Thermodynamics

THERMODYNAMICS Thermodynamics is the study of transfer of thermal energy to mechanical energy. Using this thermodynamic system is studies. A thermodynamics system may be an assembly of large number of particles having a certain value of Pressure (P), Volume (V) and Temperature (T). These are known as thermodynamic variables- Change of State of a thermodynamic system is due to the change in one or more of these variables. Thermodynamic System + Surrounding = Universe Zeroth Law of Thermodynamics Two bodies or system that is in thermal equilibrium with a third body are in thermal equilibrium with each other. In figure Heat will be exchanged between A and C also between B and C till they reach in thermal equilibrium i.e., If TA  TC and TB  TC then TA  TB Internal Energy Internal Energy of a system is a state variable it depends only on the initial and final state. It is the function of Temperature and Volume. Internal Energy U = Kinetic Energy + Potential Energy of the Molecules of the Molecules First Law of Thermodynamics The First law of thermodynamics is based on the idea that energy is neither created nor destroyed in any thermodynamic system. The heat Q added to the system can cause two changes 1. Increased the internal energy by raising the temperature of the gas. 2. Can cause the gas to expand in means allowing the gas to work. Heat added to System = Increase in internal Energy of the system + External work done by the System Q=ΔU+W If the system is isolated from the surrounding no heat exchange between system and surrounding takes place Q  0 that is why work done W  0 . Hence  U  0 or U  co n s tan t . In other words the internal energy of isolated system remains constant. In case of cyclic system there will be not change in internal energy  U  0 , heat added will be equal to work done Q  W . Ideal Gas Law The absolute pressure P of n kilo moles of gas in a volume V is related to the temperature T as PV  nRT Thermodynamic Process A thermodynamic process involves change of heat energy into mechanical energy by changing thermodynamic variables like P, V, T or internal energy etc with time. Isothermal Process It is a thermodynamic process in which the temperature remains constant. We know the ideal gas equation for 1 mole of gas is PV = RT Since T = constant PV= constant or P 1 V 1  P 2 V 2 The internal energy of an ideal gas depends only on temperature. Since temperature is constant V  0 Hence for Isothermal process 1st law of thermodynamics Q  W  PV V  W  2.303log10  2   V1  For Isothermal expansion V2  V1 work done (W) will be positive. For Isothermal compression V2  V1 work done (W) will be negative. Adiabatic Process It is a thermodynamic process in which amount of heat energy is kept constant i.e., heat is neither allowed to enter or leave a system. Application of 1st law to adiabatic process Q= ΔU+W Since Q=0 therefore ΔU = −W = −PΔV Ist Law Ideal gas equation for adiabatic process is PV y  cons tan t Where y  Cp Cv  ratio of specific heats ) Thus for change in State from ( P 1 , V 1 ) to ( P 2 , V P1 V 1 y  P 2 V y 2 2 and work done in case of Adiabatic process is W here  R (T1  T 2 ) y  1 T1  Initial temperature of the system. T2  Final temperature of the system Isobaric Process It is a thermodynamic process in which pressure is kept constant. Work done in case of Isobaric Process is W  P (V 2  V 1 ) here V2  Final volume of the gas V1  Initial volume of the gas Isochoric Process It is a thermodynamic process in which pressure is kept constant. Work done in case of Isochoric process is W  0 Since dV  0 = no change in volume of gas Quasi-Static Process It is the process in which the system changes its variables (P, V, T) so slowly that it remains in thermal and mechanical equilibrium with its surroundings throughout the process. It means Quasi-Static process is infinitely slow process. At every stage the difference in the pressure and temperature of the system and its surrounding remains infinitesimal. Limitations of 1st Law of Thermodynamics 1st Law states that the energy remains conserved but it doesn’t indicate 1. The direction of heat transfer. 2. The conditions under which heat can be converted into work. 3. It doesn’t tell why the whole of heat energy cannot be converted into mechanical work. 4. There are many irreversible process occur in nature. 1st law cannot explain the lack of reversibility ex: heat flows naturally from a hot body to cold body but heat will never of itself flow from a cold body to a hot body. Second Law of Thermodynamics Kelvin Planck Statement It is not possible to construct a heat engine which can absorb heat from a reservoir and convert the entire heat into work. Clausices Statement It is not possible to transfer heat from a cold body to hot body without the help of some external agency. Reversible process: Suppose in a process a thermodynamic system goes from an initial state i to a final state f. During the process the system absorbs heat Q from the surroundings and performs work W on it. If the process can be turned back such that both the system and the surroundings returns to their original states with no other change anywhere else in the universe then it is called reversible process. If the system and surroundings cannot be turned back to original states then the process is called irreversible process. Cause of Irreversibility: A system will be irreversible 1. If the system is passing through non-equilibrium states ex: from expansion or explosive chemical reaction. 2. Process involves dissipative effects like friction, viscosity, etc, which causes loss of mechanical energy. It is impossible to eliminate them completely.