Transcript 20-summary.ppt

Chapter 20
Entropy and the Second Law of Thermodynamics
Entropy is a state function which is a measure of the disorder
in a system.
For any process the entropy a closed system never
decreases.
f
DS = i dQr/T
Special cases:
1- Reversible process for an ideal gas:
DS = n R ln(Vf/Vi) + n cv ln(Tf/Ti)
2- Melting:
DS = m LF/Tm
3- DS for a reversible adiabatic process: zero!
4- DS for (an arbitrary) cyclic process: ZERO!!
5- Heat conduction: DS = Q/TL - Q/TH
6- Adiabatic (isolated) free expansion:
DS = n R ln(Vf/Vi)
7- Irreversible heat transfer (w/o mixing):
DS = m1c1ln(Tf/T1) + m2c2ln(Tf/T2)
Efficiency (e) of an engine is defined to be:
e = W/ QH
For a Carnot engine:
TH/TL = |QH|/ |QL|
Wc = |QH| - |QL| = area inside the T-S cycle.
ec = W/ QH = 1- |QL|/ QH = 1- TL/ TH
Even the ideal engine is not “perfect”!!
Coefficient of performance (K) for a refrigerator:
K = QL/W
For a Carnot refrigerator:
Kc = TL/(TH - TL)