Chapter 5 CSULB

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Chapter 5
Thermal Energy
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Thermal Energy
What is heat?
 Heat and Temperature
Energy Conversion 
Conservation of Energy (1st Law)
Availability of Energy (2nd Law)
Efficiency
Applications
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Thermal Energy is the energy associated with heat. 
Heat is a form of energy that flows from a hot object to a cold object, as a result of a temperature difference that exists between them. 
Nature of Heat
Historical Perspective
Aristotle (384 BC – 322 BC)
Antoine Lavoisier (1743 – 1794)
Benjamin Thompson (Count Rumford) (1753 – 1814)
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Earlier philosophers considered heat as a substance that was passed from hot objects to colder ones. This heat fluid was known as phlogiston (meaning flammable in Greek), which was thought to have a mass and the ability to flow in and out of objects during burning; it was considered to be the soul of matter. 
The concept was refined after Lavoisier, an eighteenth century French chemist, showed that mass is a conserved quantity and does not change when a substance undergoes a chemical reaction. He named the new substance caloric, a mass-less fluid thought to flow from hot to cold objects. 
It was not until the middle of the nineteenth century that Thompson and Joule showed that this theory is also wrong; heat is not a substance, but rather a manifestation of motion at the molecular level (kinetic theory). 
For example, when we rub our hands against each other, both hands get warmer, even though, initially, they were at the same cooler temperatures. If heat were a fluid, then it would have flowed from a (hotter) body with more energy to another with less energy (colder). Instead, the hands are heated because the kinetic energy of motion (rubbing) has been converted to heat in a process called “friction.”
Heat
Sensation of warmth is stronger in higher temperatures. 
Flows from hot to cold
Magnitude depends on material, temperature gradients, and thickness
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Heat is perceived as something that produces a sensation of warmth. The sensation will, of course, be stronger in higher temperatures. 
There cannot be any heat transfer between two objects that are at the same temperature. 
The amount of heat transferred depends on the temperature difference and the conductivity of the path between the two objects; the direction of heat flow is always toward the cooler object. 
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Temperature
Qualitatively by our senses
Our senses often can be unreliable and misleading. 
Quantitatively by thermometers
Calibration
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EXAMPLE-1 On a cold winter day, for example, an iron railing seems much colder to the touch than a wooden fence post, yet both are the same temperature. This “deception” by our senses arises because iron conducts heat away from our fingers much more readily than wood does. 
EXAMPLE-2 Put one hand in a glass filled with hot water (of course, not too hot!) and the second hand in a glass filled with cold water. If you now remove both hands, the hand from the hot water feels cold, while the one from the cold water feels warm. Aren't both hands feeling the same air temperature?  
In the United States, temperature is measured predominantly in degrees Fahrenheit. Most other countries use Celsius.
While our built-in senses do provide us with a qualitative characterization of temperature, our senses often can be unreliable and misleading. 
We need a reliable and reproducible method for establishing the relative “hotness” or “coldness” of objects, Thermometers have been designed.
The temperature scales use the freezing and boiling points of water as reference. 
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Thermometers
FAQ: Why do thermometers have a very narrow stem? 
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Working principle: As a thermometer heats up, the volume of the liquid in the bulb increases -- having nowhere to go, liquid climbs up the tube; the higher the temperature, the more expansion of liquid and the higher the fluid rises in the thermometer’s stem.
To be useful, thermometers have to be calibrated—which may be done by measuring the rise of a column of fluid (such as water, alcohol, or mercury) as it expands when exposed to an environment.
FAQ: Why do thermometers have a very narrow stem? 
Answer: 
Thermometers work on the principal of expansion of a fluid as a result of a rise in temperature and, thus, can be thought of as volume amplifiers. 
For a given volume expansion, the smaller the cross sectional area of the capillary tubes, the higher the height of the column of fluid -- and more accurate temperature reading is possible. 
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Temperature Scale
	Temperature Conversion Formulas
	 oF = 1.8oC +32 
 oC = (oF – 32)/1.8 
K = oC +273 
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History:
In 1948, the International Committee on Weights and Measures adopted this scale as the temperature standard, calling it Celsius, in honor of its inventor. 
Using two reference points makes it possible to divide the distance between them into a number of equal intervals. The first thermometers were divided into 360 parts, like degrees of a circle (thus, the term “degree”). In 1708, Gabriel Fahrenheit used spirits as the fluid and a mixture of ice, water, and salt to produce the lowest attainable temperature in a laboratory setting at the time, which was set as the zero mark. The second reference point he used was the temperature of the human body, which was assigned the value of 96º. The choice of 96º is believed to be due to the fact that it is divisible by 2, 3, 4, 8, 12, 16, and 32. Today, the Fahrenheit scale uses the freezing and boiling points of water as 32º and 212º. According to this scale, body temperature is 98.6ºF. 
Fahrenheit scale
Freezing and boiling points of water as 32º and 212º. 
Body temperature is 98.6ºF. 
Centigrade Scale:
Swedish astronomer Anders Celsius
0 and 100 to the freezing and boiling points of water
Lower limit: Zero Absolute Temperature 
-273.15º Celsius or zero kelvin
Temperature of a perfect crystalline lattice. 
Upper Limit
highest predicted temperature is 1038 K, (suspected temperature at the very beginning of the creation of the universe, the Big Bang)
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Thermal Properties
Heat capacity
A measures substance’s ability to retain heat. 
Definition: the quantity of heat required to raise a substance by one degree in temperature. 
Specific heat
Definition: Heat capacity per gram of substance. 
SI unit: kJ/kg-K
Thermal conductivity 
A measure of a body’s ability to conduct heat.
Cont…
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Heat Capacity:
Water has a higher heat capacity than sand because it takes more heat to warm up by the same amount than sand does. In other words, for the same amount of heat, sand temperature rises more than water temperature. 
Specific Heat:
A bigger object will have a greater heat capacity, but the same specific heat.” 
The more loosely the components of a solid are held together, the higher the substance’s specific heat.”
Thermal Conductivity:
Diamond is an excellent conductor, silver is also very good, with copper not too far behind. 
Air appears toward the end of the list, meaning it is a very poor conductor, but an effective insulator
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Thermal Properties
	Thermal Conductivities of Selected Materials (W/m.K)					
	Good Conductors		Average conductors		Poor conductors
(good insulators)	
	Diamond	1,000	Stainless Steel	14	Asbestos	0.090
	Silver	429	Ice (00C) 	2.20	Fiberglass	0.040
	Copper	400	Concrete	1.70	Glass wool	0.040
	Gold	385	Glass	1.00	Styrofoam	0.033
	Aluminum220	Water	0.60	Air (dry)	0.026
	Iron	80	Snow	0.16	Vacuum	0
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FAQ:
Which one has a higher heat capacity -- Graphite or diamond?
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Answer: Graphite has a higher heat capacity than diamond because diamond’s lattice structure is more tightly bound than the graphitic structure of carbon. 
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Heat Transfer
Heat vs. Energy
Modes of Heat Transfer
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Heat and Energy
Heat exists only when entering into or out of something (a system). After it enters the system or leaves the system, it is no longer heat -- it has become energy. 
Heat Transfer Modes
Conduction
Transfer of heat from molecule to molecule
Convection
Transfer of heat by bulk motion
Natural vs. Forced Convection
Radiation
Transfer of heat by electromagnetic waves.
Unlike conduction and convection, radiative heat transfer requires no medium.
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Conduction
Conduction is the transfer of heat, from molecule to molecule, through a substance. 
Thermal Conductivity
Heat Capacity
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Physics: If a steel bar is temporarily heated at one end, its molecules become agitated and move faster than neighboring molecules. When fast-moving molecules collide with slower molecules, energy is transferred from faster to slower molecules. The chain reaction moves along the bar until its temperature is uniform. 
Observations:
Although conduction can take place in gases and liquids, its effect is most significant when two solids come into contact. 
The effect also is more pronounced in objects with higher heat conductivities (more conductive), and larger temperature differences.
The closer the molecules are packed, the easier conduction takes place. 
Heat transfer via conduction increases in materials with greater heat conductivity and larger temperature gradients. 
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Which one is colder, an iron railing or a wooden fence post?
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Most people The central concept of thermodynamics is temperature. This word is so familiar that most of us—because of our built-in sense of hot and cold—tend to be overconfident in our understanding of it. Our “temperature sense” is in fact not always reliable. On a cold winter day, for example, an iron railing seems much colder to the touch than a wooden fence post, yet both are at the same temperature. This error in our perception comes about because iron removes energy from our fingers more quickly than wood does. Here, we shall develop the concept of temperature from its foundations, without relying in any way on our temperature sense. 
A picture from Dumb and Dumber, where the tongue gets stuck to the pole on a cold day.
Convection
Convection is energy transfer by bulk motion. 
Most prominent in gases and liquids
Classification: 
Natural convection
Forced convection
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Physics: Unlike conduction, which is a microscopic phenomenon, convection involves the macroscopic interchange of energy between two mediums. Most prominent in gases and liquids, where the distances between molecules are too large for conduction to be effective. 
In buildings, convection losses are due to air infiltration through the cracks, windows, and other openings in walls. Additional losses occur due to air movement inside and wind motion outside the exterior glass and windows. In a typical building, infiltration losses are the most significant and are comparable to losses by conduction. Common insulation materials such as fiberglass, rigid boards, cotton, and feathers work by creating tiny air pockets that slow down the convection flow of heat. 
Convection is important not only at the local level, but also plays a role in large-scale movements of the atmosphere. The major winds are convection currents driven by temperature differences caused by non-uniform heating of the Earth by solar radiation. The winds, in turn, drive the ocean currents.
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Radiation
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Radiation is the transport of energy by photons of light migrating from a hotter to a colder surface. 
Electromagnetic Wave Spectrum Different colors emit radiation differently; black objects emit the most, white objects the least. The fraction of black body radiation emitted by real surfaces is called the emissivity. 
Physics: Unlike conduction and convection, both requiring a material medium to transport heat energy, radiation transports energy via electromagnetic waves of different wavelengths, even in a vacuum. 
Wavelength is the distance between two successive crests or troughs in a wave; the shorter the wavelength, the higher its energy is. No matter what the wavelength is, they all travel at 300,000 kilometers per second, the speed of light. 
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Radiation
Solar and Terrestrial Radiations.
“Thermal radiation”
Radiative Exchange.
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Thermal Image
	
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Whether a body emits energy at one wavelength or another depends on its temperature and surface properties. An object at room temperature, say 20°C, emits nearly all its energy in infrared. The human body at ordinary temperatures also emits in the infrared region. In fact, 98% of the radiation from a bare human body ranges from 5 to 75 microns in wavelength. Using a special infrared camera it is possible to “see” a human body or a passing car in total darkness. The image will not look like what we see with our single lens cameras, but will be a contour map of constant temperature regions. An infrared (thermal) image of a man wearing a tee-shirt is shown in Figure. 
Thermal image of a man. The hotter regions (bare skin) are lighter in color, and cooler spots (tee-shirt, arm pits) are darker grey. The choice of shades of grey is arbitrary and does not imply a physical significance. (Photo courtesy of Thermotronics Inc., Brazil.)
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FAQ:
Question: Why do animals living in freezing climates often burrow into the snow to sleep?
Question: To keep a body warm in the cold of winter, would it be more beneficial to wear two layers of light clothing or one layer of clothing that is twice as thick?
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Question: Why do animals living in freezing climates often burrow into the snow to sleep?
In addition to fur and thick skin, air spaces in the snow help animals protect themselves in such harsh weather. Snow, a poor conductor, slows the loss of body heat. 
In freezing weather, an igloo would provide a warmer shelter than would a wooden shack because the snow and ice are better insulators than wood.
Question: How do gloves (or other clothing) protect us against the cold?
The temperature difference between hands (37°C) and outdoor air (say 0°C) is the same whether we wear gloves or not. However, with gloves, heat follows a path of lower resistance, so it slows down the rate of heat loss from our hands.
Question: To keep a body warm in the cold of winter, would it be more beneficial to wear two layers of light clothing or one layer of clothing that is twice as thick?
Two layers will work best because there is always some air trapped between the layers, providing additional insulation.
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FAQ
Question: You can easily feel the heat from the sun through a glass window, but behind a sheet of glass you do not feel much heat from a fireplace. Why?
 
Question: Can stirring the coffee help in lowering its temperature faster?
Question: Is it best to add creamer to coffee immediately after the coffee is poured or to add itright before drinking the coffee? Assume that coffee is most desirable when it is hot.
Question: If humans emit infrared, why do we see them in “visible” light?
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Question: You can easily feel the heat from the sun through a glass window, but behind a sheet of glass you do not feel much heat from a fireplace. Why?
Common window glass is transparent to the wavelengths of radiation between 0.3-2.5 µm. A large portion of solar radiation falls in this range, allowing both sunlight and solar heat to pass through. Flames, however, emit in wavelengths in excess of 2.5 µm, the region where window glasses are practically opaque.
 
Question: Can stirring the coffee help in lowering its temperature faster? Yes. Stirring cools coffee in two different ways, first by conducting heat away to the metallic spoon, and through convection by increasing its exposure to cooler ambient air.
Question: Is it best to add creamer to coffee immediately after the coffee is poured or to add it right before drinking the coffee? Assume that coffee is most desirable when it is hot.
Creamer should be added as soon as the coffee is poured. Two effects are of importance: First, creamer makes coffee lighter in color, reducing its emissivity and heat loss from radiation. Secondly, adding creamer sooner decreases the temperature difference between coffee and the environment, reducing the rate of conductive and convective losses. 
Question: What we see is not emitted light, but the reflection of light from other sources (sunlight, fluorescent light, etc.). This is why, in the absence of a light source (darkness), there is no light reflected to the eye and a person cannot see or be seen. 
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Thermodynamics
Thermodynamics = Therme (heat) + Dynamis (power). 
It is the science that describes the dynamics of heat and how it can be converted to power. 
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These observations have been refined and reformulated as the zeroth, first, second, and third laws of thermodynamics. These laws are important because they provide the basis for designing many machines and modern devices that change heat into work (such as an automobile engine or a power plant) or turn work into heat or cold (such as an electric heater, a refrigerator, or a heat pump). 
Thermodynamics is a phenomenological theory derived from four very simple observations: 
Heat cannot flow between bodies of the same temperature; 
Heat and work are just two different forms of energy; 
Heat always flows from a hot body to a cold body; and 
There is a temperature (called zero absolute temperature) that can never be reached. 
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The Zeroth Law of Thermodynamics (Equilibrium)
Objects at equilibrium must have the same temperatures.
It forms the basis of all measurements. 
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In other words,…
Two objects at the same temperature are at equilibrium and remain at equilibrium until the temperature of one of the objects changes. 
On the other hand, if we put two objects of different temperatures next to each other, one object heats and the other cools until both bodies reach the same temperature. 
When a mercury thermometer measures the air temperature as 0oC, in reality it invokes the zeroth law by assuming that the thermometer was in thermal equilibrium with the air, but also with a mixture of ice and water in the factory where it was calibrated. In other words, the mercury has expanded precisely as much as it did when it was placed in the ice-water mixture. When a thermometer is brought into contact with a patient's body, it compares its reading to an arbitrary scale in which one thermometer was calibrated by placing it in thermal equilibrium with two reservoirs, such as an ice bath and boiling water.
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The First Law of Thermodynamics (Conservation of Energy)
Energy can be neither created out of nothing nor destroyed into nothing, but it can only be changed from one form to another. 
But energy can be converted from one form to another 
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First law efficiency
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Energy Conversion Devices
	Examples of Different Kinds of Energy Conversions					
	FROM/TO	MECHANICAL 	THERMAL 	CHEMICAL	ELECTRICAL	LIGHT
	MECHANICAL	Bicycle, Gearbox	Friction	Matches	Wind generator, Microphone	Sparks
	THERMAL	Gas turbines	Heat pipe, Heat exchanger	Pyrometer	Thermocouple	Luminescence 
	CHEMICAL	Rockets, ICE	Metabolism	Digestion	Battery, Fuel cell	Candle 
	ELECTRICAL	Electric motor, Loudspeaker	Resistor heater	Electrolysis	Transformer	Light bulb
	LIGHT	Galvanometer	Solar collector	Photosynthesis	Solar Cells	Fluorescence
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Note:
Up to the late nineteenth century, it was believed that potential energy and kinetic energy interchanged without loss, the principle commonly known as the "conservation of mechanical energy" (See Chapter 2). In reality, the conservation was not perfect. Some energy was lost through air resistance and friction. However, if we assume that all losses eventually turn to heat and that heat is a form of energy, then the suspicion arises that total energy is conserved. 
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FAQ
If energy can be neither created nor destroyed, how can people claim that there is an “energy shortage?
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Terms such as “energy shortage” and “energy waste” are misnomers. 
According to the first law, energy can never be wasted; it may only be converted to a form not readily usable to us. 
While total energy must remain constant, useful energy -- that which can be used as fuel or to perform work -- may be in short supply. 
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The Second Law of Thermodynamics (Chaos and Disorder)
All natural processes tend to go from order (concentrated) to disorder (dispersed).
It defines the arrow of time.
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In other words, …
Events happen in a certain direction -- from order to disorder, from concentrated to diffuse, from useful to useless, from available to unavailable.
Importance: 
The second law gives us guidelines for what we can and cannot do. 
It tells us how to design better machines and provides us with a blueprint for using our resources more efficiently. In short, it gives us a sense of direction. 
Examples:
 Heat flows spontaneously from hot objects to cold ones. 
 Water will flow from high mountains toward rivers. 
 Air rushes out of a punctured rubber balloon. 
 Smells tend to diffuse outward to span greater distances. 
 Humans grow older by the minute. 
 When a house is left unattended, it quickly becomes disorganized.
Entropy
Entropy (from the Greek root meaning transformation) 
a measure of randomness, or the total number of configurations that a system can assume. 
Ludwig Boltzmann (1844-1906)
Concept: Processes happen such that the total entropy of the universe is constantly increasing. 
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FAQ
Question: It is an easy task to mix two tablespoons of salt and pepper. It takes quite a bit of work, however, to separate a mixture of salt and pepper into its constituents. Why? 
Question: An electric heater dissipates electrical energy into heat (and some light). What are the implications of the First and Second Laws of Thermodynamics?
 
Question: Ice is a more structured form of molecules of H2O than liquid water. Therefore, as water freezes during a cold, wintry night, its entropy decreases. Wouldn’t this be a violation of the law of entropy?
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Question: It isan easy task to mix two tablespoons of salt and pepper. It takes quite a bit of work, however, to separate a mixture of salt and pepper into its constituents. Why? 
Salt and pepper, by themselves, are relatively orderly, but when mixed, disorder will increase considerably. Separating salt and pepper requires work because we need to create order by reducing the mixture’s entropy.
Question: An electric heater dissipates electrical energy into heat (and some light). What are the implications of the First and Second Laws of Thermodynamics?
The first law assures that electrical energy is completely converted to heat (and light). 
The second law implies that the energy flow is from order (flow of electrons in the electric coil) to disorder (random motion of heated air molecules in the room).
 
Question: When a gas balloon is heated, it expands. What happens to the entropy?
As the balloon expands, it opens more room for gas molecules to occupy. In other words, the possibility that gas molecules take different configurations increases. Thus the balloon’s entropy increases. 
Question: Ice is a more structured form of molecules of H2O than liquid water. Therefore, as water freezes during a cold, wintry night, its entropy decreases. Wouldn’t this be a violation of the law of entropy?
Not at all. To freeze the water, heat has to be removed. This heat goes into the atmospheric air in the vicinity of the body of water. 
The increase in entropy of the air would more than compensate for the reduction of entropy in the water. 
Remember, the entropy of a system can increase or decrease. It is the entropy of the universe that constantly increases. 
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Consequences of the 2nd Law
Heat cannot, by itself flow from a low to a high temperature
Work can be completely converted to heat.
But heat cannot be completely converted to work.
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Relationship with the First Law
First Law: Quantitative
100 Btu of work = 100 Btu of heat
Second Law: Qualitative
100 Btu of work is better than 100 Btu of heat
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The First Law of Thermodynamics showed that we could convert energy from one form to another. 
When its brakes are applied, a car will slow down because of friction. As a result, both the tires and the road become a bit warmer. In this example, work has been turned to thermal energy or heat; therefore, work and heat are quantitatively the same. 
Experience tells us the reverse, cooling the tires and road to move the car backward, is not possible. This shows that thermal energy (heat) and mechanical energy (work) are qualitatively different - work can be turned entirely to heat, while the reverse is not true. 
This simple observation is a direct consequence of the Second Law of Thermodynamics, which states that processes occur naturally in a preferred direction and not the other, although energy expenditure is exactly the same in both cases. 
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Absolute Temperature (The Third Law of Thermodynamics)
There is a temperature so low that it cannot be reached; this temperature is called absolute zero (-273.15oC).
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Observation:
As substances cool, their molecules move with less speed and their kinetic energy decreases. 
Concept: 
Kinetic energy of particles is directly proportional to temperature. In other words, temperature can be seen as a measure of the kinetic energy of particles. 
It is, therefore, easy to understand that at temperatures of absolute zero, all molecular motion stops, and entropy approaches zero.
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A very simple heat engine
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A very simple heat engine. The air in (B) has been heated, increasing the molecular motion and thus the pressure. Some of the heat is transferred to the increased gravitational potential energy of the weight as it is converted to mechanical energy.
FAQ
It is a misconception among many that friction is responsible for less than 100% efficiency of heat engines and refrigerators. What do you think?
Heat Engine
Refrigerator
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Answer: Friction and other “irreversibilities” are not responsible for the unattainable nature of perfect efficiencies. 
The Second Law of Thermodynamics requires that disorder in the universe continually increases. 
Work is a more ordered form of energy than heat (heat is random motion of particles, while work is the movement of all particles in the direction that force is applied). 
Conversion of heat entirely to work requires creation of order, in violation to the second law. 
Similarly, transferring heat from a cold space to a hot space, as is the case in refrigerators, without additional energy (electric or gas) will be impossible. 
As a consequence, even under ideal conditions, efficiencies are always smaller than one. Frictional losses will make the efficiencies even lower.
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Energy efficiency vs. Energy conservation
Energy efficiency does not equal Energy conservation. 
Energy-efficiency means doing a given task with less energy. 
Energy conservation, means reducing energy use by cutting down on or abandoning a service. 
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Reasoning: 
Examples of energy efficiency include replacing an existing single-pane window of your house with a newer, double-pane window, trading a gas water heater for a solar one, or replacing incandescent lamps with fluorescent lamps of the same rating. 
Turning off a light, or lowering the room thermostat are examples of energy conservation.
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Thermal Devices – Efficiency Chart
	Typical Efficiencies of Various Conversion Devices		
	Electric motors	Electrical  Mechanical	60% - 90%
	Electric heaters	Electrical  Thermal	95% - 99%
	Candle	Chemical  Light	0.04%
	Incandescent (tungsten) lamp	Electrical  Light	3% - 5%
	Fluorescent lamp	Electrical  Light	10% - 15%
	Gas heaters (space)	Chemical  Thermal	90% - 95%
	Gas water heaters 	Chemical  Thermal	15% - 25%
	Batteries (lead-acid)	Chemical  Electrical	85% - 95%
	Steam locomotive	Thermal  Mechanical	5% - 10%
	Automobile engine	Chemical Mechanical	15% - 30%
	Coal power plant 	Chemical Thermal	30% - 45%
	Nuclear power plant 	Nuclear  Thermal	30% - 35%
	Photovoltaics	Light  Electrical	5% - 40%
	Wind turbines	Mechanical  Electrical	30% - 50%
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Carnot Efficiency
Ceiling on efficiency (is achieved when net entropy production is zero)
Under such ideal condition, the efficiency of the engine is the maximum attainable and is equal to
	(Note: Please note that Tcold and Thot are temperatures of cold and hot reservoirs and must be expressed in kelvin.)
To obtain an efficiency of unity, i.e., or 
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In an attempt to improve the efficiency of the early steam engine, the French scientist Sadi Carnot proposed an ideal engine that works between two reservoirs at different temperatures. Heat is removed from a source at temperature Thot, part of which is discarded to a sink at temperature Tcold; the remainder is supplied as work. The best this engine can do is to achieve a loss of entropy from the source that just balances the gain in entropy by the sink. 
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FAQ
Which one is a more sensible mean of heating a house: A gas heater, or an electric heater?
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Answer: Most electricity is generated in power plants by burning some form of fossil fuels. In the process, 70% of the energy is lost to the atmosphere (see “Heat Engines”). 
Additional losses occur during transmission, before the electricity is convertedback to heat -- a task that could be accomplished directly by burning gas in a gas heater. 
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Claims, Lies and bigger lies
An engine manufacturer makes the following claims: 
The heat engine operates between two reservoirs at 375 K and 225, and has efficiency of 20% 
The heat engine operates between two reservoirs at 375 K and 225 K, and has efficiency of 40% 
The heat input of the engine is 9 kW at 375 K. The heat output is 4 kW at 225 K. 
hactual = 1 - Qc/Qh = 1- 4/9 = 0.56
hideal = 1 - Tc/Th = 1 - 225/375 = 0.4
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Do you believe these claims?
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Hurricane
	
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A hurricane is an elegant example of a natural Carnot heat engine. It draws heat from the ocean, releases some of it to the atmosphere via radiative cooling, and does work during this process.
Efficiency
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First-Law Efficiency
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First-law efficiency
The first-law efficiency does not differentiate between the qualities of the energy sources. 
The first-law efficiency can be less than (heat engines), equal to (friction), or greater than (refrigerator) one. 
Worth Remembering – First law efficiency has two major flaws; it can be greater than one, and it does not reflect the limitation imposed by the second law of thermodynamics.
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 Engines, Refrigerators , and Heat Pumps
Air Conditioners
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Heat Engines
Air Conditioners
It utilizes this thermal energy by bringing it into the house and heating its interior. Of course, by removing this heat, we make the outside temperature even cooler! 
Working principle: 
A heat pump circulates a fluid called refrigerant, which absorbs heat from outdoor air and releases it inside. 
Heat pumps can be used for cooling, as well. This process is the reverse of the heating process; it removes the heat from the space to be cooled and dumps it into the already warm, outdoor environment. 
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FAQ
It is a misconception among many that friction is responsible for less than 100% efficiency of heat engines and refrigerators. What do you think?
Refrigerator
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Answer: Friction and other “irreversibilities” are not responsible for the unattainable nature of perfect efficiencies. 
The Second Law of Thermodynamics requires that disorder in the universe continually increases. 
Work is a more ordered form of energy than heat (heat is random motion of particles, while work is the movement of all particles in the direction that force is applied). 
Conversion of heat entirely to work requires creation of order, in violation to the second law. 
Similarly, transferring heat from a cold space to a hot space, as is the case in refrigerators, without additional energy (electric or gas) will be impossible. 
As a consequence, even under ideal conditions, efficiencies are always smaller than one. Frictional losses will make the efficiencies even lower.
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Cooling and Refrigeration
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Move heat away from a space in defiance of the common perception that heat moves from a higher to a lower temperature. 
The price is, of course, the expenditure of additional energy in terms of electricity and creation of disorder outside the immediate neighborhood of these devices. 
Refrigeration operation = Heat engines operating in reverse. 
Heat is removed from the space (inside refrigerated space or air-conditioned room) and dumped into a second reservoir at a higher temperature. 
For household refrigerator, the second reservoir is the kitchen. 
For air-conditioner, it is the outside air. 
The most common types use the vapor 
	compression refrigeration cycle, consisting 
	of four main components: 
a compressor, 
a condenser, 
an expansion valve, and 
an evaporator. 
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How does a heat pump work?
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Measuring Efficiency
Coefficient of Performance (COP)
Energy Efficiency Ratio (EER)
Seasonally-adjusted EER (SEER)
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COP is a measure of the performance of heat pumps.
It is defined in terms of the amount of heating or cooling that is achieved per amount of work energy (electricity) that is needed to run the compressor. 
In the United States, it is customary to specify the performance of a heat pump by two numbers, 
the COP as defined above for heating, and 
the Energy Efficiency Ratio (EER) for cooling. 
EER is expressed in Btu/Wh, defined as the ratio of cooling capacity (in Btu/h) for each unit of required electrical power (in watts). Large commercial refrigeration units are rated in kilowatts per ton.
Facts:
The COP for practical heat pumps is around 4, meaning heat pumps can provide four to five times more heating or cooling than electricity consumed .
In comparison, electric heaters have efficiencies close to 100%, oil and gas heaters have efficiencies of 50% to 90%, and wood stoves’ heating efficiencies range from 20% to 60%. 
The COP decreases markedly once outside temperatures fall below −5 to −10 °C.
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Example Problem
Question: A homeowner interested in heating his house is offered two options: an electric heater with an efficiency of nearly 100%, or a heat pump with a COP of 4. Which option makes more sense? 
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Resistance heaters work by converting electricity to heat. Even at 100% efficiency, the amount of heat is equal to the electricity consumed. 
Heat pumps, on the other hand, use electricity to extract the energy from the cold ambient air, dumping both into the house. A heat pump with a heating COP of 4, supplies four times the heat as the electricity used to run it.
The heat pump will make a lot more sense. The only thing the homeowner has to worry about is the initial cost of the unit, which is usually recovered in a few years. 
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Living Organisms and the Laws of Thermodynamics 
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Energy Efficiency Ratio (EER)
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Metabolism
Basal Metabolic Rate (BMR)
BMR~ M2/3
Metabolism Efficiency
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Chemical energy in food becomes Mechanical energy in muscles
Metabolic efficiency is not the same for all people, and can vary between 15-25%, depending on gender, weight, condition of health, athletic abilities, and age. 
Although maximum power can be quite high, average power output is not and is limited to around 1-1.5 W of mechanical energy per kilogram of body mass (power output increases to about 4 W/kg for a professional cyclist). 
By combining food production efficiency (~10-15%) and metabolic efficiency (~15-25%), a net figure for human power efficiency is calculated at around 1.5-3.75%. 
In other words, on average, each unit of mechanical (muscle) energy comes at the expense of 17 to 66 units of primary (fossil) energy. 
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Body Mass Index
Body Mass Index: 
	Table 5-7 Body Mass Index for Healthy Humans			
	Height, cm	Healthy Mass, kg	Height, feet	Healthy Mass, lbs
	150	43-56	5’ 0”	97-128
	155	46-60	5’ 2”	104-136
	160	49-64	5’ 4”	110-145
	165	52-68	5’ 6”	117-155
	170	55-72	5’ 8”	125-164
	175	58-77	5’ 10”	132-174
	180	62-81	6’ 0”	140-184
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Adults with a BMI between 19 and 25 are considered to have a healthy weight, those below 19 are underweight, those between 25 and 30 are overweight, and those above 30are considered obese. 
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FAQ
Which organism has a higher basal metabolic rate, a hummingbird or an elephant? 
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 Basal Metabolic Rate (BMR): minimum energy needed for survival and to maintain equilibrium of all vital functions (nervous, cardiovascular, respiratory, and digestive systems) when a body is at rest. 
As chemical energy in food converts to heat, body temperature tends to rise. To maintain a constant temperature, the body reacts by transferring the energy to the circulatory system and to the skin, where it eventually dissipates into the environment. 
BMR varies with the surface area through which the body must lose heat, i.e., BMR~A. 
Noting that body surface area scales with the square of length, while its mass and volume vary as length cubed, it can be easily concluded that BMR~ M2/3. 
Specific BMR (BMR per unit mass) decreases with larger animals. 
Question: Which organism has a higher basal metabolic rate, a hummingbird or an elephant? 
Answer: Hummingbirds weigh approximately 2.5-4.5 grams and have proportionately larger surfaces in relation to body mass; they can lose heat faster. Gram by gram, hummingbirds have the highest metabolic rate of any animal, roughly 12 times that of a human being and 100 times that of an elephant. Their BMR is around 29 W, which means they need to consume 600 Calories of food every day. No wonder that, each day, they have to visit hundreds of flowers to gather enough nectar to survive.
The brain and the liver, two organs which jointly make up only 4% of body weight, are responsible for half of all metabolic activity. 
 BMR is generally higher in males and for heavier, healthier, and younger animals. BMR is about 1.45 watts for a rat, 80 watts for an average sized person, and 266 watts for a medium-size cow.
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Food Units
One Calorie (with a capital C) is equal to the heat energy that is required to raise the temperature of one kilogram of water by one degree centigrade. 
A smaller unit of thermal energy is a calorie (with a small c), which is 1/1000th of the food calorie:
1 Calorie = 1 kilocalorie 
	 = 1,000 calories 
	 = 4,184 joules
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Food, Exercise, and Dieting
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Energy Expenditure
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BMR does not account for any physical activity, so additional energy is needed to carry out daily tasks. 
Physical activities = aerobic + non-aerobic activities. 
During aerobic activities, oxygen breaks down carbohydrates, fat, and protein and converts them to energy. 
Examples: dancing, jogging, swimming, and biking. 
Anaerobic activities burn carbohydrates without oxygen, with maximum bursts of energy of short duration. 
Examples: weight lifting, pushups, chin ups, and sprinting. 
The amount of daily energy needed is different for different people and can vary with gender, weight (mass), and levels of physical and mental activity. 
Rule of thumb: 
An average man requires 2,500 Calories per day, whereas 
An average woman needs only 1,800 Calories per day, in food intake. 
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