2
Physics 100 Lecture 7
Heat Engines and the 2nd Law of Thermodynamics February 12, 2018 3
Thermal Convection
Warm fluid is less dense and rises while cool fluid sinks Resulting circulation efficiently transports thermal energy 4 COLD
Convection
HOT
Turbulent motion of glycerol in a container heated from below and cooled from above. The bright lines show regions of rapid temperature variation. The fluid contains many "plumes," especially near the walls. The plumes can be identified as mushroom-shaped objects with heat flowing through the "stalk" and spreading in the "cap." The hot plumes tend to rise with their caps on top; falling, cold plumes are cap-down. All this plume activity is carried along in an overall counterclockwise "wind" caused by convection. Note the thermometer coming down from the top of the cell. Figure adapted from J. Zhang, S. Childress, A. Libchaber, Phys. Fluids 9, 1034 (1997). See detailed discussion in Kadanoff, L. P., Physics Today 54, 34 (August 2001). 5 The temperature of land changes more quickly than the nearby ocean. Thus convective “sea breezes” blow ____ during the day and ____ during the night.
A. onshore … onshore B. onshore … offshore C. offshore … onshore D. offshore … offshore 6 The temperature of land changes more quickly than the nearby ocean. Thus convective “sea breezes” blow ____ during the day and ____ during the night.
A. onshore … onshore B.onshore … offshore C.offshore … onshore D.offshore … offshore 7
Thermal radiation
Any object whose temperature is above zero Kelvin emits energy in the form of electromagnetic radiation Objects both absorb and emit EM radiation continuously, and this phenomenon helps determine the object’s equilibrium temperature 8
The electromagnetic spectrum 9
Thermal radiation
We’ll examine this concept some more in chapter 6 10 Why does the Earth cool more quickly on clear nights than it does on cloudy nights?
A. Clouds reradiate energy back to Earth. B. Clouds prevent thermal conduction like a blanket. C. Clouds promote thermal convection. D. Clouds contain latent heat energy. 11 Why does the Earth cool more quickly on clear nights than it does on cloudy nights?
A. Clouds reradiate energy back to Earth. B. Clouds prevent thermal conduction like a blanket. C. Clouds promote thermal convection. D. Clouds contain latent heat energy. 12
Heat Engines
Friction converts mechanical energy to thermal energy Can thermal energy be converted to mechanical energy? Hero’s engine demo 13
Heat engines
QQWHC W eactual QH
TC emaximum 1 TH Animated applet
animation 14
The Second Law of Thermodynamics
It is impossible to build a heat engine that converts 100% of the input energy into mechanical work e < 100%
Qc > 0 some energy is always wasted 15 Class Quiz 4b: What would be the most effective way to increase the efficiency of a heat engine?
A. Increase TC and increase TH
B. Increase TC and decrease TH
C. Decrease TC and increase TH
D. Decrease TC and decrease TH 16 Class Quiz 4b: What would be the most effective way to increase the efficiency of a heat engine?
A.Increase TC and increase TH
B.Increase TC and decrease TH
C.Decrease TC and increase TH
D.Decrease TC and decrease TH 17 What input energy is required if a heat engine performs 200 kJ of work and exhausts 600 kJ of heat?
A. 200 kJ B. 400 kJ C. 600 kJ D. 800 kJ 18 What input energy is required if a heat engine performs 200 kJ of work and exhausts 600 kJ of heat?
A. 200 kJ QQWHC B. 400 kJ 600 kJ 200 kJ
C.600 kJ QH 800 kJ D.800 kJ 19 A heat engine takes in 1000 J of energy at 1000 K and exhausts 700 J at 400 K. What is the actual efficiency of this engine?
A. 30% B. 40% C. 60% D. 70% 20 A heat engine takes in 1000 J of energy at 1000 K and exhausts 700 J at 400 K. What is the actual efficiency of this engine?
W QQ A.30% e HC actual QQ B. 40% HH 1000 J 700 J C.60% 1000 J D.70% 300 J e 30% actual 1000 J 21 A heat engine takes in 1000 J of energy at 1000 K and exhausts 700 J at 400 K. What is the maximum efficiency of this engine?
A. 30% B. 40% C. 60% D. 70% 22 A heat engine takes in 1000 J of energy at 1000 K and exhausts 700 J at 400 K. What is the maximum efficiency of this engine?
A. 30% TC emaximum 1 B. 40% TH 400 K C.60% 1 1000 K D.70% emaximum 60% 23
Internal Combustion Engines
[Only for the curious] The modern Internal Combustion Engine utilizes the Otto cycle
Animated applet