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Thermodynamics Laws and Heat Engines
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Thermodynamics Laws and Heat Engines
Thermodynamics Laws and Heat Engines
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1
Question
What is the first law of thermodynamics for a closed system?
Page 3
Answer
\(\Delta U = Q_{in} - W_{out}\) where \(\Delta U\) is change in internal energy, \(Q_{in}\) is heat added, and \(W_{out}\) is work done by the system.
2
Question
Who is Rudolf Clausius and what is his contribution to thermodynamics?
Page 3
Answer
German physicist and mathematician (1822-1888) considered one of the central founders of thermodynamics science.
3
Question
What principle does the first law represent for closed systems?
Page 4
Answer
Principle of energy conservation.
4
Question
What important definitions arise from the first law of thermodynamics?
Page 4
Answer
Heat/specific heat, work/power, energy/internal energy, enthalpy, etc.
5
Question
Why is the first law insufficient to understand energy systems fully?
Page 4
Answer
It does not predict if a process will occur or the percentage of energy convertible to work.
6
Question
In the weight system example, why do both processes satisfy the first law?
Page 5
Answer
Both have \(\Delta U = Q_{in} - W_{out}\), conserving energy.
7
Question
How does heat flow behave in a well-insulated system according to the first law?
Page 6
Answer
Both hot-to-cold and cold-to-hot flows satisfy \(\Delta U = Q_{in} - W_{out}\).
8
Question
In the coal-fired power plant example, what is the hoped-for efficiency?
Page 7
Answer
100% conversion of primary fuel to electricity.
9
Question
What is the real efficiency shown for the coal-fired power plant?
Page 7
Answer
30% to electricity, 70% waste heat.
10
Question
Why does satisfaction of the first law not ensure a process occurs?
Page 8
Answer
It only conserves energy but does not predict feasibility or direction.
11
Question
What questions does the second law of thermodynamics answer?
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Answer
Percentage of energy convertible to work, engine efficiency, COP of heat pumps/refrigerators, spontaneous process direction.
12
Question
How many different statements does the second law have?
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Answer
A few different statements including Kelvin-Planck and Clausius.
13
Question
Why are second law statements considered negative?
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Answer
They state what cannot happen, and negative statements cannot be proved.
14
Question
What is the Kelvin-Planck statement of the second law?
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Answer
Impossible for any cyclic device to receive heat from a single reservoir and produce net work.
15
Question
What implication does Kelvin-Planck have for heat engine efficiency?
Page 11
Answer
No heat engine can have thermal efficiency of 100%.
16
Question
What is a heat engine?
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Answer
Device that converts heat to mechanical work, operating cyclically between high and low temperature reservoirs.
17
Question
Describe the heat flows in a heat engine cycle.
Page 12
Answer
Receives \(Q_H\) from high-T reservoir, rejects \(Q_L\) to low-T reservoir, net work \(W_{net,out}\).
18
Question
What is the thermal efficiency of a heat engine?
Page 13
Answer
\(\eta_{th} = \frac{W_{net,out}}{Q_H} = 1 - \frac{Q_L}{Q_H}\).
19
Question
Why is thermal efficiency always less than 1 for heat engines?
Page 14
Answer
\(Q_L > 0\) per Kelvin-Planck; engine must reject heat to low-T sink.
20
Question
What are typical thermal efficiencies for real engines?
Page 14
Answer
Steam power plant ~30%, petrol 25-30%, diesel ~40%, gas turbine ~60%.
21
Question
In Example 1, what is the net power output for the heat engine?
Page 15
Answer
\(\dot{W}_{net,out} = \dot{Q}_H - \dot{Q}_L = 80 - 50 = 30\) MW.
22
Question
What is the thermal efficiency in Example 1?
Page 15
Answer
\(\eta_{th} = \frac{\dot{W}_{net,out}}{\dot{Q}_H} = \frac{30}{80} = 0.375\).
23
Question
How do you rearrange the efficiency formula to find Q_H from W_net,out?
Page 16
Answer
\(Q_H = \frac{W_{net,out}}{\eta_{th}}\).
24
Question
What are the units for heat Q and rate \(\dot{Q}\) in efficiency formulas?
Page 16
Answer
Q: J, kJ; \(\dot{Q}\): W, kW.
25
Question
In Example 2, what is the power output for the nuclear power plant?
Page 17
Answer
\(\dot{W}_{net,out} = \eta_{th} \dot{Q}_H = 0.35 \times 90 = 31.5\) MW.
26
Question
How is heat rejection calculated in Example 2?
Page 17
Answer
\(\dot{Q}_L = \dot{Q}_H (1 - \eta_{th}) = 90 (1 - 0.35) = 58.5\) MW.
27
Question
What topics does Lecture 1 cover on the second law?
Page 2
Answer
Limitations of 1st law, introduction to 2nd law, Kelvin-Planck statement, heat engine concept, heat engine efficiency.
28
Question
According to Clausius, what equals the increment in internal energy?
Page 3
Answer
Difference between heat accumulated by system and work done by it.
29
Question
Can energy be created or destroyed per first law?
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Answer
No, energy CANNOT be created or destroyed.
30
Question
In the insulated system, which heat flow is natural?
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Answer
Heat from hot to cold reservoir (green check).