Thermodynamics
Lecture 2
Basics of Thermodynamics
Dr. Sajid Hussein Alabbasi
Department of Petroleum Engineering
Engineering College – University of Basrah
Dr. Sajid Alabbasi
Dr. Sajid Alabbasi
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Dr. Sajid Alabbasi
. Useful to predict whether any change of physicist can
occur under specified conditions.
Dr. Sajid Alabbasi
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Dr. Sajid Alabbasi
Universe
Dr. Sajid Alabbasi
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Energy
In thermodynamics, we deal with change of the total energy only. Thus, the total
energy of a system can be assigned a value of zero at some reference point. Total
energy of a system has two groups: macroscopic and microscopic.
Macroscopic energy:
forms of energy that a system posses as a whole with respect to some outside
reference frame, such as kinetic and potential energy. The macroscopic energy of a
system is related to motion and the influence of some external effects such as gravity,
magnetism, electricity, and surface tension.
• Kinetic energy: energy that a system posses as a result of its relative motion
relative to some reference frame, KE
mV 2
KE = (kJ)
2
where V is the velocity of the system in (m/s).
• Potential energy: is the energy that a system posses as
a result of its elevation in a gravitational field, PE
PE = mgz (kJ)
where g is the gravitational acceleration and z is the
elevation of the center of gravity of the system relative to
some arbitrary reference plane.
Microscopic energy:
Forms of energy, are those related to molecular structure of a system.
They are independent of outside reference frames. The sum of
microscopic energy is called the internal energy, U. Internal energy is
related to the molecular structure and the degree of molecular activity
and it may be viewed as the sum of the kinetic and potential energies of
molecules.
The total energy of a system consists of the kinetic, potential, and
internal energies:
mV2
E =U +KE+PE =U + + mgz (kJ)
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where the contributions of magnetic, electric, nuclear energy are
neglected.
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• The sum of translational, vibrational, and
rotational energies of molecules is the kinetic
energy of molecules, and it is also called the
sensible energy. At higher temperatures, system will
have higher sensible energy.
• Internal energy associated with the phase of a
system is called latent heat. The intermolecular
forces are strongest in solids and weakest in gases.
• The internal energy associated with the atomic
bonds in a molecule is called chemical or bond
energy. The tremendous amount of energy
associated with the bonds within the nucleolus of
atom itself is called atomic energy.
Energy interactions with a closed system can occur
via heat transfer and work.
Equilibrium
At a given state, all the properties of a system have fixed values. Thus, if
the value of even one property changes, the state will change to different
one.
In an equilibrium state, there are no unbalanced potentials (or driving
forces) within the system. A system in equilibrium experiences no changes
when it is isolated from its surroundings.
Thermal equilibrium:
When the temperature is the same throughout the entire system.
Mechanical equilibrium:
When there is no change in pressure at any point of the system. However,
the pressure may vary within the system due to gravitational effects.
Phase equilibrium:
In a two phase system, when the mass of each phase reaches an
equilibrium level.
Chemical equilibrium:
When the chemical composition of a system does not change with time,
i.e., no chemical reactions occur.
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Polytropic Process;
Its P-V relation can be expressed as
PVn = Constant
where n is a constant for a specific process
If n = ɤ , the process is to be called adiabatic process, if the gas
is an ideal gas. Then,
PVɤ = Constant
ɤ = Cp / Cv , Where
Cp : specific heat at
constant pressure
Cv : specific heat at
constant volume
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If n = ∞ , the process is to be called Isochoric process i.e
V = Constant (P)(1/∞) = Constant
If the gas is an ideal gas. Then,
P / T = Constant
If n = 0, the process is to be called isobaric process i.e
P = Constant, if the gas is an ideal gas. Then,
V / T = Constant
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• A thermodynamic cycle is a sequence
of different processes that begins and
ends at the same thermodynamic
state.
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