Chapter 12      Laws of Thermodynamics

1.  System and surroundings

     State Variables, diathermic and adiabatic walls

      Thermal Equilibrium-  When there is no net flow of heat between two systems in thermal contact, the system are at the same temperature and in thermal equilibrium

      Work in thermodynamic processes

2.  First law of thermodynamics-  statement of conservation of energy,  relates the changes in internal energy of a system  to the heat flow in or out, and the work done on or by the system

        DU =  Q - W         sign convention, Q-positive if  heat flows in to the system, Q-negative if heat heat flows out

       W-positive when work is done by the system, W-negative if work is done on the system

       Thermal processes-  Isobaric (constant pressure)    W = PDV = P ( V_f - V_i )

                                       Isochoric (constant volume)    W = 0

                                       Isothermal (constant temperature)   W =  nRT ln(V_f/V_i)

                                       Adiabatic ( no heat in or out)    P_iV_i^g = P_fV_f^g;      

                                            W = 3nR(T_f-T_i)/2 for mono-atomic gases

  3. Second law of thermodynamics- Heat by itself will not flow from a body at a lower temperature to a body at higher temperature

     Heat engine-  A device that converts heat to useful work-  It takes in heat from a high temperature reservoir(Q_h), converts part of it to work(W) and rejects the rest of the energy to the cold reservoir(Q_c)

                        W = Q_h - Q_c

                        Efficiency    e = Work/ heat input   =  W/Q_h=  1 - Q_c/Q_h  = 1 - T_c/T_h

   Reversible and irreversible processes

                        Carnot engine- ideal engine- considers a cycle of all reversible processes. No engine can be more efficient that his ideal engine. 

    Refrigerators/ Heat pumps-  are heat engines operated in reverse

    Entropy (S)           DS  = ( Q/T)_R

                          Reversible processes- DS_universe = 0

                          Irreversible processes-DS_universe  > 0

                          Principle of increase of entropy