Problems and Solutions on Thermodynamics and Statistical Mechanics

44 Problem EI Solutiow on Thermcddynamics EI Statistical Mechanics

1-2: Adiabatic compression. The entropy is conserved.

2-3: Isothermal compression. If the working matter is an ideal gas, the

3-4: Adiabatic expansion. The entropy is conserved.

4-1: Isothermal expansion. The enthalpy is conserved.

The refrigeration efficiency is

enthalpy is conserved.

Hence

TI - T2

T2

W = 92~.

Q2 = ML is the latent heat for M = 3 kg of water at T = 0°C to become

ice. As

L = 3.35 x lo5 J/kg ,

we find W = 73.4 x lo3 J.

1049

n = 0.081 kmol of He gas initially at 27°C and pressure = 2 x 105N/m2

is taken over the path A -+ B ---t C. For He

C, = 3R/2 , C, = 5R/2.

Assume the ideal gas law.

from A -+ B?

(a) How much work does the gas do in expanding at constant pressure

(b) What is the change in thermal or internal energy of the helium

(c) How much heat is absorbed in going from A -+ B?

(d) If B -+ C is adiabatic, what is the entropy change and what is the

from A -+ B?

final pressure?

Solution:

( was co nsin)

(a) For A B, the external work is

W = PA(VB - VA) = 1.0 x lo5 J.