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Read Quiz An engine is usually a device where heat is generated by burning some kind of fuel, and this heat is then converted into work. Under ideal conditions, how much of the heat energy can be converted into work? • Some of it, but never all of it. • All of it • None of it
The internal energy U of a system is increased by the transfer of either heat or work into the system. Zeroth Law of Thermodynamics Two objects in thermal equilibium have the same temperature
The internal energy U of a system is increased by the transfer of either heat or work into the system.
What is a heat engine? • Thermal heat QH is introduced into the engine. • Some of this is converted into mechanical work, W. • Some heat is released into the environment at a lower temperature, QC.
Heat released by burning gasoline in the cylinder of an automobile engine causes the piston to move, converting some of the heat to work.
What does the First Law tell us about heat engines? U = Q - W = QH - QC -W , the internal energy U of a heat engine does not change from cycle to cycle, so U =0. Hence, Q = W. The net heat flowing into the engine equals the work done by the engine: W = QH - QC
The Second Law of Thermodynamics The Carnot engine is an ideal system which turns out to have the maximum possible efficiency: If TH is the hottest temperature in the engine, and TC is temperature outside the engine (in Kelvin), then the efficiency is: ec = (TH - TC )/ TH = W/QH This shows that it is not possible too have an efficiency of 100%. You always lose some energy into heating the environment.
This can be restated as a version of the Second Law: No engine, working in a continuous cycle, can take heat from a reservoir at a single temperature and convert that heat completely into work.
A restatement of the Second Law which turns out to be equivalent: Heat will not flow from a colder body to a hotter body unless some other process (which does work) is also involved. Another restatement to be discussed next time: The entropy of an isolated system can only increase or remain constant. Its entropy cannot decrease.
According to the Second Law of Thermodynamics, heat will not flow from a colder body to a warmer body. • True • False
A schematic representation of a heat engine. Heat is taken in at high temperatures, TH. Some heat is converted to work, and the remainder is released at a lower temperature, TC.
The arrow widths depict the quantities of energy in the sample exercise in box 11.1. Quiz What is the efficiency of this engine? 1 1/2 1/3 1/4 Not enough info.
The efficiency of a heat engine is defined as e = W/ QH . Engines are more efficient if there is a large difference between the high temperature inside and the low temperature outside. For an ideal heat engine, ec = (TH - TC )/ TH
A falling mass turns a paddle in an insulated beaker of water in this schematic representation of Joule’s apparatus for measuring the temperature increase produced by doing mechanical work on a system.
Steam issuing from the kettle makes the pinwheel turn in this simple steam turbine. Work could be done to lift a small weight with such an engine.
What is INTERNAL ENERGY? The internal energy of the system is the sum of the kinetic and potential energies of the atoms and molecules making up the system.
The First Law of Thermodynamics: The increase in the internal energy of a system is equal to the amount of heat added to the system, plus the amount of work done on the system. The increase in the internal energy of a system is equal to the amount of heat added to the system, minus the amount of work done by the system. U = Q - W Q=heat add to system W = work done by system
Quiz 1 The first Law of Thermodynamics, U = Q - W, means: A. Heat cannot be addedto a system without work being done. B. Work cannot be done without heat being added. C. The amount of work done always equals the amount of heat added. D. The total internal energy of a system is conserved. E. All of the above F. None of the above are true. CHOOSE THE TRUE STATEMENT.
Work done by a movable piston Work=F x d = F/A x d x A = P DV
Quiz 2 • If the temperature of a gas is held constant during compression or expansion it is called • Equation of state • Isobaric • Isothermal • Thermal conductivity • convection
Quiz 3 Heat can be transferred by • Thermal Conduction • Convection • Radiation • All of the above • None of the above
1. As the Moon orbits the Earth with a constant period of about 28 days, it is not accelerating. A. True B. False 2. Although the Moon is held in orbit by the force of gravity, this force does no work on the Moon. A. True B. False 3. A baseball is dropped to the ground. While it is falling, its total mechanical energy is constant. A. True B. False
4. Rocket engines work in outer space because the exhaust gases cause a recoil force on the rocket. A. True B. False 5. If an object is rotating, there must be a net torque acting on it. A. True B. False 6. Because steel is much denser than water, objects made out of steel won’t float in water. A. True B. False
7. In an ideal fluid at rest, the pressure is the same everywhere in the fluid. A. True B. False 8. Increasing the internal energy of a substance must raise its temperature. A. True B. False 9. According to the First Law of Thermodynamics, heat cannot be converted into mechanical work by a heat engine. A. True B. False 10. According to the Second Law of Thermodynamics, a heat engine cannot convert heat completely into work. A. True B. False.
11. A ball with mass 0.5 kg is tied to a string, and I swing it in a horizontal circle over my head. The radius of the circle is one meter, and the ball’s speed is 4 m/s. What is the magnitude of the force exerted by the string on the ball? A. 2 N B. 4 N C. 8 N D. 16 N E. 2 N 12. The radius of the Earth is about 6400 kilometers. If a spaceship took you up into Earth orbit at an altitude of 6400 kilometers above the surface of the Earth, by how much would your weight change? A. You would be weightless (your weight is zero). B. You would weigh half as much. C. You would weigh one-fourth as much. D. Your weight would be unchanged. E. None of the above.
13. A brick of mass 2 kg is dropped from a tower. After falling for one second, how much work was done on the brick? A. 10 J B. 100 J C. 200 J D. 2000 J E. 10000 J 14. A roller coaster car moving at 20 m/s starts to head up a hill. What is the maximum height it could reach? A. 1 m B. 2 m C. 10 m D. 20 m E. not enough information given.
15. Automobiles equipped with airbags are safer for the driver during a head-on collision, where the driver is cushioned by the airbag instead of directly striking the steering wheel. This is because: A. it reduces the maximum force on the driver. B. it reduces the driver’s change in momentum. C. it reduces the driver’s change in kinetic energy. D. it reduces the duration time of the collision. E. it reduces the torque on the driver. 16. A ball of mass 0.1 kg strikes a wall at 15 m/s, and bounces back at –10 m/s. What is the magnitude of its change in momentum? A. 0.1 kg-m/s B. 0.5 kg-m/s C. 1.0 kg-m/s D. 1.5 kg-m/s E. 2.5 kg-m/s
17. A net torque of 50 N-m is applied to a disk whose rotational inertia is 10 kg-m2. What is the angular acceleration of the disk? A. 5 rad/s2. B. 10 rad/s2. C. 50 rad/s2. D. 100 rad/s2. E. 500 rad/s2. 18. A toy top that is spinning tends to stay upright, but if it is not spinning it would quickly topple over. This can be explained by: A. its rotational inertia. B. its torque. C. conservation of angular momentum. D. conservation of linear momentum. E. conservation of energy.
19. A brick has a volume of 0.001 m3 and a density of 2000 kg/m3, while the density of water is 1000 kg/m3. When the brick is placed in water, what is the buoyant force acting on it? A. 0.1 N B. 1.0 N C. 2.0 N D. 10.0 N E. 20.0 N 20. The density of water is 1000 kg/m3. What is the water pressure in the deep end of a swimming pool at a depth of 3 m? A. 300 N/m2. B. 3,000 N/m2. C. 30,000 N/m2. D. 300,000 N/m2. E. 3,000,000 N/m2.
21. How much energy is needed to convert a 10 gram ice cube at T=0 C into steam at T=100 C? A. 720 cal B. 800 cal C. 1000 cal D. 1800 cal E. 7200 cal 22. An insulated beaker contains 100 grams of water at T=100 C. Work of 900 J is done on the water by stirring. What is the change in temperature of the water? A. about 1C. B. about 2 C. C. about 3 C. D. about 20 C. E. zero.
23. If an automobile engine performed like an ideal heat engine, would it be more efficient on a hot day or on a cold day? A. a hot day. B. a cold day. C. it would not make any difference. 24. An inventor claims to have constructed a device that will make heat flow out of a cold body into a warmer body. Which of the following is true: A. This violates the First Law of Thermodynamics (but not the Second). B. This violates the Second Law of Thermodynamics (but not the First). C. This violates both Laws of Thermodynamics. D. This need not violate either of the Laws of Thermodynamics.
25. Consider a traditional wall clock with a moving second hand. What is the angular speed of the second hand? A. about 0.01 rad/s B. about 0.10 rad/s C. about 1.00 rad/s D. about 2 rad/s E. about 1/60 rad/s