NEET Physics Notes - Chapter 14 Thermodynamics PDF

60 TThermodynamics 647 Chapter E3 14 U ID Thermodynamics D YG Thermodynamics is a branch of science which deals with exchange of heat energy between bodies and conversion of the heat energy into mechanical energy and vice-versa. Some Definitions U (1) Thermodynamic system (i) It is a collection of a...

60 TThermodynamics 647 Chapter E3 14 U ID Thermodynamics D YG Thermodynamics is a branch of science which deals with exchange of heat energy between bodies and conversion of the heat energy into mechanical energy and vice-versa. Some Definitions U (1) Thermodynamic system (i) It is a collection of an extremely large number of atoms or molecules (ii) It is confined with in certain boundaries. (iii) Anything outside the thermodynamic system to which energy or matter is exchanged is called its surroundings. ST Surrounding Gas Fig. 14.1 System (iv) Thermodynamic system may be of three types (a) Open system : It exchange both energy and matter with the surrounding. (b) Closed system : It exchange only energy (not matter) with the surroundings. (c) Isolated system : It exchange neither energy nor matter with the surrounding. (2) Thermodynamic variables and equation of state : A thermodynamic system can be described by specifying its pressure, volume, temperature, internal energy and the number of moles. These parameters are called thermodynamic variables. The relation between the thermodynamic variables (P, V, T) of the system is called equation of state. For  moles of an ideal gas, equation of state is PV = RT and for 1 mole of an it ideal gas is PV = RT (3) Thermodynamic equilibrium : In steady state thermodynamic variables are independent of time and the system is said to be in the state of thermodynamic equilibrium. For a system to be in thermodynamic equilibrium, the following conditions must be fulfilled. (i) Mechanical equilibrium : There is no unbalanced force between the system and its surroundings. (ii) Thermal equilibrium : There is a uniform temperature in all parts of the system and is same as that of surrounding. (iii) Chemical equilibrium : There is a uniform chemical composition through out the system and the surrounding. (4) Thermodynamic process : The process of change of state of a system involves change of thermodynamic variables such as pressure P, volume V and temperature T of the system. The process is known as thermodynamic process. Some important processes are (i) Isothermal process : Temperature remain constant (ii) Adiabatic process : No transfer of heat (iii) Isobaric process : Pressure remains constant (iv) Isochoric (isovolumic process) : Volume remains constant (v) Cyclic and non-cyclic process : Incyclic process Initial and final states are same while in non-cyclic process these states are different. (vi) Reversible and irreversible process : 648 Thermodynamics (5) Indicator diagram : Whenever the state of a gas (P, V, T) is changed, we say the gaseous system is undergone a thermodynamic process. The graphical representation of the change in state of a gas by a thermodynamic process is called indicator diagram. Indicator diagram is plotted generally in pressure and volume of gas. Zeroth Law of Thermodynamics If systems A and B are each in thermal equilibrium with a third system C, then A and B are in thermal equilibrium with each other. (iii) Change in internal energy does not depend on the path of the process. So it is called a point function i.e. it depends only on the initial and final states of the system, i.e. U  U f  Ui (3) Work (W) : Suppose a gas is confined in a cylinder that has a movable piston at one end. If P be the pressure of the gas in the cylinder, then force exerted by the gas on the piston of the cylinder F = PA (A = Area of cross-section of piston) Piston Conducting System System System System A B A B System System C C Conducting 60 Insulating dx F Insulating (B) Fig. 14.3 (1) The zeroth law leads to the concept of temperature. All bodies in thermal equilibrium must have a common property which has the same value for all of them. This property is called the temperature. For a finite change in volume from V to V i Total amount of work done W  ID (2) The zeroth law came to light long after the first and seconds laws of thermodynamics had been discovered and numbered. It is so named because it logically precedes the first and second laws of thermodynamics. Fig. 14.4 When the piston is pushed outward an infinitesimal distance dx, the work done by the gas dW  F.dx  P( A dx )  P dV E3 (A) Heat, Internal Energy and Work in Thermodynamics D YG (i) Heat is a path dependent quantity e.g. Heat required to change the temperature of a given gas at a constant pressure is different from that required to change the temperature of same gas through same amount at constant volume. (ii) For gases when heat is absorbed and temperature changes  Q  CT At constant pressure (Q)P  C P T  f Vf Vi P dV  P(Vf  Vi ) (i) If we draw indicator diagram, the area bounded by PV-graph and volume axis represents the work done P P U (1) Heat (Q) : It is the energy that is transferred between a system and its environment because of the temperature difference between them. Gas 2 1 V2 V V1 P Work = Area = P(V – V ) A Fig. 14.5 Work  P  V2 V1 PdV  P(V2  V1 ) At constant volume (Q)V  C V T (2) Internal energy (U) : Internal energy of a system is the energy possessed by the system due to molecular motion and molecular configuration. U The energy due to molecular motion is called internal kinetic energy U and that due to molecular configuration is called internal potential energy U i.e. Total internal energy U  UK  UP V1 dV V2 V Fig. 14.6 P K Work = 0 P ST (i) For an ideal gas, as there is no molecular attraction U p  0 i.e. internal energy of an ideal gas is totally kinetic and is given by U  UK  3 3 RT and change in internal energy U  R T 2 2 (ii) In case of gases whatever be the process U     R(Tf  Ti ) R f T  R T  C V T   (  1)  1 2 RT f  RTi  1  (Pf V f  Pi Vi )  1 V Fig. 14.7 P P2 Work = Area of the shown trapezium  P1 V2 V1 1 (P1  P2 ) (V2  V1 ) 2 V Fig. 14.8 (ii) From W  PV  P(V f  Vi ) If system expands against some external force then V f  Vi  W = positive TThermodynamics 649 Q = U + W If system contracts because of external force then V f  Vi  W = negative B P B P Positive work (3) It makes no distinction between work and heat as according to it the internal energy (and hence temperature) of a system may be increased either by adding heat to it or doing work on it or both. Negative work (4) Q and W are the path functions but U is the point function. A (5) In the above equation all three quantities Q, U and W must be expressed either in Joule or in calorie. A V (A) Expansion (B) Compression V Fig. depends 14.9 (iii) Like heat, work done is also upon initial and final state of the system and path adopted for the process 60 (7) Limitation : First law of thermodynamics does not indicate the direction of heat transfer. It does not tell anything about the conditions, under which heat can be transformed into work and also it does not indicate as to why the whole of heat energy cannot be converted into mechanical work continuously. P A P 1 2 A B A1 (6) The first law introduces the concept of internal energy. Table 14.1 : Useful sign convention in thermodynamics B 1 E3 V (A) Less area Quantity P V 2 A Q A2 B (B) More area A