SOLUTIONS TO EXERCISE 2


(a) One liter of water has a mass of 1 kg.  The freezer must first remove the
heat stored in the liquid water to reduce its temperature to 0 deg C, and then 
remove still more heat to decrease the entropy in the liquid water until it can
change its phase from liquid to solid.

  dU = mCdT + (mass * heat of fusion)

     = (1000 g * 1 cal/g/deg K * (20-0) deg K)  + (1000 g * 80 cal/g)

     = 20000 cal + 80000 cal  = 100000 cal * 4.2 J/cal = 4.2 * 10^5 J.

(b) The stove must add heat to the liquid water to increase its temperature to 
100 deg C, and then add still more heat to increase the entropy in the water
until the water changes phase from liquid to solid.

  dU = mCdT + (mass * heat of vaporization)

     = (1000 g * 1 cal/g/deg K * (100-20) deg K) + (1000 g * 540 cal/g)

     = 80000 cal + 540,000 cal = 620,000 cal * 4.2 J/cal = 2.6 * 10^6 J.


(c) This answer is left as an exercise to the reader.