Heat Engines
When a thermodynamic system goes through a complete cycle the system's change in internal energy and change in entropy are both zero.
The work done by the system is the enclosed area on the P-V diagram. Clockwise cycles give positive work (e.g., car engines); counterclockwise cycles give negative work (e.g., air conditioners).
Applying the First Law to a complete cycle gives:
|QH| = W + |QL|
| Efficiency: e
| = |
| W
|  |
| |QH|
|
|
= 1 - |
| |QL|
| |
| |QH|
|
|
| Ideal engine: eCarnot
| = 1 - |
| TL
| |
| TH
|
|
Second Law: ΔS ≥
0
Determining changes in entropy:
| ΔS = Sf - Si |
= |
∫ |
| dQ
| |
| T
|
|
If the heat transfer takes place over a small range of temperatures:
| ΔS
| = |
| Q
| |
| Tavg
|
|