University Physics Volume 2

# Summary

### 4.1Reversible and Irreversible Processes

• A reversible process is one in which both the system and its environment can return to exactly the states they were in by following the reverse path.
• An irreversible process is one in which the system and its environment cannot return together to exactly the states that they were in.
• The irreversibility of any natural process results from the second law of thermodynamics.

### 4.2Heat Engines

• The work done by a heat engine is the difference between the heat absorbed from the hot reservoir and the heat discharged to the cold reservoir, that is, $W=Qh−Qc.W=Qh−Qc.$
• The ratio of the work done by the engine and the heat absorbed from the hot reservoir provides the efficiency of the engine, that is, $e=W/Qh=1−Qc/Qh.e=W/Qh=1−Qc/Qh.$

### 4.3Refrigerators and Heat Pumps

• A refrigerator or a heat pump is a heat engine run in reverse.
• The focus of a refrigerator is on removing heat from the cold reservoir with a coefficient of performance $KR.KR.$
• The focus of a heat pump is on dumping heat to the hot reservoir with a coefficient of performance $KP.KP.$

### 4.4Statements of the Second Law of Thermodynamics

• The Kelvin statement of the second law of thermodynamics: It is impossible to convert the heat from a single source into work without any other effect.
• The Kelvin statement and Clausius statement of the second law of thermodynamics are equivalent.

### 4.5The Carnot Cycle

• The Carnot cycle is the most efficient engine for a reversible cycle designed between two reservoirs.
• The Carnot principle is another way of stating the second law of thermodynamics.

### 4.6Entropy

• The change in entropy for a reversible process at constant temperature is equal to the heat divided by the temperature. The entropy change of a system under a reversible process is given by $ΔS=∫ABdQ/TΔS=∫ABdQ/T$.
• A system’s change in entropy between two states is independent of the reversible thermodynamic path taken by the system when it makes a transition between the states.

### 4.7Entropy on a Microscopic Scale

• Entropy can be related to how disordered a system is—the more it is disordered, the higher is its entropy. In any irreversible process, the universe becomes more disordered.
• According to the third law of thermodynamics, absolute zero temperature is unreachable.
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