Physics Program, Lecture Notes, problems and quesions

1. Sheet-2
Thermodynamics
221PHYS

2. 1 At a constant temperature, an ideal gas is compressed from 6.0 liters to 4.0 liters by
constant external pressure of 5.0 atm. How much work is done on the gas?
w = +10 liter atm (b) w = -10 liter atm (c) w = +30 liter atm (d) w = -30 liter atm
The answer cannot be calculated
Q-2 A system suffers an increase in internal energy of 80 J and at the same time has 50 J of
work done on it. What is the heat change of the system?
(a) +130 J (b) +30 J (c) -130 J (d) -30 J (e) 0 J
Q-3 A refrigerator operates in a cyclic and reversible manner (Carnot cycle). The
temperature inside the refrigerator is 0°C and the room temperature is 27°C. The absorbed
heat from the interior of the refrigerator Q1 = 334.72x103
J . If the coefficient of
performance of the refrigerator, COP= Q1/W .the magnitude of heat Q2 rejected in the
room is,
A) 455.8J B) 310.6 J C) 367.8J D) 340 J e) None

3. Q-5 If the gas in a container absorbs 300 J of heat, has 100 J of work done on it, and then
does 200 J of work on its surroundings, what is the increase in the internal energy of the
gas?
(A) 600 J (B) 400 J (C) 0 J (D) 500 J (E) 200 J
Q-4 A nuclear power plant absorbs heat at a rate of 300 MW from its reactor core and
exhausts heat to the atmosphere at a rate of 200 MW. What is the efficiency of the power
plant?
(A) 0.33 (B) 0.50 (C) 0.67 (D) 1.50 (E) 2.00
Q-6 The efficiency of a Carnot engine is 35.0%. What is the temperature of the cold
reservoir if the temperature of the hot reservoir is 500 K?
A) 175 K B) 325 K C) 269 K D) 231 K
Q-7 A heat engine takes in 200 J of thermal energy and performs 50 J of work in each
cycle. What is its efficiency?
A) 500 % B) 400 % C) 25 % D) 20 % E) 12 %

4. Q-9 If the theoretical efficiency of a Carnot engine is to be 100%, the heat sink must be
(A) at absolute zero. (B) at 0°C. (C) at 100°C. (D) infinitely hot.
Q-8 A Carnot cycle consists of :
(A) two adiabats and two isobars. (B) two isobars and two isotherms.
(C) two isotherms and two isomets. (D) two adiabats and two isotherms.
Q-10 The second law of thermodynamics leads us to conclude that
(A) the total energy of the universe is constant.
(B) disorder in the universe is increasing with the passage of time.
(C) it is theoretically possible to convert heat into work with 100% efficiency.
(D) the average temperature of the universe is increasing with the passage of time.
Q-11 The entropy will usually increase when
I. a molecule is broken into two or more smaller molecules.
II. a reaction occurs that results in an increase in the number of moles of gas.
III. a solid changes to a liquid.
IV. a liquid changes to a gas.
(a) I only (b) II only (c) III only (d) IV only (e) I, II, III, and IV

5. Q-13 Which one of the following thermodynamic quantities is not a state function?(a)
Gibbs free energy (b) enthalpy (c) entropy (d) internal energy (e) work
Q-14 In the equation G = H - TS, the term G stands for
A)Entropy B) the reactants C) enthalpy D) free energy E) the products
Q-15 The energy available to do work in a system is called
A)Entropy B) activation energy C) thermodynamics D) free energy E)heat
Q-12 The law of entropy states that
(A) heat always flows spontaneously from a colder body to a hotter one.
(B) every natural system will tend toward lower entropy.
(C) heat lost by one object must be gained by another.
(D) the specific heat of a substance cannot exceed a certain value.
(E) every natural system will tend toward disorder.

6. Q-16 Which statement is incorrect?
(a) At constant pressure,( H = U + P V)
(b) The thermodynamic symbol for entropy is S.
(c) Gibbs free energy is a state function.
(d) For an endothermic process, H is negative.
(e) If the work done by the system is greater than the heat absorbed by the
system, U is negative.

7. Pb-1 A vessel containing 1.8kg of water is placed on a hot plate. Both the water and hot plate being
initially at . The temperature of the hot plate is raised very slowly to at which point the water
begins to boil. Find of the water during this process?
Sol:
c
100
S∆
c
20

8. Pb-2: A heat source at 800 K loses 2000 kJ of heat to a sink at (a) 500 K and (b) 750 K.
Determine which heat transfer process is more irreversible.

9. Pb-3 A “perfect” heat engine with η = 0.4 is used as a refrigerator (the heat
reservoirs remain the same). How much heat QC can be transferred in one cycle
from the cold reservoir to the hot one if the supplied work in one cycle is W =10 kJ?

10. Pb-4 A household refrigerator, η =4.5, extracts heat from the food chamber at the rate of
(a) how much work per cycle is required to operate the refrigerator?
(b) how much heat per cycle is discharged to the room?
cycleJ /250

11. Pb-5 : A Carnot engine operates between a hot reservoir at 650 K and a cold reservoir at 300
K. If it absorbs 400 J of heat at the hot reservoir, how much work does it deliver?
Th= 650 K
Tc= 300 K
Engine
Qh
Qc
W = ?
= 400 J

12. Pb-6 : The temperature inside the engine of a helicopter is 20000
C, the temperature of the
exhaust gases is 9000
C. The mass of the helicopter is M = 2×103
kg, the heat of
combustion of gasoline is Qcomb = 47×103
kJ/kg, and the density of gasoline is ρ = 0.8
g/cm3
. What is the maximum height that the helicopter can reach by burning V = 1 liter of
gasoline?

13. Pb-7 Consider two engines, the details of which are given in the following diagrams.
For both engines, calculate the heat flow to the cold reservoir and the changes in
entropy of the hot reservoir, cold reservoir and engine. Which engine violates the
Second Law? What is the efficiency of the working engine?

14. The vapor pressure of ethanol is 115 torr at 34.90
C. If ∆Hvap of
ethanol is 40.5 kJ/mol, calculate the temperature (in 0
C) when the
vapor pressure is 760 torr.
Pb-8

15. 1. Derive an expression for the entropy change for ideal
gases?
2. Derive the relationship of the entropy change in an ideal gas ?
3. Derive the maximum efficiency for a Carnot heat engine (show
the cycle diagram)
4. Show the diagram of Stirling engine and drive its efficiency?
5. Derive the The Clapeyron Equation in terms of enthalpy
6. Using diagram show the difference between Carnot engine and
the refrigerator
Derivation questions