useful for FLIRs.

Infrared telescopes launched into orbit around the Earth require extreme sensitivity to observe distant and cool cosmic objects. As a result, they are often built as (very large) double-Dewar flasks known as "cryostats", with an infrared telescope built inside the inner Dewar flask.

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[5.8] MAXWELL'S DEMON & THE SECOND LAW

Most of these attempts have been the artless works of crackpots, who were quickly apprehended by the physics police. However, there are also a number of physicists who like to play the perpetual-motion game as a thought experiment to see what interesting insights it may provide.

The most famous of these theoretical perpetual motion machines is Maxwell's Demon, originally devised by 19th-century Scots physicist James Clerk Maxwell. In this scheme, the system consists of two chambers containing gas molecules. The chambers are separated by a door that is opened or shut by a tiny "demon". When the demon spots a fast-moving molecule in the first chamber, it opens the door to let it into the second, and keeps the door closed for slow-moving particles.

In this way, the demon can build up a pressure difference that could be used to do useful work. While obviously there is no demon available to do the job, it was still hard to understand why such a scheme could not be used in principle to build perpetual-motion machines, and through the invention of his demon, Maxwell was posing the question to later generations as to why it wouldn't work.

Modern theoretical analysis finally demonstrated that the act of information processing in making a decision to open the door or not used a certain inescapable minimum amount of energy, enough to ensure that the Second Law was not violated. Incidentally, Maxwell did not name the demon after himself. It was named by others later in honor of Maxwell.

If the Feynman motor were placed in a gas or fluid, molecules would strike the paddle wheel. Since the gear could only move in one direction, the paddle wheel could only turn one way, and useful work could be obtained from random (Brownian) molecular motion. Feynman figured that his motor could lift fleas.

Having presented this idea, Feynman then demolished it. The weak point was the spring-loaded ratchet. Thermal action would also cause the ratchet to bounce out of place, and the skewed configuration of the gear teeth would make it more likely the gear would move in reverse if that happened. The jerk back through the paddle wheel as the ratchet snapped back into place would dump heat back into the medium.

Feynman's motor was a theoretical toy, but others have taken it a bit more seriously. In 1997, researchers at Boston College actually built something like a Feynman motor, using benzene molecules to build a three-blade paddle wheel with a ratchet. A group at IBM devised a similar structure. As Feynman predicted, the motors simply spun uselessly in both directions. However, researchers are now trying to modify them to use chemical reactions to perform useful work, if on a very, very small scale.

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