Recitation ch 21
Answers at the end
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.
Five moles of an ideal monatomic gas are taken though the cycle
shown in the Figure (1). Calculate the efficiency of the cycle.
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].
b + 0.20 kcal/K
c + 0.82 kcal/K
d - 0.82 kcal/K
e - 0.20 kcal/K
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
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
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
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.
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.
An automobile engine operates with an overall efficiency
of 20%. How many gallons of gasoline is wasted for each
10 gallons burned?
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
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 ?
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.
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