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PHYS 1110 Lecture 10 Professor Stephen Thornton September 27, 2012
Reading Quiz A) 0°C 1 kg of water at 100°C is poured into a B) 20°C bucket that contains 4 kg of water at C) 50°C 0°C. Find the equilibrium temperature D) 80°C (neglect the influence of the bucket). E) 100°C
Reading Quiz A) 0°C 1 kg of water at 100°C is poured into a B) 20°C bucket that contains 4 kg of water at C) 50°C 0°C. Find the equilibrium temperature D) 80°C (neglect the influence of the bucket). E) 100°C Because the cold water mass is greater, it will greater have a smaller temperature change! change The masses of cold/hot have a ratio of 4: 1, so the temperature change must have a ratio of 1: 4 (cold/hot). Q 1 = Q 2 m 1 c D T 1 = m 2 c D T 2 D T 1 / D T 2 = m 2 / m 1
Midterm exam next Tuesday. Chapters 1 -4. Thermodynamics will not be on the exam. Homework, including the electric motor, is due today. I will count off 10 points a day for late homework.
Heat Exchange § Conduction – molecules touch each other and exchange energy. § Convection – hot fluids rise § Radiation – electromagnetic radiation like light, infrared, ultraviolet radiation; all frequencies. These are very important!!
Heat conduction If we put a torch to a piece of metal, the molecules in the metal have increased kinetic energy. They collide with adjacent molecules, and the heat moves down the material via these collisions. Some materials transport heat energy more easily than others. Metals are good heat conductors. Wood and plastics are poor.
Heat Conduction Through a Rod Q is proportional to A and temperatures T 2 – T 1 Q is proportional to 1/L where k is called thermal conductivity W/(m K)
The constant k is called thermal conductivity. Materials with large k are called conductors; those with small k are called insulators. Note: materials that are good heat conductors are also good electrical conductors. Why? Copyright © 2009 Pearson Education, Inc.
Note in the table on thermal conductivities that air is a very poor heat conductor. In fact, we could say it is a good heat insulator. This is why double pane windows are such good insulators both in the summer and winter. Glass panes are thin and conduct heat much better than air. The layer of air does wonders!
Building materials are measured using Rvalues rather than thermal conductivity: Here, Copyright © 2009 Pearson Education, Inc. is the thickness of the material.
Heat convection Well known phenomenon because hot fluids rise due to their lower density. We take advantage of this by putting heat ducts on the floor. Did demo – convection chimney
Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced; both these examples are natural convection. Copyright © 2009 Pearson Education, Inc.
Heat radiation Have you ever sat in front of a campfire and wondered why your face is so warm, and your behind so cold? All objects emit electromagnetic radiation. Waves easily carry energy in the form of light, radar, microwave (cell phone), etc. Our existence depends on heat radiation from the Sun.
Did light the match (wood) demo. Example of radiation.
Heat radiation is noted in terms of radiated power P
e = 1 is a perfect emitter and absorber, and is called a blackbody. e = 0 is an ideal reflector. Inside of a thermos bottle is shiny and is a good reflector. The heat of the container emits radiation, but it is not absorbed by the outer wall.
The Thermos Bottle
If you are in the sunlight, the Sun’s radiation will warm you. In general, you will not be perfectly perpendicular to the Sun’s rays, and will absorb energy at the rate: Copyright © 2009 Pearson Education, Inc.
This cos θ effect is also responsible for the seasons. Copyright © 2009 Pearson Education, Inc.
Consider a system: Automobile engine Human body Simple piston and cylinder We want to consider what happens if we add heat to our system or take heat away. Also let the system do work or have work done on it. What happens to internal energy? The internal energy is the sum of all the kinetic and potential energies.
The Internal Energy of a System Ei Ef = Ei + Q If we add heat Q to a system having internal energy Ei, the new internal energy of the system is Ef = Ei + Q. E = Ef – Ei = Q
Work and Internal Energy E Ef = Ei -W i If the system does work W on the outside, then the system loses internal energy. Ei – Ef = W E = Ef – Ei = -W
First Law of Thermodynamics (Conservation of Energy) Let’s combine the last two equations: E = Ef – Ei = Q E = Ef – Ei = -W Because both heat flow and work can occur, the change in internal energy of a system depends on both Q and W. First law of thermodynamics
Signs of Q and W *** Q positive System gains heat Q negative System loses heat W positive Work done by system W negative Work done on system The convention for W is opposite of that in chemistry.
The internal energy E depends on the state of the system (P, V, T, m, n). They are called state functions. Heat flow Q and work W are not state functions. They depend on how the system is changed.
A Constant-Pressure Process System does work to push piston in cylinder at constant pressure. Volume expands.
In a general problem like this example, the area under the curve is equal to the work done by the system. Area here is work
We add heat to a system at constant volume. What is the work done? W= P V = 0 Because volume doesn’t change, the work done W must be zero.
Isotherms on a PV diagram
In an adiabatic process, the system is well insulated thermally, and no heat flows (Q = 0). When the piston compresses the volume, the pressure and temperature must both go up.
Adiabatic Heating If we push down quickly, there is no time for heat to flow, and the process is adiabatic. Temperature rises quickly.
When the piston moves up, the volume expands, and the pressure and temperature decrease. Adiabatic process occurs often when the process is rapid, and there is no time for heat to flow.
Conceptual Quiz Two equal-mass liquids, initially at the same temperature, are heated for the same A) the cooler one time over the same stove. You measure B) the hotter one the temperatures and find that one liquid has a higher temperature than the other. Which liquid has a higher specific heat? C) both the same
Conceptual Quiz Two equal-mass liquids, initially at the same temperature, are heated for the same A) the cooler one time over the same stove. You measure B) the hotter one the temperatures and find that one liquid has a higher temperature than the other. C) both the same Which liquid has a higher specific heat? Both liquids had the same increase in internal energy, because the same heat was added. But the cooler liquid had a lower temperature change. Because Q = mc. DT, if Q and m are both the same and DT is smaller, then c (specific heat) must be bigger.
Thermodynamic Processes and Their Characteristics Constant pressure W = P V Q = Eint + P V Constant volume W = 0 Q = Eint Isothermal (constant W = Q Eint = 0 temperature) Adiabatic (no heat W = – Q = 0 flow) Eint = Q – W
Work Done by Thermal Systems
If the first law of thermodynamics is about energy conservation, then the 2 nd law is about the way in which energy flows. Examples: A bowl of water sitting in this room does not spontaneously freeze. It is impossible to construct an engine that can extract thermal energy from a system and turn all that energy into work. Thermal systems spontaneously change in only certain ways.
2 nd Law of Thermodynamics We can discuss this law in a number of ways. The law basically states the way in which heat flow occurs. Heat flow between two objects brought together in thermal contact always goes from the hotter object to the colder object. Lots of ways to say the same thing!
Heat Engines An engine is a device that converts heat into mechanical work. Engines must operate in cycles in order to be useful. A piston and cylinder must return to original position. The change in internal energy is zero. An engine operates between two thermal reservoirs.
Schematic Diagram of Heat Engine
Heat Engines A steam engine is one type of heat engine. Copyright © 2009 Pearson Education, Inc.
Do demos • Heat engine • Steam engine
Our favorite heat engine. Reversible processes. X
Carnot Cycle • Carnot’s cycle represents the most efficient engine possible. • It operates between two heat reservoirs. • All the processes are reversible – two isothermals and two adiabatics. • We can show for the Carnot cycle.
Conceptual Quiz: A heat engine absorbs 150 J of heat from a hot reservoir and rejects 90 J of it to a cold reservoir. What is the efficiency of this engine? A) 20% B) 40% C) 60% D) 67% E) 90%
Conceptual Quiz: For the previous heat engine, you are told the temperature of the hot reservoir is 200 o. C and that of the cold reservoir is 11 o. C. Your response is to A) believe that this is possible. B) laugh at the idea. C) contact a patent lawyer immediately.
Another statement of 2 nd Law of Thermodynamics It is not possible to construct an engine whose sole effect is to transform a given amount of heat completely into work!
Heat engine and refrigerator
This figure shows more details of a typical refrigerator. Copyright © 2009 Pearson Education, Inc.
We analyze refrigerators differently. We want to remove as much heat Qc as possible for the least amount of work. Coefficient of Performance or COP
Air conditioner and heat pump inside house Maximize Qh Maximize Qc Heat house
A heat pump can heat a house in the winter: Copyright © 2009 Pearson Education, Inc.
For an ideal, reversible heat pump (i. e. Carnot cycle), we have To minimize W we want temperatures to be similar.
Various Engines http: //www. animatedengines. com/ Look at Four Stroke Diesel Two Stroke Steam Locomotive Newcomen Atmospheric Engine Two Cylinder Stirling Engine
Entropy There are several ways to look at entropy, but eventually they are all equal. Entropy is related to disorder in a system. A messy bedroom has more entropy than a clean one. The natural order of the universe is to increase entropy. Your bedroom never naturally becomes clean; it always naturally becomes messy.
Entropy is also related to probability. There is a higher probability that a block of ice will melt at room temperature than it will get colder. Thermodynamics does not prevent either action. The probability of the latter is incredibly small.
Conceptual Quiz: Humpty Dumpty falls off and breaks. Can he get back together again? A) Yes, very easily. B) Yes, but with extremely low probability. C) No, there is no possibility. D) Are you kidding us? Humpty Dumpty sat on a wall, Humpty Dumpty had a great fall; All the King's horses and all the King's men, Couldn't put Humpty together again.
Answer: B From what we just learned, this is only a question of probabilities. And no, I am not kidding!
Entropy is a very fundamental property, and is a state variable. It is determined by the heat flow Q divided by the temperature T.
For a reversible heat engine, the total entropy of the engine cycle is
Entropy and the Second Law of Thermodynamics The total entropy always increases when heat flows from a warmer object to a colder one in an isolated two-body system. The heat transferred is the same, and the cooler object is at a lower average temperature than the warmer one, so the entropy gained by the cooler one is always more than the entropy lost by the warmer one. Copyright © 2009 Pearson Education, Inc.
The fact that after every interaction the entropy of the system plus the environment increases is another way of putting the second law of thermodynamics: The entropy of an isolated system never decreases. It either stays constant (reversible processes) or increases (irreversible processes). Copyright © 2009 Pearson Education, Inc.
The total entropy of the universe increases whenever an irreversible process occurs. The total entropy of the universe is unchanged whenever a reversible process occurs. This is another way to state the 2 nd Law of Thermodynamics.
There is some really bad news here. Because the universe actually works through irreversible processes, the entropy is gradually increasing. There will eventually be a gradual “heat death” of the universe. The universe will be full of energy which cannot be used to perform work! We are doomed!
Order, Disorder, and Entropy As we have stated, entropy is related to disorder. As the entropy of a system increases, its disorder increases as well. GOOD NEWS: When you go home, and your mother fusses about how messy your bedroom is, tell her it is because entropy is increasing, and it is the natural order of the universe. There is little you or your mother can do about it (without doing a lot of work!). She will be impressed by how much physics you have learned!
Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law: Natural processes tend to move toward a state of greater disorder. Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet. No amount of stirring will get the milk and sugar to come back out of solution. Copyright © 2009 Pearson Education, Inc.
Another example: When a tornado hits a building, there is major damage. You never see a tornado approach a pile of rubble and leave a building behind when it passes. Copyright © 2009 Pearson Education, Inc.
Statistical Interpretation of Entropy and the Second Law The most probable distribution of speeds in a gas is Maxwellian: Highly unlikely Copyright © 2009 Pearson Education, Inc. The most probable state is the one with the greatest disorder, or the greatest entropy. With k being Boltzmann’s constant and W the number of microstates, Boltzmann showed
Statistical Interpretation of Entropy and the Second Law In this form, the second law of thermodynamics does not forbid processes in which the total entropy decreases; it just makes them exceedingly unlikely. Copyright © 2009 Pearson Education, Inc.
Thermal Pollution, Global Warming, and Energy Resources Over 90% of the energy used in the U. S. is generated using heat engines to drive turbines and generators—even nuclear power plants use the energy generated from fission to heat water for a steam engine. The thermal output QL of all these heat engines contributes to warming of the atmosphere and water. This is an inevitable consequence of the second law of thermodynamics. Copyright © 2009 Pearson Education, Inc.
Conceptual Quiz A) positive In the closed thermodynamic B) zero cycle shown in the P-V diagram, C) negative the work done by the gas is: P V
Conceptual Quiz In the closed thermodynamic A) positive cycle shown in the P-V diagram, B) zero the work done by the gas is: C) negative The gas expands at a higher pressure and compresses at a lower pressure. In general, clockwise = positive work; P counterclockwise = negative work. V
Conceptual Quiz Given your experience of what feels colder when you walk on it, which of the surfaces would have the highest thermal conductivity? A) a rug B) a steel surface C) a concrete floor D) has nothing to do with thermal conductivity
Conceptual Quiz Given your experience of what feels colder when you walk on it, which of the A) a rug B) a steel surfaces would have the C) a concrete floor highest thermal conductivity? E) has nothing to do with thermal conductivity The heat flow rate is k A (T 1 − T 2)/L. All things being equal, bigger k leads to bigger heat loss. From the book: Steel = 40, Concrete = 0. 84, Human tissue = 0. 2, Wool = 0. 04, in units of J/(s. m. C°).