Revision Notes on Thermodynamics Thermodynamics It is the branch of physics which deals with process involving heat work and internal energy Thermodynamics is concerned with macroscopic behavior rather than microscopic behavior of the system Basic Terminology System Part of the universe under investigation Open System A system which can exchange both energy and matter with its Closed System A system which permits passage of energy but not mass across its Isolated system A system which can neither exchange energy nor matter with its Surroundings Part of the universe other than system which can interact with it Boundary Anything which separates system from surrounding State variables The variables which are required to be defined in order to define state of any system i e pressure volume mass temperature surface area etc State Functions Property of system which depend only on the state of the system and not Example Pressure volume temperature internal energy enthalpy surroundings boundary surrounding on the path entropy etc Intensive properties Properties of a system which do not depend on mass of the system i e Temperature pressure density concentration Extensive properties Properties of a system which depend on mass of the system i e Volume Process Path along which state of a system changes energy enthalpy entropy etc Isothermal process Process which takes place at constant temperature Isobaric process Process which takes place at constant pressure Isochoric process Process which takes place at constant volume Adiabatic process Process during which transfer of heat cannot take place between system Cyclic process Process in which system comes back to its initial state after undergoing Reversible process Process during which the system always departs infinitesimally from the state of equilibrium i e its direction can be reversed at any moment and surrounding series of changes Kinetic Energy Energy possessed by the atoms or molecules by virtue of their motion is called kinetic energy Internal Energy U Sum total of kinetic and potential energies of atoms molecules constituting a system is called the internal energy of the system a U is taken as positive if the internal energy of the system increases b U is taken as negative if the internal energy of the system decreases Heat Heat is the part of internal energy which is transferred from one body to another an account of the temperature difference Work Work is said to be done when a force acting on a system displaces the body in its own direction dW Fdx PdV W P Vf Vi therefore W will be positive a If the gas expands work is said to be done by the system In this case Vf Vi b If the gas is compressed work is said to be done on the system In this case Vf Vi therefore work done is negative Thermodynamic variables or parameters The thermodynamic state of system can be determined by quantities like temperature T volume V pressure P internal energy U etc These quantities are known as thermodynamic variables or the parameters of the system Equation of state A relation between the values of any of the three thermodynamic variables for the system is called its equation of state Equation of state for an ideal gas is PV RT Equilibrium of a system A system is said to be in equilibrium if its macroscopic quantities do not change with time Relation between joule and calorie 1 joule 4 186 cal First law of thermodynamics If the quantity of heat supplied to a system is capable of doing work then the quantity of heat absorbed by the system is equal to the sum of the increase in the internal energy of the s ystem and the external work done by it dQ dU dW Thermodynamic Process A process by which one or more parameters of thermodynamic system undergo a change is called a thermodynamic process or a thermodynamic change a Isothermal process The process in which change in pressure and volume takes place at a constant temperature is called a isothermal change It may be noted that in such a change total amount of heat of the system does not remain constant b Isobaric process The process in which change in volume and temperature of a gas take place at a constant pressure is called an isobaric process c Isochoric process The process in which changes in pressure and temperature take place in such a way that the volume of the system remains constant is called isochoric process d Adiabatic process The process in which change in pressure and volume and temperature takes place without any heat entering or leaving the system is called adiabatic change e Quasi static process The process in which change in any of the parameters take place at such a slow speed that the values of P V and T can be taken to be practically constant is called a quasi static process f Cyclic process In a system in which the parameters acquire the original values the process is called a cyclic process g Free expansion Such an expansion in which no external work is done and the total internal energy of the system remains constant is called free expansion Reversible isothermal and adiabatic curve Application of first law of thermodynamics a Cooling caused in adiabatic process dT PdV Cv b Melting dU mLf c Boiling dU mLv P Vf Vi d Mayer s formula Cp Cv R Specific heat capacity of gases Specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of a unit mass of substance through 1 C a Specific heat capacity at constant volume cv Specific heat capacity at constant volume is defined as the amount of heat required to raise the temperature of 1 g of the gas through 1 C keeping volume of the gas constant Molar specific heat capacity at constant volume Cv is defined as the amount of heat required to raise the temperature of 1 mole of gas through 1 C keeping its volume constant Cv Mcv b Specific heat capacity at constant pressure cp Specific heat capacity at constant pressure is defined as the amount of heat required to raise the temperature of 1 g of gas through 1 C keeping its pressure constant Gram molecular specific heat capacity of a gas Cp at constant pressure is defined as the amount of heat required to raise the temperature of 1 mole of the gas through 1 C keeping its pressure constant Cp Mcp Difference between two specific heat capacities Mayer s formula a Cp Cv R J b For 1 g of gas cp cv r J c Adiabatic gas constant Cp Cv cp cv Relation of Cv with energy Cv 1 m dU dT a Mono atomic gas 3 degree of freedom Total energy U mN 3 1 2 KT Here m is the
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