Chapter 18: The Laws of Thermodynamics  Thermodynamics is the study of heat and work  We have already discussed the 3 rd law - It is impossible to lower the temperature of an object to absolute 0 K.  Again, consider two isolated objects which are insulated from their surroundings (no heat flow in/out) with temperatures T 1 and T 2 - they are surrounding by adiabatic walls  They are brought into contact (still enclosed by adiabatic walls), but with a connecting wall which allows heat flow (diathermal wall)

But if Q=0, then T 1 =T 2 The two objects are in Thermal Equilibrium If a third object with temperature T 3 is brought into contact with object 1 and Q is measured to be zero, then T 1 =T 3 and the two objects are in thermal equilibrium (TE) also This implies objects 2 and 3 are in TE and that no heat will flow between them The 0 th law of thermodynamics- if objects 1 and 2 are in TE, and objects 1 and 3 are in TE, then objects 2 and 3 must also be in TE T1T1 T2T2 T1T1 T2T2 Q

The 1 st Law of Thermodynamics Consider a system (a gas enclosed within rigid walls). It has some initial temperature T i or initial internal energy (sum of kinetic plus potential energies) U i or E i int If heat is added to it, its internal energy increases to E f int, giving a change in internal energy   E int = E f int - E i int = Q Next consider the same system, but enclosed by adiabatic walls (Q=0), but one wall is a moving piston, so the volume can expand The system then does work on the surroundings, so its internal energy decreases E i int Q

  E int = E f int - E i int = -W Combining the two gives the 1 st Law of Thermodynamics We must be careful about the signs of Q and W - Q positive  system gains heat - Q negative  system loses heat - W positive  Work done by system on surroundings - W negative  Work done on system by surroundings E int is a state function or variable of the system, only initial/final values important. Q and W are not!