Recitation ch 21
Answers at the end

Question 1
A heat engine operates between 600 K and 300 K. In each cycle
it takes 100 J from the hot reservoir, loses 25 J to the cold
reservoir, and does 75 J of work. This heat engine violates:

      a   The second law but not the first law of thermodynamics.
      b   The first law but not the second law of the thermodynamics.
      c   Both, the first law and the second law of thermodynamics.
      d   Conservation of energy.
      e   Neither the first law nor the second law.
Question 2
Five moles of an ideal monatomic gas are taken though the cycle
shown in the Figure (1). Calculate the efficiency of the cycle.

      a  0.17.
      b  0.06.
      c  0.28.
      d  0.45.
      e  0.83.
Question 3
A 4.0-kg piece of iron at 800 K is dropped into a lake
whose temperature is 280 K. Assume that the lake is so
large that its temperature rise is negligible.
Find the change in the entropy of the lake.
[specific heat of iron = 0.11 kcal/kg.K].

      a   zero
      b   + 0.20 kcal/K
      c   + 0.82 kcal/K
      d   - 0.82 kcal/K
      e   - 0.20 kcal/K
Question 4
50.0 g of water at 15.0 degrees-C are converted slowly
into ice at - 15.0 degrees-C.
What is the change of entropy of water ?
specific heat of water = 4190 J/kg.K.
specific heat of ice = 2220 J/kg.K.
Lf = 333kJ/kg, Lv = 2256kJ/kg

      a   + 78.5 J/K
      b   - 78.5 J/K
      c   + 83.4 J/K
      d   - 17.5 J/K
      e   - 83.4 J/K
Question 5
A Carnot refrigerator has a coefficient of performance
equal to 5. The refrigerator absorbs 120 J of heat from a
cold reservoir in each cycle.
How much heat is expelled to the hot reservoir ?

      a   600 J
      b   480 J
      c   144 J
      d   720 J
      e   125 J
Question 6
A Carnot heat engine operates between two reservoirs
whose temperatures are 27 degrees-C and 127 degrees-C.
If we want to double the efficiency of the heat engine,
what should be the temperature of the hot reservoir?
Assume the temperature of the cold reservoir is kept

      a   800 K
      b   600 K
      c   1500 K
      d   1200 K
      e   900 K
Question 7
A sample of an ideal monatomic gas undergoes
the reversible process A to B displayed in the
T-S diagram shown in figure 6. The process is :

      a   an isothermal compression.
      b   a change of phase.
      c   a free expansion.
      d   a constant-volume process.
      e   an isothermal expansion.
Question 8
Five moles of an ideal gas undergo a reversible isothermal
compression from volume V to volume V/2 at temperature
30 degrees C. What is the change in the entropy of the gas?

      a   -18 J/K.
      b   18 J/K.
      c   -29 J/K.
      d   29 J/K.
      e   -81 J/K.
Question 9
An automobile engine operates with an overall efficiency
of 20%. How many gallons of gasoline is wasted for each
10 gallons burned?

      a   6.
      b   12.
      c   10.
      d   2.
      e   8.
Question 10
A Carnot heat engine absorbs 70.0 kJ as heat and expels
55.0 kJ as heat in each cycle. If the low-temperature
reservoir is at 120 degrees-C, find the temperature of
the high-temperature reservoir.
      a   35.8 degrees-C
      b   153 degrees-C
      c   227 degrees-C
      d   450 degrees-C
      e   393 degrees-C
Question 11
One mole of an ideal gas is taken through the reversible
cycle shown in figure 1, with Q1 = 6.0 kJ, Q2 = 30 kJ,
Q3 = 18 kJ and Q4 = 10 kJ.
What is the efficiency of this cycle ?

      a   0.54
      b   0.29
      c   0.33
      d   0.22
      e   0.44
Question 12
240 grams of water at 8 degrees-C are cooled to ice at
at - 5 degrees-C. Calculate the change in entropy of the
c(ice)=2090 J/kg*K, c(water)=4186 J/kg*K, Lf=3.33*10**(5) J/Kg.

      a   254 J/K.
      b  -172 J/K.
      c  -254 J/K.
      d  -331 J/K.
      e   331 J/K.
Question 13
Which one of the following statements is WRONG?

      a  A refrigerator works like a heat engine in reverse.
      b  No heat engine has higher efficiency than Carnot efficiency.
      c  Thermal energy cannot be transferred spontaneously from a
        cold object to a hot object.
      d  After a system has gone through a reversible cyclic process,
        its total entropy does not change.
      e  The total entropy of a system increases only if it absorbs

1     a
2     a
3     c
4     b
5     c
6     b
7     e
8     c
9     e
10     c
11     d
12     d
13     e