Physics 211 lecture 35: The 2nd Law of Thermodynamics

1
Physics 211 lecture 35 The 2nd Law of
Thermodynamics

• What is a Heat Engine?
• Definition A device that operates on a cycle by
  taking in energy in the form of heat to produce
  an output in the form of work.
• Diagram

Hot reservoir
Qh heat input
engine
W work output
Qc heat output (exhaust)
Cold reservoir
2
Efficiency of Heat Engines
2nd Law of Thermodynamics

• Statement
• It is impossible to make a heat engine that
  operates on a cycle and produces an amount of
  work equal to the energy input
• Meaning
• Efficiency can never reach 100
• You can never reach absolute zero
• Entropy (a measure of disorder or how much heat
  is spread out) always increases

3

• Example Ch22 2
• 30 efficient heat engine performs 200J of work
  in each cycle. How much heat is
• required (taken in) per cycle?
• expelled (exhausted) per cycle

4
Example Ch22 6 Liquid Al (660C, Lf
3.97x105J/kg) and solid Hg (-38.9C, Lv
1.18x104J/kg) are heat reservoirs for engine
that runs by freezing 1g Al and melting 15g Hg
per cycle. What is the engine efficiency?
5
Questions for Groups

• 22.1 What are some factors affecting efficiency
  of automobile engines.
• 22.4 Is it possible to construct a heat engine
  that creates no thermal pollution? What does
  this say about environmental considerations for
  an industrialized society?