1. VII. The second low of Thermodynamics
1. Reversible and irreversible processes
Mechanics
Thermodynamics
Heat can flow spontaneously from a hot object to a cold object;
heat will not flow spontaneously from a cold object to a hot object.
2. Heat engines
H.E. transforms heat partly into work or mechanical energy
Working substance
Cycle process: ΔU = 0
ΔU = Q - W = 0 W = Q = QH + QC = |QH| - |QC| TH
efficiency:
|QH| W
engine
3. Refrigerators
|QC|
coefficient of
performance: TC

2. 4. The second law of thermodynamics and heat engines
It is impossible to build a heat engine with 100% efficiency
5. The Carnot Cycle (reversible)
Maximum efficiency: P a b TH d c TL V
6. The Carnot Cycle and the second law of thermodynamics
No device is possible whose sole effect is to transform a given amount of heat completely into work
No device is possible whose sole effect is to transform heat from one system at temperature TL into a second system at higher temperature TH

3. Example 1: A nuclear power plant generate steam at 6300 C.
The steam is sent through a series of turbines and exits at 1000 C.
What is the maximum possible efficiency of the power station?
TH = 630ºC
TC = 100ºC
εC - ?

4. Example 2: An ice-making machine operates in a Carnot cycle.
It takes heat from water at 0.0C and rejects heat to a room at 24.0C.
Suppose that 50.0 kg of water at 0.0C are converted to ice at 0.0C.
How much energy must be supplied to the device?
Heat of fusion of water is 334*103 J/kg.
TH = 24.0C
TC = 0.0C
m = 50.0 kg
L = 334*103 J/kg
W - ?

5. The thermal efficiency of the heat engine is ___ %.
Question
In one cycle, a heat engine does 10 J of work and exhausts 20 J of waste heat.
The thermal efficiency of the heat engine is ___ %. 20 33 50

6. The temperature of the hot reservoir is ___ K.
Question
In a Carnot heat engine, the cold temperature reservoir is at 300 K (= 27 ˚C) and the thermal efficiency is 50%.
400
600
800
1000
The temperature of the hot reservoir is ___ K.

7. Question
A refrigerator has a coefficient of performance K = Work is done on the refrigerator at a rate Pwork = dW/dt = 100 Watts. 50
100
200
400
The rate at which heat is removed from inside the refrigerator is d|QC|/dt = ___ Watts.

8. The internal-combustion engine
A fuel vapor can be compressed, then detonated to rebound the cylinder, doing useful work.

9. r - compression ratio γ – CP/CV 6. Otto cycle
Example: An automobile engine has a compression ratio of 10.0.
The maximum theoretical thermal efficiency of the engine is ___ %. ( = 1.40 for air)