heat and thermodynamics

heat and thermodynamics - Heat & Thermodynamics Eng...

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Unformatted text preview: Heat & Thermodynamics Eng 010: How Things Work (images from Hyperphysics) 1 Internal Energy (U) It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. TOTAL INTERNAL ENERGY = INTERNAL KINETIC ENERGY + INTERNAL POTENTIAL ENERGY 2 A graphical description for Internal Energy Internal energy involves energy on the microscopic scale. For an ideal monoatomic gas, this is just the translational kinetic energy of the linear motion of the "hard sphere" type atoms , and the behavior of the system is well described by kinetic theory. However, for polyatomic gases there is rotational and vibrational kinetic energy as well. Then in liquids and solids there is potential energy associated with the intermolecular attractive forces. 3 Recall that temperature only measures the translational Kinetic energy 4 A consequence of internal energy concept When the sample of water and copper are both heated by 1C, the addition to the kinetic energy is the same, since that is what temperature measures. But to achieve this increase for water, a much larger proportional energy must be added to the potential energy portion of the internal energy. So the total energy required to increase the temperature of the water is much larger, i.e., its specific heat is much larger. 5 Outstanding points amt of heat required to raise temperature Sand feels hotter than water under the same sunlight of 1 g of water is much more than Copper 6 Specific heat The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The specific heat of water is 1 calorie/gram C = 4.186 joule/gram C which is higher than any other common substance. Higher the specific heat, greater is the resistance a substance offers to change in his temperature. In this 7 specific heats of some solids matter Water Ice (-10 C) Glass Iron Copper Gold cal/ (gmC) 1 0.49 0.2 0.12 0.09 0.03 Sand (specific heat 0.2 cal/gm C) gets five times hotter than water under the same sunlight low specific heats of metal is due to proportionately lower potential energy change required (microscopic description of specific heat...) it would take twice as many calories to heat some water one degree than it would to heat the same mass of ice one degree. 8 What is Heat Heat may be defined as energy in transit from a high temperature object to a lower temperature object. An object does not possess "heat"; the appropriate term for the microscopic energy in an object is internal energy. The internal energy may be increased by transferring energy to the object from a higher temperature (hotter) object - this is properly called heating. Stated differently, it's a thermal analog of mechanical concept of work A traditional unit of heat is Calorie (= 4.18 joules) 9 Both heat and work are means of transferring energy the temperature of a gas can be raised either by heating it, by doing work on it, or a combination of the two. 10 Thermodynamics The study of heat and its transformation to mechanical energy is called Thermodynamics Serves as basic theory of heat engines, steam turbines, nuclear reactors, refrigerators, and pumps It is interesting to appreciate that the field of thermodynamics began to develop even before the atomic theory of matter was accepted Its foundations are energy conservation and heat flow 11 System work 12 First law of thermodynamics The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes. It states that heat added to a system is exactly equal to the change in internal energy of the system plus the work done by the system. Alternatively, when heat flows to or from a system, the system gains or loses an amount of energy equal to the amount of heat transferred. The energy change is retained by the system as a net change in its internal energy and any work done by the system. Q = U + W 13 Special cases (Q = U + W) If no heat is added, and we do work on the system, it raises its internal energy If no heat is added to the system and system does work on the surroundings, it loses some internal energy (Adiabatic) all heat energy into mechanical work (2nd Law of thermodynamics) We will see that it's not possible to convert 14 Adiabatic processes have fascinating effects Blow air on your hand first with your mouth wide open, then with puckered lips so that air expands...Now compare the temperature of your breath... Rub your palms together... pump a bicycle pump handle... mechanical work done on the system raises their internal energy 15 Meteorologists' Version of the first law (Q = U + W) Air temperature rises as pressure is increased or heat is added. Sources of heat include... (+) solar radiation, moisture condensation, contact with warm ground (-) evaporation of rain and contact with cold surface 16 Adiabatic processes in air Air temperature rises as pressure increases... Expanding air cools... Compressing air warms... 17 Air Parcels An air parcel is an imaginary volume of air used by meteorologists to conceptualize the thermodynamic fluid motions of the atmosphere for use in weather forecasting. (Wiki) Tens of meters to a kilometer wide air "clouds" 18 Air parcel behavior: Adiabatic cooling Normal lapse rate: 6.4 C/km 19 http://eesc.columbia.edu/courses/ees/slides/climate/adiabatic.jpg 20 smog: temperature inversion 21 2nd Law of Thermodynamics The entropy (disorder, randomness, chaos) of the universe increases with each process. The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. 22 multiplicity entropy time's arrow ice melting (Clausius) 23 Maxwell's demon ! ... if we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is impossible to us. For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics. [MAXWELL, TAKEN FROM WIKIPEDIA] 24 heat by itself never flows from cold to hot, the natural direction of heat flow is from hot to the cold. 2nd Law 25 Heat Engine A device which converts internal energy into mechanical work. When work is done by a heat engine running between two temperatures, only some of the input heat can be converted to work, the rest is expelled to the low temperature sink. 26 Thermal Bottleneck If the first law of thermodynamics says you can't win, then the Second Law of Thermodynamics says you can't even break even. The First Law is essentially a statement of conservation of energy and asserts that you can't get more energy out of a heat engine than you put in. But the Second Law says that no heat engine can use all the heat produced by a fuel to do work. The Carnot cycle sets the ideal efficiency which can be obtained if there is no friction, mechanical losses, leakage, etc., but real machine efficiencies are much less. 27 engine efficiency X 28 pumps, refrigerators Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles. 29 Statements of 2nd Law Heat will not flow spontaneously from a cold object to a hot object Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. You cannot create a heat engine which extracts heat and converts it all to useful work. No perpetual motion machines of the second kind. No 100 % efficient heat engines. There is a thermal bottleneck which contrains devices which convert stored energy to heat and then use the heat to accomplish work. For a given mechanical efficiency of the devices, a machine which includes the conversion to heat as one of the steps will be inherently less efficient than one which is purely mechanical. Maxwell's demon is a hard working individual, doing work on the system by information processing and while opening the gate... 30 ...
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This note was uploaded on 04/05/2008 for the course ENG 10 taught by Professor Atul-parhak during the Spring '08 term at UC Davis.

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