Problems and Solutions on Thermodynamics and Statistical Mechanics

40 Problem Ec Solutions on Thermafyamics €4 Statistical Mechunics

(c) When the air conditioner works 30% of the time, we know from (b)

When it operates continuously, we have

100 100

p = pSon. 30 239

= A. -. - w 1.13774.

With T2 = 20°C = 293K, we get

Tl = T2 + = 293 +

= 293 + 18.26 K = 38.26"C.

Id) When the cycle is reversed in winter, we have 92 = P + Q1 and

• Qz = ' Qi -. At equilibrium, Qz = A(T2 - Ti)) SO that
  T2 Tl

Thus Ti = T2 - -T2 =^293 - (1.14 x 293)'/2 = 275K = 2°C.

K

1044

Calculate the change of entropy involved in heating a gram-atomic

weight of silver at constant volume from 0" to 30°C. The value of C, over

this temperature may be taken as a constant equal to 5.85 cal/deg.mole.

( wis ca nsin)

Solution:

The change of entropy is

7'2 30 + 273

273

• = nC, In - = 5.851n
  as = n6 cgT Ti
  = 0.61 cal/K.