
 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 U_{1 }to U_{2} and some useful work is also performed by the system. Increase in internal energy is given by: DU = U_{1} – U_{2} 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 DW = 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 V_{1} and initial internal energy is U_{1}. Let DQ_{P}_{ }the gas is heated from


T1 K to T2 K. Addition of heat causes the following changes in the system: 
Internal energy increases from U_{1} to U_{2}. Volume of the system increases from V_{1} to V_{2}. Temperature increases from T_{1} K to T_{2} 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 DQ_{P}_{} = DU+ PDV As DV = (V_{2} – V_{1}) DQ_{P}_{} = DU+ P (V_{2} – V_{1}) 
GRAPHICAL REPRESENTATION  
Graph between P & V for an isobaric process is a straight line which is parallel to Vaxis. 
