Thermodynamics

1
Thermodynamics

• Thermal Processes
• The 2nd Law of Thermodynamics
• Entropy

2
Isovolumetric Processes

• No change in volume so no work done on system
• Change in system energy equals heat transfer

3
Isothermal Process

• Temperature of system remains constant
• Internal energy of system remains constant
• Integrating P?V gives the work done on the
  environment

4
Heat Engines

• Working substance goes through cyclic process
• Energy transferred by heat from hot reservoir
• Work done by engine
• Energy expelled by heat to cold reservoir
• ?U 0 so

1
2
3
Fig. 12.9, p. 399
5
Heat Engine Efficiency

• If working substance is a gas, work done in one
  cycle is area enclosed in PV diagram
• Thermal efficiency is work done divided by energy
  absorbed

Fig. 12.10, p. 399
6
Refrigerators and Heat Pumps

• Heat engine operating in reverse
• Can be used to cool the cold reservoir or warm
  the warm hot reservoir

Fig. 12.12, p. 403
7
2nd Law of Thermodynamics

• No heat engine operating in a cycle can absorb
  energy from a reservoir and use it entirely for
  the performance of an equal amount of work.
• The efficiency of heat engines is always less
  than one.
• If two systems are in thermal contact, net
  thermal energy transfers spontaneously by heat
  from the hotter system to the colder system.

8
Reversible vs. Irreverisble

• Reversible Process every state along the path is
  an equilibrium state.
• System can revert to initial state by reversing
  path.
• Real processes are always irreversible (although
  some are almost reversible).

9
Carnot Cycle
Fig. 12.15, p. 406
Fig. 12.16, p. 407
10
Entropy

• A state variable related to the 2nd Law of
  Thermodynamics
• Energy absorbed/expelled during a reversible,
  isothermal process between 2 equilibrium states
  defines change in entropy
• The entropy of the universe increases in all
  natural processes

11
Entropy and Disorder

• Disorderly arrangement is more probable
• Isolated systems tend toward greater disorder
• Entropy is a measure of disorder

• W is the probability that a system has a
  particular configuration