 What is the Second Law of Thermodynamics and how does it affect my life?
 What is a Heat Engine?
 What is its Efficiency?
 How about a Steam Engine? ...an Internal Combustion
Engine?
 What is the Otto Cycle?
 The Carnot Cycle, why is it so important?
 What about Refrigerators?...Heat Pumps?...Air Conditioners?
What is Entropy and how is it related to The Second Law?
How is it related to Order and Disorder?
How is it related to Probability?
20-1 The Second Law of Thermodynamics - Introduction
20-2 Heat Engines
20-3 Reversible and Irreversible Processes; the Carnot Engine
20-4 Refrigerators, Air Conditioners, and Heat Pumps
20-5 Entropy
20-6 Entropy and the Second Law of Thermodynamics
20-7 Order to Disorder
20-8 Unavailability of Energy; Heat Death
*20-9 Statistical Interpretation of Entropy and the Second Law
*20-10 Thermodynamic Temperature Scale; Absolute Zero and the
Third Law of Thermodynamics
*20-11 Thermal Pollution, Global Warning, and Energy Resources
Second Law of Thermodynamics
Heat Engines
Operating temperature
Working substance
Efficiency
The Kelvin-Planck statement
Reversible and Irreversible processes
The Carnot engine

Refrigerators, Air Conditioners, and Heat Pumps
Coefficient of performance
The Clausius statement
SEER rating
Entropy
Times Arrow
Order to disorder
Biological evolution
Heat death
Statistical interpretation of entropy
Thermodynamic probability
Thermodynamic Temperature Scale
The Third Law of Thermodynamics
Absolute zero
Thermal Pollution
Global Warming
Energy Resources
This chapter is the last of the four chapters on thermal physics. The spontaneous transfer of energy, the concept of entropy, and the limits on the ability to use the energy of a system to do useful work are presented using the second law of thermodynamics. The chapter concludes with optional sections on applications of thermodynamics to current topics.
An engine changes heat into work. In the engine of a car the heat is generated by the combustion of an air-gas mixture. The heat is used to expand the gas, and lift the piston. Although work can be completely changed into heat, the reverse is not true (second law of
thermodynamics). The first form of the second law is:
" It is not possible to completely change heat into work with no other change taking place "

Let us look at an ideal gas confined by a piston in a volume V i
. The gas is in contact with a heat reservoir with temperature T. The gas is permitted to expand by removing some weight of the piston. Its volume will increase while its temperature remains constant. Since the internal energy of the gas only depends on its temperature, the internal energy of the gas will not change. From the first law of thermodynamics we conclude that
This shows that we have converted all extracted heat into work. However, in the process we have also changed the state of the gas. Its volume and pressure have changed. In order to return the gas to its internal state we need to do work and extract heat from the system. In that case, the net work and heat will be zero.
The basic operation cycles of an engine consist of extracting heat from a reservoir, performing work, and supplying heat to another reservoir (see Figure 20.1). After one complete cycle the gas in the system returns to its original state. Since it has the same temperature at the beginning and at the end of the cycle, its internal energy did not change. The first law of thermodynamics tells us that the net work done per cycle therefore must equal the net heat transferred per cycle:
A measure of the performance of an engine is its thermal efficiency e, which is defined as the ratio of the work done per cycle and the heat absorbed per cycle

A 100% efficiency will be obtained if and only if Q
C
= 0. However, the second law of thermodynamics clearly states that it is impossible to change heat completely into work. Thus, there are no perfect engines.
Figure 20.1. The engine.
A refrigerator is a device that causes heat to flow from a cold place to a warm place. Another formulation of the second law of thermodynamics states that
" It is not possible for heat to flow from one body to another body at a higher temperature
with no other change taking place "
The performance of a refrigerator is specified by the coefficient of performance K
An example of a series of processes which would create a reversible engine is the carnot cycle. One full cycle consists of two isothermal and two adiabatic processes. The initial situation is

specified by a volume V a
, a pressure p a
, and a temperature T a
. The gas is confined in a cylinder by a piston which has weight on top.
Step 1: the system is in contact with a high temperature reservoir. Thus, T a
= T
H
. Part of the weight of the piston is removed. As a result the gas will expand; its volume will increase from V a
to V b
and its pressure will decrease. During this process a total heat equal to Q
H
is extracted from the reservoir. Since the temperature of the gas is kept constant at T
H
all the heat extracted from the reservoir is transformed into work
Step 2: the cylinder is removed from the high temperature reservoir and put into an insulating stand. Some more weight is removed from the piston. As a consequence, the gas will further expand, and since there is no heat transfer during the expansion this is adiabatic expansion. During the expansion the temperature drops to T
C
and the volume of the gas increases from V b
to V c
. In chapter
21 we derived the following relation between temperature and volume for adiabatic processes
Thus gives the following relation between the initial and final temperature and volume
Step 3: The cylinder is put in contact with a colder heat reservoir and weight is added to the piston. As a result, the gas is slowly compressed (and the volume is decreased from V c
to
V d
). During this compression, heat is transferred from the gas to the colder reservoir. Since there is no change in the temperature of the gas, its internal energy will not change. Therefore, during the

isothermal compression the heat transferred to the colder heat reservoir is equal to the work done on the gas
Step 4: the cylinder is put in an insulating stand and more weight is added to the piston. The gas is compressed and its volume is decreased from V d
to V a
. The temperature of the gas increases from
T
C
to T
H
. Since there is no transfer of heat during the compression, the process is adiabatic. This means that
Thus gives the following relation between the initial and final temperature and volume
This relation can be rewritten as
A similar relation can be obtained for the parameters involved in the adiabatic expansion of the gas
Combining these two equations for T
H
/T
C
we conclude that or

The heat extracted from the hot reservoir is given by
The heat delivered to the cold reservoir is given by
Thus,
The efficiency of the cannot engine is therefore given by
No real engine operating between two specified temperatures can have a greater efficiency than that of a carnot engine operating between the same two temperatures.