Entropy and the Second Law of Thermodynamics

Question 1

Entropy Change (irreversible)

Answer 1

ΔS = Sf - Si = ∫ dQ/T [from i to f]

Question 2

Entropy Change (reversible)

Answer 2

ΔS = Sf - Si = Q/T

Question 2

Entropy Change * relative small change of temp

Answer 2

ΔS = Sf - Si = Q/Tavg

Question 3

Change in entropy (when ideal gas changes reversibly)

Answer 3

ΔS = Sf - Si = nR ln (Vf/Vi) + nCv ln (Tf/Ti)

Question 4

Second Law of Thermodynamics

Answer 4

if a process occurs in a closed system, the entropy of the system increases for irreversible processes and remains constant for reversible processes. It never decreases.

ΔS ≥ 0

Question 5

Efficiency of an engine

Answer 5

ε = energy we get / energy we pay for = |W|/|QH|

Question 6

Efficiency of an ideal engine (Carnot engine)

Answer 6

ε = 1 - (|QL|/|QH|) = 1 - (TL/TH)

Question 7

coefficient of performance K of a refrigerator

Answer 7

K = what we want / what we pay for = |QL|/|W|

Question 7

coefficient of performance K of a Carnot refrigerator

Answer 7

Kc = |QL| / (|QH|- |QL|) = TL / (TH - TL)

Question 8

multiplicity (entropy from a statistical view)

Answer 8

W = N! / n1! n2!