SCIEDP 204
st
The 1 LAW of
THERMODYNAMICS
CHAPTER 3
THERMODYNAMIC SYSTEMS
-> any collection of objects that is convenient to
regard as a unit, and that may have potential to
exchange energy with its surroundings
THERMODYNAMIC PROCESS
-> a process where there are changes in the state of
a thermodynamic system
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THERMODYNAMIC SYSTEMS
Signs for Heat and Work in Thermodynamics
+ Q Heat is absorbed by (or entering) the system
− Q Heat is released by (or leaving) the system
+ W Work is done BY the system ON the surroundings
− W Work is done ON the system BY the surroundings
Work Done During Volume Changes
V2
W =  pdV
V1
W = p(V2 − V1 )
Work done in a volume
change at constant
pressure
Paths Between Thermodynamic States
WORK DONE in a THERMODYNAMIC PROCESS -> depends on the path between two states
Paths Between Thermodynamic States
HEAT ADDED in a THERMODYNAMIC PROCESS
-> also depends on the path between the initial and final state
Free
expansion
1 st LAW of THERMODYNAMICS
INTERNAL ENERGY (U)
-> sum of the kinetic energies of all the constituent particles, plus the sum of all the
potential energies of interaction among these particles
FIRST LAW OF THERMODYNAMICS
U = Q − W
-> The change in internal energy of a system is the difference between the heat
absorbed by the system and work done by the system
-> generalization of the principle of conservation of energy
-> depends only on temperature dU = nCvdT
dU = dQ − dW
nCV dT = nC P dT + pdV
1 st LAW of THERMODYNAMICS
-> the change in internal
energy of a system does
not depend on the path,
but only on the initial
and final state of the
system
1 st LAW of THERMODYNAMICS
CYCLIC PROCESS -> a process that eventually returns a system to its initial state
U = 0
Q =W
ISOLATED SYSTEM
-> a system where no work and no
heat flows to or from the
surroundings
U = 0
Q =W = 0
1 st LAW of THERMODYNAMICS
Example: Working off your dessert
You propose to eat a 900 calorie hot fudge sundae (w/ whipped cream) and then run up
several flights of stairs to work off the energy you have to take in. how high do you have
to climb? Assume that your mass is 60kg.
Example: Thermodynamics of Boiling Water
One gram of water (1cm3) becomes 1671 cm3 of steam when boiled at a constant pressure
of 1 atm. The heat of vaporization of water at this pressure is Lv=2.256x106J/kg.
Compute (a) the work done by the water when it vaporizes and (b) its increase in internal
energy.
Example:
Five moles of an ideal monatomic gas with an initial temperature of 127oC expand and in
the process absorbs 1200 J of heat and do 2100J of work. What is the final temperature
of the gas?
Kinds of THERMODYNAMIC PROCESSES
ADIABATIC PROCESS
-> no heat transfer into or out of a system, Q=0
U = −W
ISOCHORIC PROCESS
-> constant volume process, W=0
U = Q
Q = nCV T
Molar heat capacity at constant volume
Kinds of THERMODYNAMIC PROCESSES
ISOBARIC PROCESS
-> constant pressure process, W=p(V2-V1)
U = Q − W
Q = nC p T
Molar heat capacity at constant pressure
ISOTHERMAL PROCESS
-> constant temperature process, ΔU=0
Q =W
-> cyclic process and isolated systems
Kinds of THERMODYNAMIC PROCESSES
Kinds of THERMODYNAMIC PROCESSES
PROCESS
PATH NAME
ADIABATIC
ADIABAT
ISOCHORIC
ISOCHOR
ISOBARIC
ISOBAR
ISOTHERMAL
ISOTHERM
-> adiabatic, isobaric and isothermal
processes can undergo expansion or
compression
Kinds of THERMODYNAMIC PROCESSES
Example: Comparing thermodynamic processes
A series of thermodynamic processes is shown
in the pV-diagram. In process a->b, 150 J of
heat is added to the system, and in process
b-> d, 600 J of heat is added. Find (a) the
internal energy change of the system in process
a->b. (b) the internal energy change in a->b-> d;
and (c) the total heat added in the process
a->c-> d.
Kinds of THERMODYNAMIC PROCESSES
Example:
In an experiment to simulate conditions inside an automobile engine, 0.185 mol of air at
a temperature of 780 K and pressure of 3x106Pa is contained in a cylinder of volume
40cm3. Then 645 J of heat is transferred to the cylinder. (a) If the volume of the
cylinder is constant while the heat is added, what is the final temperature of the air?
Assume the air is essentially nitrogen gas. Draw a pV diagram for this process. (b) If
instead the volume of the cylinder is allowed to increase while the pressure remains
constant, find the final temperature of the air. Draw a pV diagram for this process.
Adiabatic Process for an Ideal Gas
 −1
1 1
TV
= T2V2
 −1

1 1
p V = p2V2

W = nCV (T1 − T2 )
1
( p1V1 − p2V2 )
=
 −1
Ratio of heat capacities
Adiabatic Process for an Ideal Gas
Example: Adiabatic compression in a diesel engine
The compression ratio of a diesel engine is 15 to 1;
this means that air in the cylinders is compressed
to 1/15 of its original volume. (a) If the initial
pressure is 1 atm and the initial temperature is
27oC, find the final temperature and pressure after
compression. (b) How much does the gas do during
the compression if the initial volume of the cylinder
is 1L?
EXERCISES
1.
One half mole of an ideal gas is taken from state a to
state c as shown in the figure. (a) calculate the final
temperature of the gas. (b) Calculate the work done on (or
by) the gas as it moves from state a to state c. (c) does
heat leave the system or enter the system during this
process? Hom much heat?
2.
When a system is taken from state a to state b in the figure along
the path acb, 90 J of heat flows into the system and 60J of work
is done by the system. (a) How much heat flows into the system
along the path adb if the work done by the system is 15J?
(b) When the system is returned from b to a along the curved
path, the absolute value of the work done by the system is 35J.
Does the system absorb or liberate heat? How much heat?
(c) If Ua=0 and Ud=8J, find the heat absorbed in the process ad
and db.
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EXERCISES
3.
A thermodynamic system is taken from state a to state c in
the figure along either path abc or path adc. Along path abc,
the work done by the system is 450 J. Along path adc, W is
120 J.The internal energies of each of the four states shown
are Ua=150J, Ub=240J, Uc=680J and Ud=330J. Calculate the
heat flow for each of the four processes ab, bc, cd and dc.
In each process, does the system absorb or liberate heat?
4.
Three moles of an ideal monatomic gas expands (by heating) at constant pressure of 2.50 atm
from a volume of 3.20x10-2m3 to 4.50x10-2m3. (a) Calculate the initial and final temperature of
the gas. (b) Calculate the amount of work the gas does in expanding. (c) Calculate the amount
of heat added to the gas. (d) Calculate the change in internal energy of the gas.
5.
During an adiabatic expansion, the temperature of 0.450 mol of argon drops from 50oC to 10oC.
The argon may be treated as an ideal gas. (a) Draw a pV-diagram for this process. (b) How
much work does the gas do? (c) What is the change in internal energy of the gas?
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