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Thermodynamics
•The First Law of Thermodynamics
•Thermodynamic Processes (isobaric, isochoric, isothermal, adiabatic)
Thermodynamic
•Reversible and Irreversible Processes
•Heat Engines
•Refrigerators and Heat Pumps
•The Carnot Cycle
•Entropy (The Second Law of Thermodynamics)
•The Third Law of Thermodynamics

1

The Zeroth Law of Thermodynamics
If A is in thermal
equilibrium with C and B in
thermal equilibrium with C
th
l
ilib i
ith
then A and B have to be in
thermal equilibrium
...
wikipedia
...


In thermodynamics the internal energy of a
thermodynamics,
thermodynamic system, or a body with well-defined
boundaries, denoted by U, or sometimes E, is the total of
the kinetic energy due to the motion of molecules
(translational, rotational, vibrational) and the potential
energy associated with the vibrational and electric energy
of atoms within molecules or crystals
...


3

The First Law of Thermodynamics 
The First Law of Thermodynamics
The first law of thermodynamics says the change in
internal energy of a system is equal to the heat flow into the
system plus the work done on the system
...

ΔU= Uf ‐ Ui = Q ‐ W
Where:
Uf = internal energy of system @ end
Ui = internal energy of system @ start
i
l
f
@
Q = net thermal energy flowing into 
y
gp
system during process
Positive when system gains heat
Negative when system loses heat
W = net work done by the system
W net work done by the system
Positive when work done by the system
Negative when work done on the system

5

Thermodynamic Processes
Thermodynamic Processes
A state variable describes the state of a system at time t,
but it does not reveal how the system was put into that
state
...

Thermal processes can change the state of a system
...

In other words: All processes are reversible
(Reversible means that it is possible to return system and
surroundings to the initial states)
REALITY: irreversible
6

“Humpty Dumpty sat on a wall
wall
...


The area under a PV curve
gives the magnitude of the
work done on a system
...


8

To go from the state (Vi, Pi) by the path (a) to the state (Vf,
Pf) requires a different amount of work then by path (b)
...


The work done on a system depends on the path taken in
the PV diagram
...

9

An isothermal process
implies that both P and
p
V of the gas change
(PV∝T)
...

For an ideal gas (provided the
number of moles remains
ΔU constant), the change in internal
= Q −W = Q − 0
Q = ΔU = n C ΔT
energy is
V

12

For a co s a p essu e ( soba c) p ocess, the c a ge in
o constant pressure (isobaric) process, e change
internal energy is

ΔU = Q − W
where

W = PΔV = nRΔT

and Q = nC P ΔT
...

For an ideal gas CP
= CV+R
...

⎜V ⎟
⎜V ⎟
⎝ i⎠
⎝ i⎠
ΔU = 0 ⇒ Q = W

14

We have found for a monoatomic gas
ΔU = 3/2 n R ΔT
Constant volume:

ΔU= Q

3/2 n R ΔT = n CV ΔT

CV= 3/2 R

Constant pressure:

Q = ΔU + W

n CP ΔT = 3/2 n R ΔT + n R ΔT
CP= 5/2 R
CV – CP = R (always valid for any ideal gas)
15

Adiabatic (“not passable”) processes
(no heat is gained or lost by the system Q=0 i e system
Q 0, i
...

perfectly isolated )

Q=0 and so ΔU= -W

P V = constant (isothermal)
P Vγ = constant (adiabatic)
γ = CP/CV
For a monoatomic gas
therefore γ = 5/3

16

Example: An ideal gas is in contact with a heat reservoir so
that it remains at constant temperature of 300 0 K The gas
300
...

is compressed from a volume of 24
...
0 L
...
00
f
energy
...
7 mol
l
8
...
00 kJ
...

20

An ice cube placed on a countertop in a warm room will
melt
...


21

Any process that involves dissipation of energy is not
reversible
...


The second law of thermodynamics
(Clausius Statement): Heat never flows
spontaneously from a colder body to a hotter
body
...
The net work
y
g
done by an engine
during one cycle is
equal to the net heat
flow into the engine
during the cycle (ΔU= 0)
...
g
...
8
means 80% of the heat is
converted to mechanical
work)

Note: Qnet = Qin - Qout

net work output Wnet
e wo ou pu
e=
=
QH
heat input
QH − QC
QC
=
= 1−

...

Pump

Refrigerator

K = Coefficient of performance
25

26

Reversible Engines and Heat Pumps
Reversible Engines and Heat Pumps
A reversible engine can be
used as an engine (
g
(heat
input from a hot reservoir
and exhausted to a cold
reservoir) or as a heat
pump (heat is taken from
cold reservoir and
exhausted t a h t
h
t d to hot
reservoir)
...

The efficiency of this ideal reversible engine is

TC
er = 1 −
...

engine
The Carnot cycle has four steps:
1
...

2
...

3
...

4
...


30

The Carnot
engine model was
graphically
expanded upon
by Benoit Paul
Émile Clapeyron
p y
in 1834 and
mathematically
elaborated upon
by Rudolf
Clausius in the
1850s d 60s
18 0 and 60
from which the
concept of
p
entropy emerged

The Carnot cycle
illustrated

31

The Otto cycle
Its power cycle consists of adiabatic
compression,
compression heat addition at constant
volume, adiabatic expansion and
rejection of heat at constant volume and
characterized by four strokes, or
reciprocating movements of a piston in a
cylinder:
intake/induction stroke
compression stroke
power stroke
exhaust stroke

32

Entropy 
Entropy
Heat flows from objects of high temperature to objects at
low temperature because this process increases the
disorder of the system
...
Entropy is a state variable
...

T
Every i
E
irreversible process i
ibl
increases th t t l entropy of th
the total t
f the
universe
...


36

The
Th second law of thermodynamics
dl
f th
d
i
(Entropy Statement): The entropy of the
universe never decreases
...
0 °C is slowly melting
...
00 g of ice
that melts?
To melt ice requires Q = mLf joules of heat
...
7
energy
...
7 J
ΔS = =
= 1
...

T
273 K

38

Q − 300 J
ΔS h t = =
= −1 J/K
...

T
5K

300K
Q
5K

http://www
...
com/watch?v=Xa6Pctf23tQ
htt //
t b
/ t h? X 6P tf23tQ
39

Statistical Interpretation of Entropy

A microstate specifies the state of each constituent particle
in a thermodynamic system
...


40

probability of a macrostate =

number of microstates corresponding to the macrostate
total number of microstates for all possible macrostates

41

The number of microstates for a given macrostate is related
g
to the entropy
...


42

The Third Law of Thermodynamics
The Third Law of Thermodynamics

It is impossible to cool a system to absolute zero

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