2. The first law of thermodynamics is an extension of the law of conservation of
“The change in internal energy of a system is equal to the difference between heat
added to the system and the work done by the system”
The first law of thermodynamics
the internal energy of a system can
be changed by doing work on it or by
From the microscopic point of view, this
statement is equivalent to a statement of
conservation of energy.
Note: The first law of thermodynamics tells us that in order to change the internal energy of a
system we must add (or remove) heat and/or do work on (or have work done by) the system.
ΔU = Q - W
3. +=Change in internal
Energy supplied to
system as heat
Energy supplied to
system as work
U= Q (heat) + w (work)
Or we can say….
U like reserves of a bank: bank accepts deposits
or withdrawals in two currencies (Q & w) but
stores them as common fund, U.
U– Change in internal
Q – Heat added or lost
by the system [J]
W – Work done on or
by the system [J]
4. Heat and work are forms of energy transfer and energy is conserved.
The First Law of Thermodynamics, In general :
U = Q + Won
on the system
U = Q - Wby
State Function Path Functions
statement of energy conservation for a thermodynamic system
The internal energy of a system tends to increase if energy is added
via heat (Q) and decrease via work (W) done by the system.
The signs on Q and W depend
on the way the internal
energy is changed. If Q and W
are positive the internal
energy increases, but if Q and
W are negative the internal
5. heat is random molecular motion while work is force times distanced moved
under its influence
Exothermic Processes release heat and have Q<0
Endothermic Processes absorb heat and have Q>0
Energy: The SI unit is joule (J) although we will frequently use calorie ;
1 cal = 4.2 J
The work is not necessarily associated with the volume changes – e.g.,
in the Joule’s experiments on determining the “mechanical equivalent of
heat”, the system (water) was heated by stirring.
ΔU = Ufinal - Uinital = Q – W
From the first Law of Thermodynamics: Energy is Conserved
Q = heat absorbed by the system from the surroundings
W = work done by the system on the surroundings
6. Special Cases of the 1st Law of Thermodynamics
Isolated System The system doesn’t interact with the environment.
What does this mean in terms of the 1st law of thermodynamics?
Q = 0 -------- No heat is transferred into or out of the system.
W = 0 ------- No work is done on the system.
U Q W
0U i fU U
Cyclic Process The system starts and ends in the same state (same internal energy).
The system is not necessarily isolated.
The function that describes the changes in the state on a PV - diagram would be a
U = 0 ------- No net change in the internal energy.
U Q W
WQ 0 WQ
7. Adiabatic Process
This process considers a system where there is no
loss or gain of energy through heat.
This can be accomplished by:
1. Thermally insulating the chamber
2. Performing the process very rapidly
– no time for heat to be transferred.
• Expansion of hot gases in an internal combustion engine
• Liquefaction of gases in a cooling system
Adiabatic Free Expansion This is a special case of an adiabatic process,
where the gas expands into free space.
U Q W
U Q W
If we do work to compress the gas the
temperature of the gas should increase.
The increase in the temperature of the gas
corresponds to an increase in the internal
energy of the system.
Applications of 1st law of thermodynamic
8. Isobaric Process Constant pressure process.
U Q W 0Q 0W if VVPW
Isovolumetric (Isochoric) Process Constant volume process.
U Q W 0Q 0W
If volume doesn’t change work cannot be done to compress the gas.
9. Isothermal Process
U Q W 0U WQ
Constant temperature process.
If you do work to compress a gas the energy you put in is released through heat.
On a PV – diagram it is common to use isotherms to show how the
temperature changes for a process.
Isotherm – Hyperbolic line of constant temperature on a PV – diagram.
W = F y
W= PA y
V = A y
W = P V
Work in Thermodynamic Processes
During a compression:
Work done on a gas is
As stated previously, pressure, temperature and volume are
considered state variables and are used to define the particular
state of the system.
Work and Heat are called transfer variables. These describe
changes in the state. They do not describe the state. We know
how to describe the work done on a system.
For example let us look at applying a force to a piston in order to
compress the gas inside a container.
11. External force is equal and
opposite to force gas exerts on
piston. Work done on gas!
dW F d s
PdVW -F is parallel to y
Total Work done to change the
volume of a gas
If the gas is compressed slowly enough for all
of the system to remain in thermal equilibrium.
Work is the transfer of energy that takes
place when an object is moved against an
The sign: if the volume is decreased, W is positive
(by compressing gas, we increase its internal
if the volume is increased, W is negative (the gas
decreases its internal energy by doing some work
on the environment).
VW P dV
The work done by an external force to compress a
gas enclosed within a cylinder fitted with a piston:
W = (PA) dx = P (Adx) = - PdV
W = PdV
applies to any
boundary dU = Q – PdV
A – the
piston area force
13. Work Done by an Expanding Gas
Gas expands slowly enough to maintain thermodynamic equilibrium.
+dV Positive Work (Work is
done by the gas)
-dV Negative Work (Work is
done on the gas)
Energy leaves the system
and goes to the environment.
Energy enters the system
from the environment.
PdVW Total Work done to change
the volume of a gas
14. What type of process is
described by each of the
This is a PV – diagram showing
15. A PV-Diagram is a plot of pressure vs. volume.
The work done during the process shown by the PV-diagram
can be determined by looking at the area under the curve.
Remember this is the same as the integral expression for the
Example: In the three figures shown (a), (b) and (c), rank the
amount of work done by each of the processes shown from
largest to smallest. How much work is done in each case?
(b) > (c) > (a)
The amount of work done during a process depends
on the path you take from your initial point to your
final point. In other words it depends on how you
change your pressure and volume!
(a) W=-Pi(Vf-Vi) (b) W=-Pf(Vf-Vi) (c)
16. Shaded area is the work done by the system
P depends on V in general
to take the
i to f.
The work W
done and Q
17. W and Q are not State Functions
The work is negative for the “clockwise” cycle; if
the cyclic process were carried out in the reverse
order (counterclockwise), the net work done on the
gas would be positive.
We can bring the system from state 1 to state 2 along infinite
number of paths, and for each path P (T,V ) will be different.
U is a state function, W - is not
Q is not a state function either.
U = Q + W
Since the work done on a system depends not only on the initial and final
states, but also on the intermediate states, it is not a state function.
21. Change of State: implies one or more properties of the system has
How these properties would change outside of time is curiously
outside the framework of equilibrium thermodynamics!
The best way to think of them is that the changes are slow relative
to the underlying molecular time scales.
Processes and cycles
22. • Process: is a succession of changes of state.
Assuming processes are all sufficiently slow such that each stage of
the process is near equilibrium.
Certain common processes are isos, meaning “equal”:
A. Adiabatic: no heat transferred
B. Isothermal: constant temperature,
C. Isobaric: constant pressure, and
D. Isochoric: constant volume.
An important notion in thermodynamics is that of a
• Cycle: series of processes which returns to the original state.
The cycle is a thermodynamic “round trip.”
24. A. Adiabatic Process
• An adiabatic process transfers no heat
ΔU = Q – W
– But Q = 0 , → ΔU = – W
• When a system expands adiabatically, W is positive (the system does
work) so ΔU is negative.
• When a system compresses adiabatically, W is negative (work is done
on the system) so ΔU is positive.
Adiabatic processes can occur when the system is well insulated or a very
rapid process occurs so that there is not enough time for a significant heat
to be transferred (e.g., rapid expansion of a gas; a series of compressions
and expansions as a sound wave propagates through air).
25. •For an ideal gas, and most real gasses,
•dQ = dU + PdV
•dQ = CVdT + PdV,.
•Then, when dQ = 0,
26. Adiabatic Processes
Remember: In adiabatic process there is no thermal energy transfer to or
from a system (Q = 0)
A reversible adiabatic process involves a “worked” expansion in which we can
return all of the energy transferred.
A Polytropic process is a thermodynamic process that
obeys the relation:
Atmospheric processes which lead to changes
in atmospheric pressure often adiabatic: HIGH
pressure cell, falling air is compressed and
warmed. LOW pressure cell, rising air expands
and cooled condensation and rain.
A Polytropic process
27. 2 2
2 1 2 1 1 2
p 2 1
2 21 1 2 2 2 1
1 2 1 1 1
; ln( / ) ln( / ) ln ( / )
T Vv v
U W nC dT P dV dV
dT R dV R
T T V V V V
T C V C
C T VR
R C C
C C T V
P VP V T PV V
T T T PV V
cons an, t t
Q = 0Adiabatic Processes, Derivation
3 5 5
Monatomic: ; ;
2 2 3
5 7 7
Diatomic: ; ;
2 2 5
Adiabatic expansion of a perfect gas
For a reversible
CVdT = -pdV
along the path.
Now, per mole,
for an ideal
gas, PV = RT
28. Thermo & Stat Mech - Spring 2006 Class 3
PV PdV VdP
PdV PdV VdP
For an ideal gas, PV=nRT, so
PdV PdV VdP
nR C VdP
C nR nR
29. Thermo & Stat Mech - Spring 2006 Class 3 29
PdV VdP nR C C
PdV VdP PdV VdP
0, which can be integrated,
ln ln constant
ln ln ln constant
V P PV
30. Thermo & Stat Mech - Spring 2006 Class 3 30
for “Ideal Gasses”
monatomic: 1 1.67
diatomic: 1 1.40
polyatomic: 1 1.33
31. Adiabatic Process in an Ideal Gas
work in adiabatic process 021 Q
1 2 ( , )
W P V T dV dV
1+2/31.67 (monatomic), 1+2/5 =1.4 (diatomic),
and 1+2/6 1.33 (polyatomic)
(again, neglecting the vibrational degrees of freedom)
1 1 1
P V V
32. B. Isothermal Process
• An isothermal process is a constant temperature process.
• Any heat flow into or out of the system must be slow
enough to maintain thermal equilibrium
• For ideal gases, if ΔT is zero, ΔU = 0
• Therefore, Q = W
– Any energy entering the system (Q) must leave as work
34. Isothermal change T = 0
U = 0, PV = n R T
2 1 1
1 1 2 2
1 2 2
2 2 2
1 1 1
Q W PdV dV nRT
V P P
PV P V Q W n RT
V P P
V V PW
e and e
V nRT V P
Boyle’s Law (1627 -1691)
T1= T2 ------- P1V1 = P2 V2
35. c. Isochoric Process
• An isochoric process is a constant volume process. When the volume of a
system doesn’t change, it will do no work on its surroundings. W = 0
ΔU = Q
Heating gas in a closed container is an isochoric process
isochoric ( V = const )
021 W TCTTNkQ VB 0
36. D. Isobaric Process
• An isobaric process is a constant pressure process.
• ΔU, W, and Q are generally non-zero, but calculating the work done by an
ideal gas is straightforward
W = P·ΔV
Water boiling in a saucepan is an example of an isobar process
1 2 2 1
0W PdV P V V
TCTTNkQ PB 0
37. Isobaric Process : constant P
2 1W P V V P V
Q U W U P V
isobaric processesPQ n C T
CP = molar specific heat at constant pressure
P Vn C T n C T P V Ideal gas, isobaric :
Vn C T n R T
P VC C R Ideal gas
Isobaric Processes & Specific Heat
38. Summary, Workand ideal gases
W P dV P V V
W PdV nRT nRT
𝑊 = 0 Isochoric
1 1 1
1 2 2
W P V T dV dV
P V V
39. The heat Qp absorbed in a constant-pressure
process equals the system’s enthalpy change.
2 1 2 1
2 2 1 1 2 1
( ) ( )
constant P, closed system, P-V work only
p pV V
H U PV
U U Q W Q PdV Q P dV Q P V V
Q U PV U PV H H
It is a thermodynamic quantity equivalent to the total heat
content of a system.
Δ Δ Δ Δ Δ
heat capacity at constant pressure
heat capacity at constant volume
TP P p P
TV V V V
H nC T E U nC T
C ( ) ΔH Q C dT
C ( ) ΔU Q C dT
C -C R
The universal gas constant
42. EXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction) that releases
A release of heat corresponds to a decrease in enthalpy.
Exothermic process: H < 0 (at constant pressure).
Endothermic process: a change (e.g. a chemical reaction) that requires
(or absorbs) heat.
Absorption of heat corresponds to an increase in enthalpy.
Endothermic process: H > 0 (at constant pressure).
e.g: Evaporation, fusion, melting of solids
e.g: Condensation, crystallization of liquids
Energy has to be supplied to a liquid to enable it to overcome forces that hold
molecules together (Endothermic process (H positive))
Energy is supplied to a solid to enable it to vibrate more vigorously until molecules
can move past each other and flow as a liquid-- Endothermic process (H
Liquid releases energy and allows molecules to settle into a lower energy state and
form a solid --Exothermic process (H negative)
(we remove heat from water when making ice in freezer)
44. Changes in Thermal Systems
Example of a Reversible Process:
Cylinder must be pulled or pushed slowly enough that
the system remains in thermal equilibrium
45. Changes in Thermal Systems
Example of an Irreversible Process:
The gas expands freely when
the valve is opened.