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APPLICATIONS
OF THE FIRST LAW OF THERMODYNAMICS
Heat can be supplied to a thermodynamic system under
the following conditions:
-
ISOBARIC PROCESS
-
ISOCHORIC PROCESS
-
ISOTHERMAL PROCESS
-
ADIABATIC PROCESS
ISOBARIC
PROCESS
A
thermodynamic process in which pressure of the system remains constant
during the supply of heat is called an ISOBARIC PROCESS.
EXPLANATION
Consider
a cylinder fitted with a frictionless piston. The piston is free to move
in the cylinder. An ideal gas is enclosed in the cylinder.
ISOCHORIC
PROCESS
A
thermodynamic process in which the volume of the system remains constant
during the supply of heat is called an ISOCHORIC PROCESS.
 EXPLANATION
Consider
a cylinder fitted with a frictionless piston. An ideal gas is enclosed
in the cylinder. The piston is fixed at a particular position so that
the volume of cylinder remains constant during the supply of heat.
Let DQ amount
of heat is added to the system. Addition of heat causes the following
changes in the system:
-
Internal energy increases from U1 to U2.
-
Volume of the system remains unchanged.
-
Temperature increases from T1 K to T2 K.
-
Pressure increases from P1 to P2.
-
No work is performed.
According
to the first law of thermodynamics:
DQ
= DU+
DW
But DW
= PDV
Thus
DQ
= DU+
PDV
As
DV
= 0
DQ
= DU+
P (0)
DQ = DU
DQ
= DU+
DW
But DW
= PDV
Thus
DQ
= DU+
PDV
As
DV
= 0
DQ
= DU+
P (0)
DQ
= DU
This
expression indicates that the heat supplied under isochoric process is
consumed in increasing the internal energy of the system but no work is
performed.
GRAPHICAL
REPRESENTATION
Graph
between P & V for an isochoric process is a straight line, which is
parallel to P-axis.
ISOTHERMAL
PROCESS
A
thermodynamic process in which the temperature of the system remains
constant during the supply of heat is called an ISOTHERMAL PROCESS.
ISOTHERMAL
COMPRESSION
Consider
a cylinder of non-conducting walls and good heat conducting base. The cylinder
is fitted with a frictionless piston. An ideal gas is enclosed in the
cylinder. In the first stage pressure on the piston is increased and the
cylinder is placed on a cold body. Due to compression, the temperature of the
system increases but at the same time DQ amount of heat is removed from the
system and the temperature of the system is maintained.
According
to the first law of thermodynamics:
DQ
= DU+
DW
Since temperature is constant, therefore, there is no change in internal
energy of the system. i.e. DU
= 0
As the work is done on the system, therefore, DW is negative,
DQ
= 0+ (-DW)
DQ
= -DW
ISOTHERMAL
EXPANSION
in
another situation the cylinder is placed over a hot body and the pressure on
the system is decreased. Due to expansion, the temperature of the system is
decreased but at the same time DQ amount of heat is absorbed from the hot body
and the temperature of the system is again maintained.
According to the first law of thermodynamics:
 DQ
= DU+
DW
Since
temperature is constant, therefore, there is no change in internal energy of
the system. i.e. DU = 0
As the work is done by the system, therefore, DW is positive,
DQ
= 0+ (DW)
DQ
= +DW
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