Chapter 15
Thermodynamics
15.1 Thermodynamic Systems and Their Surroundings
Thermodynamics is the branch of physics that is built
upon the fundamental laws that heat and work obey.
The collection of objects on which attention is being
focused is called the system, while everything else
in the environment is called the surroundings.
Walls that permit heat flow are called diathermal walls,
while walls that do not permit heat flow are called
adiabatic walls.
To understand thermodynamics, it is necessary to
describe the state of a system.
15.2 The Zeroth Law of Thermodynamics
Two systems are said to be in thermal
equilibrium if there is no heat flow
between then when they are brought
into contact.
Temperature is the indicator of thermal
equilibrium in the sense that there is no
net flow of heat between two systems
in thermal contact that have the same
temperature.
15.2 The Zeroth Law of Thermodynamics
THE ZEROTH LAW OF THERMODYNAMICS
Two systems individually in thermal equilibrium
with a third system are in thermal equilibrium
with each other.
15.3 The First Law of Thermodynamics
Suppose that a system gains heat Q and that is the only effect occurring.
Consistent with the law of conservation of energy, the internal energy
of the system changes:
Heat is positive when the system gains heat and negative when the system loses heat.
15.3 The First Law of Thermodynamics
If a system does work W on its surroundings and there is no heat flow,
conservation of energy indicates that the internal energy of the system
will decrease:
Work is positive when it is done by the system and negative when it is done
on the system.
15.3 The First Law of Thermodynamics
THE FIRST LAW OF THERMODYNAMICS
The internal energy of a system changes due to heat and work:
Work is positive when it is done by the system and negative when it is done
on the system.
Heat is positive when the system gains heat and negative when the system
loses heat.
15.3 The First Law of Thermodynamics
Example 1 Positive and Negative Work
In part a of figure, the system gains 1500J of heat
and 2200J of work is done by the system on its
surroundings.
In part b, the system also gains 1500J of heat, but
2200J of work is done on the system.
In each case, determine the change in internal energy
of the system.
15.3 The First Law of Thermodynamics
(a)
(b)
15.3 The First Law of Thermodynamics
Example 2 An Ideal Gas
The temperature of three moles of a monatomic ideal gas is reduced
from 540K to 350K as 5500J of heat flows into the gas.
Find (a) the change in internal energy and (b) the work done by the
gas.
15.3 The First Law of Thermodynamics
(a)
(b)
15.4 Thermal Processes
A quasi-static process is one that occurs slowly enough that a uniform
temperature and pressure exist throughout all regions of the system at all
times.
isobaric: constant pressure
isochoric: constant volume
isothermal: constant temperature
adiabatic: no transfer of heat
15.4 Thermal Processes
An isobaric process is one that occurs at
constant pressure.
Isobaric process:
15.4 Thermal Processes
Example 3 Isobaric Expansion of Water
One gram of water is placed in the cylinder and
the pressure is maintained at 2.0x105Pa. The
temperature of the water is raised by 31oC. The
water is in the liquid phase and expands by the
small amount of 1.0x10-8m3.
Find the work done and the change in internal
energy.
15.4 Thermal Processes
15.4 Thermal Processes
15.4 Thermal Processes
isochoric: constant volume
15.4 Thermal Processes
Example 4 Work and the Area Under a
Pressure-Volume Graph
Determine the work for the process in
which the pressure, volume, and temp-
erature of a gas are changed along the
straight line in the figure.
The area under a pressure-volume graph is
the work for any kind of process.
15.4 Thermal Processes
Since the volume increases, the work
is positive.
Estimate that there are 8.9 colored
squares in the drawing.
15.5 Thermal Processes Using an Ideal Gas
ISOTHERMAL EXPANSION OR COMPRESSION
Isothermal
expansion or
compression of
an ideal gas
15.5 Thermal Processes Using and Ideal Gas
Example 5 Isothermal Expansion of an Ideal Gas
Two moles of the monatomic gas argon expand isothermally at 298K
from an initial volume of 0.025m3 to a final volume of 0.050m3. Assuming
that argon is an ideal gas, find (a) the work done by the gas, (b) the
change in internal energy of the gas, and (c) the heat supplied to the
gas.
15.5 Thermal Processes Using and Ideal Gas
(a)
(b)
(c)
15.5 Thermal Processes Using and Ideal Gas
ADIABATIC EXPANSION OR COMPRESSION
Adiabatic
expansion or
compression of
a monatomic
ideal gas
Adiabatic
expansion or
compression of
a monatomic
ideal gas
15.7 The Second Law of Thermodynamics
THE SECOND LAW OF THERMODYNAMICS: THE HEAT FLOW STATEMENT
Heat flows spontaneously from a substance at a higher temperature to a substance
at a lower temperature and does not flow spontaneously in the reverse direction.
The second law is a statement about the natural tendency of heat to
flow from hot to cold, whereas the first law deals with energy conservation
and focuses on both heat and work.
15.12 The Third Law of Thermodynamics
THE THIRD LAW OF THERMODYNAMICS
It is not possible to lower the temperature of any system to absolute
zero in a finite number of steps.