Summary

• First law of thermodynamics
• Thermodynamic processes
• Heat engines

• Reading Quiz



• Carnot heat engines
  • diagram and applet
  • max theoretical efficiency
    Example #1
    Example #2
    Example #3
    
    
  • second law of thermodynamics
  • conservation of entropy
  
  
• Refrigerators
  • diagram and applet
  • coefficient of performance
  • Carnot cycle
    Example #4
    Example #5
    Example #6
    Example #7
    Example #8
    Example #9
    
    
  
  
  
  
  
  
  
  
  
  

Walker4e
Reading quiz Suppose you have an ideal heat engine that can operate in one of two different modes.
In mode 1, T_h = 400 K and T_c = 200 K.
In mode 2, T_h = 600 K and T_c = 400 K.
The efficiency of mode 2 is _______ the efficiency of mode 1.
A. greater than
B. less than
C. the same as
Answer















gc6 15.q10mod
How should you set the operating temperatures in order to maximize the efficiency of a heat engine?
A. Make T_L as warm as possible.
B. Make ΔT as large as possible.
C. Make ΔT as small as possible.
D. Make T_H as cool as possible.
Answer















kw4
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 heat engine?
A. 30%
B. 40%
C. 60%
D. 70%
Answer















kw4
What is the theoretical maximum efficiency of this heat engine?
A. 30%
B. 40%
C. 60%
D. 70%
Answer















klm
How should you set the operating temperatures in order to maximize the performance of a refrigerator?
A. Make T_L as warm as possible.
B. Make ΔT as large as possible.
C. Make ΔT as small as possible.
D. Make T_H as cool as possible.
Answer















kw4
Can the coefficient of performance of a refrigerator be greater than one?
A. Yes
B. No

Answer















sj6 22.28
An ideal refrigerator runs between 0°C and 25°C. It removes 8000 kJ/h from the cold region. What minimum power is required?
A. 204 W
B. 320 W
C. 400 W
D. 733 W
Answer















sj6 22.28
An ideal refrigerator runs between 0°C and 25°C. It removes 8000 kJ/h from the cold region. What is the rate at which it exhausts energy?
A. 0.320 kW
B. 2.22 kW
C. 2.43 kW
D. 8.74 kW
Answer
















 



B. less than

The maximum efficiency is 1 − T_c/T_h. For mode 1 this amounts to 1 − (200 K)/(400 K) = 50%. For mode 2 it is 1 − (400 K)/(600 K) = 33%.
























 



B. Make ΔT as large as possible.





















 



A. 30%





















 



C. 60%





















 



C. Make ΔT as small as possible.





















 



A. Yes
The second law of thermodynamics only states that the work must be greater than zero, but that means the coefficient of performance must be less than infinity, not one. Good COP values for actual refrigerators can be 3 or more.




















 



A. 204 W





















 



C. 2.43 kW