TITLE.PM5

SECOND LAW OF THERMODYNAMICS AND ENTROPY 299

dharm
/M-therm/th5-4.pm5

but when the cycle is not reversible

Cycle

∑

F

HG

I

KJ

δQ

T < 0.’’

4. ‘Entropy’ is a function of a quantity of heat which shows the possibility of conversion of that heat into
   work. The increase in entropy is small when heat is added at a high temperature and is greater when heat
   addition is made at lower temperature. Thus for maximum entropy, there is a minimum availability for
   conversion into work and for minimum entropy there is maximum availability for conversion into work.


5. Entropy changes for a closed system (per kg) :
   (i)General case :

(a) cv loge TT^2

1

+ R loge vv^2

1

(in terms of T and v)

(b) cv loge pp^2

1

+ cp loge vv^2

1

(in terms of p and v)

(c) cp loge TT^2

1

• R loge p
  p

2

1

(in terms of T and p)

(ii)Constant volume : cv loge TT^2

1

(iii)Constant pressure : cp loge TT^2

1

(iv)Isothermal : R loge vv^2

1

(v)Adiabatic : zero

(vi)Polytropic : cv Fnn−−

HG

I

KJ

γ

1 loge^

T

T

2

1

6. Entropy change for an open system

dS ≥ dQT

0

+ Σsi. dmi – Σs 0. dm 0

where, T 0 = Temperature of the surroundings.

Subscripts i and 0 refer to inlet and outlet conditions.

Objective Type Questions

Choose the Correct Answer :

1. Second law of thermodynamics defines
   (a) heat (b) work (c) enthalpy
   (d) entropy (e) internal energy.


2. For a reversible adiabatic process, the change in entropy is
   (a) zero (b) minimum (c) maximum
   (d) infinite (e) unity.


3. For any reversible process, the change in entropy of the system and surroundings is
   (a) zero (b) unity (c) negative
   (d) positive (e) infinite.


4. For any irreversible process the net entropy change is
   (a) zero (b) positive (c) negative
   (d) infinite (e) unity.