First Law of Thermodynamics



THERMODYNAMICS
THERMODYNAMICS
Thermodynamics is the branch of science which deals with the interconversion of heat energy and mechanical energy. All those problems that are related to the interconversion of heat energy and work done are studied in thermodynamics. In thermodynamics we discuss different cycles such as Carnot cycle, Rankine cycle, Otto cycle, diesel cycle, refrigerator, IC engines, EC engines, Compressors, turbines and air conditioners.
FIRST LAW OF THERMODYNAMICS
STATEMENT:                                                                                                                      
“During any process total energy of a system and its surroundings is constant.”
OR
“It is impossible to construct a machine which performs work
continuously with taking energy from an external source.” 

OR
“Energy can neither be created nor destroyed but it can be
converted from one form of energy to another form of energy.”
MATHEMATICAL REPRESENTATION
Let a system absorbs DQ amount of heat energy. Addition of heat energy increases the internal energy of system from Uto U2 and some useful work is also performed by the system.
Increase in internal energy is given by:
DU = U1 – U2
and
work done is DW
According to the first law of thermodynamics:
                                                                                                    DQ = DU+ DW
 
SIGN CONVENTION:
DQ = positive if heat is added to a system
DQ = negative if heat is released from a system
D= positive if work is done by the system

DW = negative if work is done on the system
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.


Let the initial volume of the system is V1 and initial internal energy is U1Let DQP the gas is heated from

T1 K to T2 K. Addition of heat causes the following changes in the system:
 Internal energy increases from U1 to U2.
 Volume of the system increases from V1 to V2.
 Temperature increases from T1 K to T2 K.
 Work (DW) is done by the gas on the piston.
According to the first law of
thermodynamics:
                                                                                             DQ = DU+ DW 
But DW = PDV
Thus
                                                                                          DQP = DU+ 
PDAs 
DV = (V2 – V1)
                                                                                          DQP = DU+ 
P (V2 – V1)
GRAPHICAL REPRESENTATION
Graph between P & V for an isobaric process is a straight line which is parallel to V-axis.
isobaric process