Thermodynamics

2. Thermodynamics-I
Lecture No.03
Akbar Ali Qureshi
Lecturer
Email: akbaraliqureshi@bzu.edu.pk
Contact no: 0335-6387138
Mechanical Engineering Department
UCE&T, BZU Multan.

3. Thermal Equilibrium
Two bodies are said to be at thermal equilibrium if they are at the same
temperature. This means there is no net exchange of thermal energy between the
two bodies. The top pair of objects are in contact, but since they are at different
temps, they are not in thermal equilibrium, and energy is flowing from the hot side
to the cold side.
hot coldheat
26°C 26°C
No net heat flow
The two purple objects are at the same temp and, therefore are in thermal
equilibrium. There is no net flow of heat energy here.

4. 4
Temperature and Zeroth Law of Thermodynamics
• The zeroth law of thermodynamics: If two bodies are in thermal
equilibrium with a third body, they are also in thermal equilibrium with
each other.
• By replacing the third body with a thermometer, the zeroth law can
be restated as two bodies are in thermal equilibrium if both have the
same temperature reading even if they are not in contact.
Two bodies reaching
thermal equilibrium after
being brought into contact
in an isolated enclosure.

5. The Zeroth Law of Thermodynamics
We have already discussed the zeroth law, and
include it here for completeness:
If object A is in thermal equilibrium with object
C, and object B is separately in thermal
equilibrium with object C, then objects A and B
will be in thermal equilibrium if they are placed
in thermal contact.

6. 6
Temperature Scales
• All temperature scales are based on some easily
reproducible states such as the freezing and boiling
points of water: the ice point and the steam point.
• Ice point: A mixture of ice and water that is in
equilibrium at 1 atm pressure (0°C or 32°F).
• Steam point: A mixture of liquid water and water
vapor (with no air) in equilibrium at 1 atm pressure
(100°C or 212°F).
• Celsius scale: in SI unit system
• Fahrenheit scale: in English unit system
• Thermodynamic temperature scale: A temperature
scale that is independent of the properties of any
substance.
• Kelvin scale (SI) Rankine scale (E)
• A temperature scale nearly identical to the Kelvin
scale is the ideal-gas temperature scale.

7. 7
Pressure
The normal stress (or “pressure”) on
the feet of a chubby person is much
greater than on the feet of a slim
person.
Pressure: A normal force exerted by a
fluid per unit area
68 kg 136 kg
Afeet=300c
m2
0.23
kgf/cm2
0.46
kgf/cm2
P=68/300=0.23 kgf/cm2

8.  In thermodynamics, an equation of state is a relation
between state variables. More specifically, an equation
of state is a thermodynamic equation describing the
state of matter under a given set of physical conditions.
It is a constitutive equation which provides a
mathematical relationship between two or more state
functions associated with the matter, such as its
temperature, pressure, volume, or internal energy.

9. Boyle’s law & Charles law
 Boyle's law
 Boyle noted that the gas volume varied inversely with the pressure. In
mathematical form, this can be stated as:
 Charles's law
 In 1787 Jacques Charles indicated a linear relationship between volume and
temperature:

10.  The ideal gas law is the equation of state of a hypothetical
ideal gas. It is a good approximation to the behavior of
many gases under many conditions. The ideal gas law is
often introduced in its common form:
 where P is the absolute pressure of the gas, V is the volume of the gas, n is
the amount of substance of gas (measured in moles), T is the absolute
temperature of the gas and R is the ideal, or universal, gas constant.
 In SI units, P is measured in Pascal, V is measured in cubic meters, n is
measured in moles, and T in Kelvin (273.15 Kelvin = 0 degrees Celsius). R
has the value 8.314 J·K−1·mol−1 or 0.08206 L·atm·mol−1·K−1 if using pressure
in standard atmospheres (atm) instead of Pascal, and volume in liters
instead of cubic meters.

11. Molar form
 By replacing n with m / M, and subsequently introducing density
ρ = m/V, we get:

12. Ideal Gas Equation

14. Work
Work is a Mechanical form of Energy:
DistanceForceWork 
xFdW 
pdVdW 
∆X
Force
e.g. Raising
Piston,

15. Work
 Work is the energy transferred
between a system and
environment when a net force
acts on the system over a
distance.
 Work is positive when the force
is in the direction of motion.
 Work is negative when the
force is opposite to the
motion.

16. Heat
• The energy transferred in a thermal interaction is called heat.
• The symbol for heat is Q.
• The energy equation now becomes
ΔEsys = ΔEmech + ΔEth = Wext + Q
Quick quiz: A gas cylinder and piston are covered with heavy insulation. The piston is
pushed into the cylinder, compressing the gas. In the process. The gas temperature
• Increases
• Decreases
• Doesn’t change

17. Heat and Thermal interactions

18.  Thermal energy is an energy of the system due to the motion
of its atoms and molecules. Thermal energy is a state variable,
it may change during a process. The system’s thermal energy
continues to exist even if the system is isolated and not
interacting thermally with its environment
 Heat is energy transferred between the system and the
environment as they interact. Heat is not a particular form of
energy, nor is it a state variable. Heat may cause the system’s
thermal energy to change, but that does not mean that heat
and thermal energy are the same thing.
 Temperature is a state variable, it is related to the thermal
energy per molecule. But not the same thing.

19. Internal energy (also called thermal energy) is the energy an object or
substance is due to the kinetic and potential energies associated with the
random motions of all the particles that make it up. The kinetic energy is, of
course, due to the motion of the particles. To understand the potential energy,
imagine a solid in which all of its molecules are bound to its neighbors by
springs. As the molecules vibrate, the springs are compressed and stretched.
The hotter something is, the faster its molecules are
moving or vibrating, and the higher its temperature.
Temperature is proportional to the average kinetic
energy of the atoms or molecules that make up a
substance.

20. Internal Energy

21. The term heat refers is the energy that is transferred from one body or location
due to a difference in temperature. This is similar to the idea of work, which is
the energy that is transferred from one body to another due to forces that act
between them. Heat is internal energy when it is transferred between bodies.
Technically, a hot potato does not possess heat; rather it possesses
a good deal of internal energy on account of the motion of its molecules. If that
potato is dropped in a bowl of cold water, we can talk about heat: There is a heat
flow (energy transfer) from the hot potato to the cold water; the potato’s internal
energy is decreased, while the water’s is increased by the same amount.

22. Temperature and internal energy are related but not the same thing. Temperature
is directly proportional to the average molecular kinetic energy.
Consider a bucket of hot water and a swimming pool full of cold water. The hot
water is at a higher temperature, but the pool water actually has more internal
energy! This is because, even though the average kinetic energy of the water
molecules in the bucket is much greater than that of the pool, there are thousands
of times more molecules in the pool, so their total energy is greater.

23. Energy is always conserved. It can change forms: kinetic, potential,
internal etc., but the total energy is a constant. Another way to say it is
that the change in thermal energy of a system is equal to the sum of the
work done on it and the amount of heat energy transferred to it.

24. First Law of Thermodynamics

25. PdV Work
Let the Piston be moving from
Thermodynamic Equilibrium State 1 (P1, V1)
to State 2 (P2, V2).
Let the values at any intermediate
Equilibrium State is given by P and V.
State 2State 1
P1 V1
P2 V2
Area A
For an Infinitesimal displacement, dL, the Infinitesimal Work done is;
Similarly, for Process 1 – 2; we can say that;

2
1
21
V
V
PdVW
Volume
Pressure
Quasi-Static
Process Path
P1
P2
V1 V2
dW = F * dL = P*A*dL = PdV

26. PdV Work
)( 1221
2
1
VVPPdVW
V
V
 
Pressure(P)
Volume (V)
P=Const
Isobaric
W1-2
State 1 State 2
V2V1
• 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
• Isobaric Process

27. Pressure(P)
V=Const
Isochoric
Volume (V)
State 1
State 2P2
P1
0
2
1
21  
V
V
PdVW
PdV Work
• 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 Process

28. 2
1
11
1
2
111121
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21
lnln
2
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P
P
VP
V
V
VP
V
dV
VPW
V
VP
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V
V
V
V







PdV Work
Isothermal
Process:
Pressure
PV = C
Quasi-Static
Volume
State 1
State 2
P2
P1
V1 V2
 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 (W)

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PdV Work
Pressure
PVn = C
Volume
1
2
P2
P1
n =1
n =3 n =2
n =∞
 An adiabatic process transfers no heat
therefore Q = 0
 ΔU = Q – 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.

30. The End